Wednesday, January 30, 2008

12 comments:
HOW HAPPY WILL YOUR FRIDAY BE?

Uncertainty abounds in the latest round of winter weather set to affect the Mid-Atlantic Thursday night into Friday. High pressure settling into place for the next 2 days will allow overnight temperatures to refreeze the ground and make all surfaces quite cold as the next storm system approaches from the Tennessee Valley. Overnight Thursday into Friday it seems likely that west of I-95 in Maryland will see several hours of freezing rain. Rapid arrival of the storm will allow low level cold air to hang tough into the early morning Friday, and will take until well after sunrise for the southeast and easterly flow to scour out that cold air. However, low dewpoints also mean the potential for a delayed start of precip, which for those hoping to get school delayed, could be a "mixed" blessing, no weather puns intended.


Since many of you come to this site for the "inside scoop" on weather events like this one, here's the breakdown: A tricky call for school districts in central Maryland. Baltimore County especially will face a challenging decision for several reasons:
- There will be ice accretion in roughly two-thirds of the county, north and west of a line from Fork to Towson to Catonsville, enough to disrupt morning traffic only. South and west of a line from White Marsh to Glen Burnie, expect a brief period of freezing rain overnight turning to all rain. If your commute keeps you along or east of 95, icing will be minimal.
- All frozen precip will change over to rain by 10 AM, with exception of far N and W, such as Hereford Zone and obviously into the more rural counties like Carroll and Frederick.
- Despite this changeover likelihood, the issue becomes... will it occur quickly enough (say by 8AM) so as to let a 2 hour delay stand. We all know that if a school system announces the call at 5:30, they can't wait until 8AM to change it, because by then buses would already be rolling to pickup high school students.

- My theory on the logistics of this (based on witnessing similar situations in Feb 2005), is that any decision to upgrade a 2 hour delay to a closing really has to be made by 6:30-7:00 AM. While it may still be icing throughout the area (except for east of 95)...I fully expect temperatures to warm above 32 by mid morning SO THAT were buses to start rolling at 8AM...things theoretically would be fine.
- The problem with this whole setup is the call to stick with a 2 hour delay will have to be made on a HOPE that the forecast pans out as projected. The danger is that if warming to well above 32 has not occured by 8AM, the whole area remains a disastrously treacherous commute right at the critical time of 8 to 9 AM, only to clear up perfectly at 10.


For simplicity sake, let's hope that:
1. In eastern areas (the immediate Baltimore Metro area) Any freezing rain/sleet quickly turns to rain just in time to let school systems feel confident the 2 hour delay call was a good one.
2. in central and western areas (Carroll County on west)...the weather forecast at least makes it easy for schools to make a sensible call that won't cause political or parental consternation.


What do I think will happen?
Honestly, I can see Baltimore County closing the Hereford Zone, (or perhaps allowing them a 2 hour delay) and leaving everyone else with a 1 hour delay. I say this because I sense we'll have a later start time, (5AM instead of after midnight) limiting how much can fall and freeze. The city may not even have a delay. Carroll, Frederick and west-ward are facing a significant ice storm that could result in .25" or greater, and strong winds behind the storm are going to knock down trees and powerlines. They're going to heed the call and close outright in my opinion. From Howard County on south, expect mostly rain and nothing in the way of delays or closings, sorry Anne Arundel, PG and Montco, you'll have to sit out this one. Harford County likely ends up with a 2 hour delay. Will be interesting to see what the morning brings, so goodnight all!

Friday, January 25, 2008

8 comments:
IN MARKING PERIOD GRADE MODE UNTIL MONDAY

Sorry for not being able to post recently, what little brainpower is left in the evenings this week has been diverted to preparing quarterly grades. Sometime during the weekend, I hope to post an overview of what's lurking for February. I see the potential for a little surprise snow Saturday, and some computer models are having trouble resolving the development of a possible rogue coastal on Sunday. At least there wasn't much Mid-Atlantic weather to talk about this past week. But as we head into February, I foresee some "weather whiplash" coming up, with wild swings in temperature and storm types. Feel free to post any questions you have about the upcoming pattern and I will answer this weekend as time permits.

Tuesday, January 22, 2008

3 comments:
WILL THE TREND BE YOUR FRIEND?

Tuesday 1/22 thoughts on the clipper: Think over the trend of the past 2 storms that approached us from the west/south...both ended up as "overperformers" due to more dynamic development once in the Mid-Atlantic. The Dec 5 and Jan 17 events produced more snowfall than was anticipated by NWS and Accuweather. I believe today's event will continue that trend, for 2 reasons:

1) While dewpoints are again low presently, southwest winds ahead of the storm will raise temperatures a few degrees faster than normal, and moistent the atmosphere quickly ahead of arrival. This will enable snow to once again start suddenly as it did in the Jan 17 storm.
2) This system is slower moving, and while moisture content is low, the surface and boundary layer temperarures will below freezing until late in the day, allowing whatever falls to freeze on contact. This will create instant and widespread slippery conditions.

WHAT ABOUT SCHOOL? Easy call there...another round of parental pandemonium.
MOST LIKELY: 1 HOUR EARLY DISMISSAL IF the precip arrives sometime after 10 am in the Baltimore/DC Metro areas, and is generally light but persistent for several hours. A changeover at or after 3 PM won't help high and middle schools which let out in the 2:15 to 2:45 period.
LESS LIKELY: 2 or 3 hour dismissal. The mess that resulted from the 2 hour call recently I don't think will be reattempted again for sometime. There is not enough heavy precip potential to warrant a big call for that. THEN AGAIN... do you think there's a chance that backlash from last week might have some districts running scared and they'll over-react, going with a 3 hour when a 1 hour would be sufficient? Regardless of the call, I can forecast widespread parental pandemonium if the trend turns out to be our friend and this event becomes another over-performer.

Monday, January 21, 2008

12 comments:
WILL THE FUN BE LASTING?

This forecast graphic just made me smile this morning, and for powderhounds in the area, I'm sure it'll have the same effect. If this pans out, look for a highly disrupted week considering the overnight temperatures. Keep in mind now I use places like downtown Baltimore or Dundalk as my bellweathers... because if warming influences of the urban heat island/bay are not enough to overcome frozen precip potential, then it's a sure bet outlying areas north and west are going to see more. If not "more" they'll at least see generally frozen precip.
Will The Fun Be Lasting
Right on schedule, the Baltimore/DC NWS is coming around to the ideas I originally posted late last week: 3 snow events on tap for the Mid-Atlantic over the next 7-8 days. Now Saturday's Vlizzard could have been counted as one of those 3, so that leaves Tuesday and Wed-Thu as candidates. There is still potential for a weekend system, and if the timing/amounts are right, is there a chance this might deliver TEACHERS as well as students in some school systems (namely Baltimore County) a shot at a 3 day weekend? The answer is: I suspect region-wide accumulations next Sunday would have to be 5"+, or at least fall very late overnight into Monday. Obviously that is too far distant right now for any solid projections, but the potential is there and I'll be watching it carefully.


Final note: Very busy today trying to get grades, paperwork done along with attempts to resurrect the "forecast computer." I may have spilled some water into the laptop's wireless port (of all the dumbest places!) and now, the laptop just sits on my lap and hums. It is not a pretty tune, so I'm hoping our President will deliver on his promise to send us all bonus money borrowed from China that I can use ot help stimulate the economy. (Just don't tell my wife!)Anyway, will try to respond to your comments as time and family permit.

Sunday, January 20, 2008

5 comments:
IT WAS FUN WHILE IT LASTED

1/20/2008: Until we get to the next round of winter weather (first minor shot Tuesday, next major shot Saturday-Sunday), I wanted to share with everyone these delightful images sent to me by 2 of our readers..they're white-blooded powderhounds, and teachers in Baltimore County. If you're looking for a wintery screen saver to tide you over until next time, these are worthy candidates:

Frostry branches adorning the Hereford Zone of northern Baltimore County, MD
frosty branches
I think it's Par for course that no one will be hitting any birdies here.
golf course
Robert Frost would have been proud: "Stopping by a woods on a snowy evening..."

Julee's bench

In case no one noticed yesterday, you went about you day underneath one of the longest-duration "vlizzards" I have ever seen. (That's a virga blizzard). I mean it snowed like mad for at least 9 hours straight, giant invisible flakes coating everything in a blanket of vapor. Actually, it was a virga storm. All that beautiful white and blue on the radar yesterday was snow falling but evaporating before reaching the ground. Whodunit? Dewpoint depression. Boundary layer moisture content was too low to be overcome, and the dryness prevented any of that snow from reaching the ground. Had it reached the surface, we could have called it a "flizzard."


Also of note is one of my personal first accomplishments with this website. After 4 years of blogging, I finally figured out ...totally by accident mind you... how to embed animation in the post. I've been wanting to show radar and satellite loops for the longest time, and as long as I can remember how I figured it out, there'll be more internet weather fun like that for you in the next storm. I'll stake out some time today to revisit our still-on-the-table Rumble Storms..the Tuesday quick shot and the Saturday-Sunday big shot.

Saturday, January 19, 2008

5 comments:
DE JA VU ALL OVER AGAIN?
Twice in the past 7 years, I've heard the famous words:
"No need for concern, the storm is going to stay mostly south of DC."

Results:
(1) January 25, 2000. Initial forecast for Baltimore: 1.5" Actual: 17"
(2) February 16, 2003: Initial forecast for Baltimore: Light rain/snow mixed. Actual: 24"

The map below was a projection by the GFS on conditional snowfall for Saturday evening. Of course no one bought this solution, because it was (and still is) believed the approaching cold front in the mid-west would kick the Southeast storm out to sea before affecting the Mid-Atlantic and Northeast. Now scroll down to see the current radar and compare. Call me a wishcaster, but it seems that the western edge of precipitation is a tad bit west of DC.
Saturday Surprise 1

Saturday Surprise 2

Thursday, January 17, 2008

9 comments:
"LLLLLET'S GET READY TO RUMBLLLLLLLE..."

-The catch phrase popularized by Wrestling announcer Michael Buffer
The Middle Atlantic States may experience two or possibly three snow events over the next 7-8 day period, combined with very cold Arctic air this weekend with another round of cold later next week. One point to make clear is that by Saturday morning, sidestreets and parking lots will likely be slick and "cruncy" as Justin Berk of ABC2 News noted. This may impact school activities and events planned for tomorrow.


ON THE WEEK AHEAD: (I'ved moved graphics to bottom of the post, as they are blocked by some school servers.) The Saturday storm has potential to deliver anywhere from a glancing blow to a similar amount of snow as occured on Thursday. The second event next Tuesday will feature some winter precipitation but does not appear to be significant. Next Thursday into Friday could very possibly contain the necessary ingredients for a major coastal snowstorm. The setup for this third and final storm of the month looks similar in orientation to that which we saw prior to the February 2003 Blizzard. I know that sounds like a hypecast statement, but bear with me on the explanation as to why I am seeing this.


The meteorologists whom read this site are going to chuckle and say, "Yes, but that's the GFS 7 days from now. Come on, you know a lot can change between now and then." I must agree that they would be right. To the contrary, they would also agree that the GFS has sometimes shows a big storm in the long range, then loses it for a few days. We've looked back at model verification to see that the final solution occasionally ended up looking a lot like the initial projection. With that precedent in mind, allow me to explain the dynamics of a Mid-Atlantic Big Kahuna # 1.

If you're not able to view the graphic below, I'll summarize the main features:
1. The next two events are going to orient the upper atmospheric flow in such a way as to leave behind a "piece of energy" or a closed 500mb Low in the Southwest, as alluded to in HPC discussions of late.
2. Between Monday and Wednesday, this upper level feature will begin to rotate east and eventually eject into the Gulf of Mexico. An active southern stream will provide the impetus for a surface low to develop.
3. High pressure in northern Canada should settle to Ontario and expand over the Northeast and Great Lakes by Thursday morning.
4. Easterly and southerly flow off the Gulf and Atlantic will allow precipitation to break out over the Lower Mississippi valley Wednesday into Thursday as the developing storm moves northeast. A slightly neutral to postive NAO and generally slightly positive PNA should allow for any Southeast Canadian High Pressure to remain in place as the surface low travels through the Tennessee Valley and toward the Carolina coast by Friday morning.


IN CONCLUSION FOR NOW: While I'm not saying the map that some of you can't see will exactly verify, what I am saying is it represents the general trend of pressure systems over the next week. One of the most important features necessary for a solid East Coast snow event is a strong, stationary High Pressure in SE Canada. I think the climate teleconnectors are indicating the environment becomes most favorable for this arrangement by middle to late next week. How much snow is too early to tell, but the potential is there for a large scale winter weather event across much of the Mid-Atlantic and Northeast.

Thu 1-24 Banana High

28 comments:
BE CAREFUL WHAT YOU WISH FOR

THU 1/17 - 4:45 PM. SO...ABOUT THAT "CHANGEOVER" IF ANYONE OUT THERE ACTUALLY WITNESSING ACTUAL RAIN? In Dundalk, it was all snow until about 4:30. Preliminary snow reports from across MD are interesting, and considering that Anne Arundel has some of the highest totals in the Metro area, and they weren't even part of the original advisory package. My temp has dropped 2 degrees, now back to 32, when we were supposed to be rising by now. The rain/snow line according to the AccuWx Radar has been firmly established along and east of the Bay all day, and it's only now begun to move west. Could this turn into the "Big Storm That Would?" I think it's becoming clear to most of you that even light rain for a few hours won't be enough to wash away all the slush, and with temps holding near 32 overnight, delays tomorrow I think are IN THE BAG.

THU 1/17 - 1:30 PM As I write this, snow is softly falling outside my classroom window while I enjoy the peace and quiet of seeing our school's front yard glistened over with white. I am sorry to all of you who have to commute and face snarled traffic instead of a tranquil ride home. So in honor of all those commuters out there, I will walk the grueling, downhill trek of my endless 400 yards home, and think of you among the winter wonderland.

FOLLOWUP OBSERVATIONS ON TODAY'S EVENT:

1. My start time was much earlier than news and weather outlets, and I can see why it took until mid morning (at least in southeast Baltimore County). I would surmise the immediate boundary layer was still quite dry, and several hours of virga (when precip is falling but evaporating before reaching the surface), delayed the onset.

2. However, the very nature of what "virga" does is what I believe enabled the snow to reach greater intensity once it began. The atmosphere became sufficiently chilled and moistened to the point that snowfall rates were very high from the start. Combined with the fast moving shield of snow that overspread the area, I knew that once it got started, the snow would make up for lost time by accumulating more quickly.

3. I wore a special tie I reserve for these occasions.. the dark blue one featuring my 2 skiing snowmen. My wife asked what the tie meant this time, and I told her that the 2 snowmen on the tie indicated schools (at least the ones that have some sense to make the right call ...Frederick County...ahem.. Bueller...anyone?) would close 2 hours early. To this she remarked: "Of course, when I bought this tie for you 4 years ago, I had this exact date in mind, January 17, 2008. Riiiight."

HAVE WE LEARNED OUR LESSON FROM PREVIOUS STORMS?

It's obvious now this storm is turning out to mimick it's December 5 cousin, aptly named "The Little Storm That Could" by faithful powderhound Mr. Ligner, the Athletic Director at Sparrows Point High School. That means the 2-4" could end up closer to 5" in some parts of northern and western Baltimore County. I suspect Carroll and out-in-left-field-they-must-have-hired-their-transportation-folks-from-Buffalo-
Frederick County will see amounts on the high end of their Warning criteria... with widespread 6-7-8" likely. Of course I adore my job, my students, my employer and the nice summer 2-month bonus I get every year, so you can be sure that MY COUNTY will always make the right call, no matter what. ;-) I leave all the bad weather related decision making to other places, like Howard and Anne Arundel Counties, for starters. No offense and kudos to my Ho Co colleagues who quickly pointed out they are on an exam schedule this week..so they were scheduled to get out early ANYWAY!

There have been several times in the past 5 years that school systems were burned for making a call based on the BELIEF that dire predictions of weather agencies would come to fruition, and didn't. The one that sticks out in my mind most recently is late February 2005, when NWS had posted a Heavy Snow Warning for most of Maryland. You might have thought it was going to be "Day After Tomorrow, Part 2." Everyone closed from bow to stern. The snow started... at 10 AM. Oh it was heavy all right, and started to stick on the parking lot here in southern Balto County, around 3 PM. I know because I was working in the school greenhouse that day, watching the heavy white clumps land on the grass and MELT. It was so unfortunate how that that storm turned out, because in reality we could have squeezed a full school day out of that. The ground was too warm to support accumulate UNTIL the sun angle decreased, once it did, snow started to stick like glue, but roads were still just slushy and not frozen over by any means.

Since that time, I believe school systems have been following a Revolutionary Era to making the call. You've heard the old saying from the Battle of Bunker Hill in 1775.. "Don't fire until you see the whites of their eyes!" I think we are seeing proof of that proverb here in Maryland. We can identify 3 storms tied to the dubious distinction of being a "Whites of their Eyes" Storm:
- Feb 12-14 Valentine's Day Massacre of 2007
- December 5, 2007 Little Storm That Could
- January 17, 2008 Frederick Co. Honorable Mention "What's a Winter Storm Warning?" Storm

GET TO THE POINT WILL YOU?

Oh sorry, got carried away with reminiscing about climatology. Okay, the point is for tomorrow:

If two-thirds of your geographical scope of authority receive at or above the projected snow amounts, and a changeover to rain is delayed or denied, the likelihood of your school system calling for a delay or closing is inversely porportional to the decision made the day before and the parent reaction to that decision. Without delving too far into the political gooeyness (sp?) of this, let's break it down like such:

IF YOUR SCHOOL SYSTEM WAS.. TODAY YOU RECEIVED... TOMORROW YOU WILL BE..
a Winter Storm Warning area... No early dismissal... Closed, for various reasons.
a Winter Weather Advisory area... A 2 hour early dismissal... 1-2 hour late opening
a Winter Weather Advisory area... No early dismissal... On time, No late opening
Not in any warning or advisory area... Snow but no dismissal... 1-2 hour late opening

WHEN'S THE NEXT STORM?

There is potential for a coastal system to surprise the Mid-Atlantic yet again on Saturday. With cold air in place and models continuing to back the shield of precip more westward with each run, I could see a Special Weather Statement coming out on Friday for the Saturday system which has the potential to deliver the same amount or greater than what the Mid-Atlantic saw Thursday.


And then there's Kahuna potential lurking in the near future, as early as Tuesday and perhaps again on Friday. I will endeavor to stay on top of these developing winter weather events and bring you the latest scoops of truth as time and family permit. As I said in the heading, those of you who were honking for snow, be careful what you wish for, because Mother Nature is about to make up for lost time.

As always, please post your observations in the comments, and feel free to send me a snow pic that I can use to beautify the site: rdfoot@comcast.net. Enjoy the snow while it lasts!

Sincerely,
Forecaster Foot

4 comments:
AFTERNOON HEADACHES ON TAP

With precip not starting as early as I expected, it's obvious why all area Baltimore and DC Metro schools are heading in on time. I'm sure they're grumbling out in Frederick and Carroll Counties, being under a Winter Storm Warning and yet in school. Once snow and sleet begin this morning, roads will become slick since whatever falls is going to freeze on contact. I envision widespread early dismissals being announced in the mid-morning, making for a very headache filled day. In fact, it seems as though this might even turn out similar to the Dec 5 "Little Storm That Could" which brought many area schools an early dismissal followed by delays the next day.

Wednesday, January 16, 2008

10 comments:
WILL POWDERHOUNDS BE PLEASANTLY SURPRISED?

WED 1/16 - 10:00 PM UPDATE: I am now expecting an earlier start time, based on rising dewpoints and faster than anticipated movement of the southern system. I believe you'll see snow and sleet mixed outside your window at 5 AM in the Baltimore/DC area. If I'm right and you see precip before daybreak, this will be the field goal that nets you that day off you've been hoping for so long (if you're in Baltimore County or City, Harford, Carroll, Frederick and yes even lil' ole' stodgy Howard County too, believe it or not.) The map below is the 10:30 PM Radar, and you can see precipitation is already intruding southwest Virginia. Even though much of that may not be reaching the ground, the speed of arrival tells me that it should overspread the area between 4:00 and 5:00 AM. Late-breaking addition...NWS Sterling, VA has upped the amounts on your Winter Weather Advisory, and upgraded large swaths of Virginia to Winter Storm Warnings. If that all comes to pass, the school forecast is, as George Tenet might say, a SLAM DUNK.

Thursday Surprise 3

Now compare how hugely different this map has become from just one hour previously. Didn't your grandfather always used to say "the biggest storms always came from the south." Here's a another weather rule that never fails: If it's snowing in Atlanta, it'll be snowing in Baltimore.

Thursday Surprise 2

WED 1/16 - 7:00 PM UPDATE: This projection from the GFS model indicates snow and/or frozen precip may still be falling even at 1 PM Thursday across most of central and western Maryland.

Thursday Surprise 1


WED 1/16 - 6:00 PM: I figured you'd be back here before long. Perhaps you were wondering if I would ever be back, but since the weather has provided us a new topic, as Captain Kirk might say: "Well, now you have something new to talk about." As many of you already know, the Baltimore/DC NWS issued a Winter Weather Advisory for the most of Maryland west of the Chesapeake Bay, including the DC and Baltimore Metro areas. I've been watching their discussions, the observations, satellite, and of course our old friend, the computer models, for several days now in anticipation this storm was going to deliver some sort of winter precipitation on Thursday. Those in this region whom follow winter weather are beginning to suspect that this system is not turning out as expected, and I anticipate a diversity of surprise closings and delays throughout the DC-Baltimore Metro areas are on tap Thursday morning. Low level cold air appears entrenched into the deep South, as evidenced by the extent of NWS winter weather advisories from here to Georgia.

This fast-moving system will display unique impacts across the Mid-Atlantic area (specifically in Maryland west of I-95, south central Pennsylvania, Northern Virginia, the West Virginia Panhandle, and of course the Shenandoah region):

1. Persisent high pressure "centers" in the Mid-Atlantic will allow temperatures overnight to drop below freezing. By daybreak, whatever precip that begins to fall will freeze on contact, allowing slippery travel conditions to develop quickly, and catch unsuspecting motorists/walkers off guard.
2. A rapid onset of precipitation from southwest to northeast in the pre-dawn hours.
3. The timing of snow and sleet is such that it will disrupt and delay traffic in the critical hours from 5 to 9 AM, and a changeover to "liquid precip" may not occur until afternoon.
4. Changing and unresolved precipitation timing concerns that may disrupt school schedules.
5. A fast moving system that WOULD become a major snowstorm were it not for lack of access to an Arctic high in Southeast Canada. (actually there is one there, but the timing and orientation are not right this round)

The biggest concern which prompted NWS to hoist advisories for the Maryland region is that computer models do not see to be handling the presence of low level cold air across the region. Secondly, the eventual intrusion of warm air aloft may be delayed such that frozen precip is now more likely over a wider area than originally thought, and for a longer period of time. Accuweather, on the other hand has been downplaying the potential for ice or mixed precip. As I remind my students in situations like this, the amounts sometimes does not matter if we have a mixture of frozen precip, because even one tenth of an inch of ice as you all know can wreak far greater havoc than just one inch of snow. The movement of this storm can be seen in a computer model animation from a site I've recently discovered: coolwx.com


For those who have wanted to know what's been going on: The lack of activity on this site is due mainly to the challenge of managing young children, on going winter illness and teaching high school. Those of you with children, or who teach children, or even just know how to spell children understand how rewardingly drained one feels at the end of the day sometimes. Hence there has been little energy left over to post on our recent weather events. I am grateful for the ongoing support of the many loyal readers out there, and feel bad I could not deliver more frequent updates the past month. Especially for my Pennsylvania, New Jersey and New England colleagues who must think we're no longer on speaking terms, as I stopped forecasting altogether even for Boston's recent snows, sorry guys! I will make it up to you. There are big things in the offing for next week and I have a special post in preparation for the announcement that "something is brewing out there...and it's not just the coffee."


Overnight and tomorrow morning: Please post your observations and location in the comments if you are able and willing.

Sunday, December 16, 2007

19 comments:
GUESS WHO'S COMING FOR DINNER?

Good thing I kept my expectations low for this morning, as I saw outside what some of you weren't expecting to see...windswept rain in the 40's. But scroll down to the set of 3 maps to see who I think may be coming to dinner.

I see that many location east of the Blue Ridge that were icing last night changed over to rain. While I expected that too (remember the "frozen-liquid-frozen" bit from yesterday), I can still hear the collective sigh from across the lands. It does just seem wrong and sacreligious to have a nice pretty coating of ice get washed away so quickly. So what happened? To put it in simple terms, strong southerly flow ahead of the primary low sent warm air intruding aloft, which overnight worked down to the surface, scouring out the marginal cold air east of the mountains. But from Hagerstown on west along the PA-MD line, surface cold air remained locked in, providing a constant supply of freezing rain. Secondly, Influence of subtropical moisture really revved up the secondary low as some of you observed with the heavy rain overnight.

What looks apparent to me on the national radar is proof of my long standing rule.. the primary low always seems to stay stronger longer. The benefit of that is areas east of the blue ridge are caught in this warm "occluded zone" keeping temps in the upper 30's to around 40. While that's depressing for those of you wanting ice (odd bunch they are) this setup also prevents surface moisture from evaporating. Theoretically, once the primary low decays, cold air will rush in to fill the gap left by it and the departing secondary low. For those in school systems (or elsewhere) wishing to have a delay or closing, we'll have to see rapid refreezing in the overnight hours.

First map is known as Surface Streamlines. Note the position of 2 lows and the counter-clockwise flow. What's interesting is the pull of air from New England and New York into PA on backside of the coastal low... now go on to the next map.

Surface Streamlines..note the 2 Lows

Surface Map for this morning, or view a current animation. Observe how the warmer air is being held to the coast and not intruding across PA, as evidenced by the precip type seen by the radar. Those of you along I-95 or in a blue zone here might say, "Well it's raining at my house but your map shows it's snowing." That rain may have been snow above for a brief time before melting on the way down. For the cold air to arrive, either the secondary low has to pull it in from the north, or the front to our west has to sweep through.

Surface Map as of 7:30 AM 12/16

The most interesting observations come from this map. See how the 30's are entrenched across PA, and the 40's are being held to the coastal plain. In fact, look at the contrast between northern and central NJ.. 29 vs. 41. Also note the lobe of 20's nosing down through eastern PA and the depth of near-freezing temps extending through central VA. This tells me there's a slow gravy train of cold air being setup by the developing secondary Low that's taking over air flow as the primary Low decays slowly. Will also be interesting to see how that large area of 40+ is dealt with as cold invades from the west and north during the day.

Is the cold air trying to sneak back in?

So who's coming to dinner? For areas east of I-81 in PA, MD and VA...whatever precip you have will changeover to snow showers with strong north winds late, followed by temps in the upper teens in central and eastern PA, and the 20's in MD west of the Bay, 30's east of the Bay. Too early to tell how temps and precip will impact schools, that may not become apparent until very late tonight. Until then, please consider posting your observations IF YOU NOTICE A SHARP CHANGE OVER A SHORT TIME, I think you'll start to see it soon...

Saturday, December 15, 2007

31 comments:
ICE, THEN RAIN, THEN ICE,
FOLLOWED BY A COLD WIND

It's Here, Bringing No Good Cheer

SAT 10PM UPDATE: While the uncertainty of this storm reminds me of the Valentine's Day Massacre of this past February, the precipitation type does not. I love the first comment posted by Terpguy of Baltimore region's TV weathercasters. I can simply the whole arrangement for you quite easily: In my opinion, what your National Weather Service Forecast office has posted is outlined well for this storm, and they seem to have delineated nicely the differences in precipitation type, timing and amounts. It is a complicated storm for sure, but does not have to be difficult to understand at our level. I hope my headline makes the big picture more clear for you. Areas west of Frederick, MD will see the highest precip amounts, mostly sleet and freezing rain versus snow. East of Frederick and north of the Baltimore City/County border, light sleet and freezing rain will give way to all rain overnight. In the larger view, it appears our anticipated secondary low is forming along the Carolina coast, and bringing with it deep moisture already convectively active. For areas at or below freezing which have a pocket of this energy cross over from the warm sector to cold sector, you may be woken tonight to the sound of thunder-sleet or thunder-freezing rain!


Think of it this way...most locations in the Northeast, except for the deep interior mountain areas, will see frozen precip changeover to liquid for a time, all then back to frozen before ending with stiff north and northwest winds on the backside. The intrusion of warm air aloft from a stubborn Southeast Ridge combined with influence of subtropical moisture from Olga's remnants are the main culprits to blame for robbing you of a good snowstorm. At this time, I will only be able to focus on the storm's impacts to the North and Central Maryland/Northern VA, Central and Eastern PA, On Sunday, I will start an analysis of potential impact to the Monday school schedule. I will try to take a look at the impact to our readers in the Northeast and New England.

For everyone in this storm, the one event that seems near certain is a refreezing of all standing water Sunday night, coupled with strong winds creating power outages overnight. A second Arctic High will arrive as the storm departs Sunday, sending temperatures into the 20's across the region, encasing all untreated roads, parking lots, sidewalks and your front step with a glaze of ice. My biggest concern is the school schedule situation for Monday, considering the potential for strong gusty winds, downed power lines, slick roads and widespread icy spots due to the refreezing. Right now, delays seem quite likely from Carroll County, MD east. In areas experiencing the sleet/freezing rain, temps will remain lower and re-freezing occurs more easily tomorrow night, so I could see Northern VA schools closed, as well those in and west of the Blue Ridge.


In the comments, please consider posting a run down of what your local TV weathercasters are projecting, as well as your observations on when precip began at your location, and what type it was. Sunday afternoon, once we see the winds materialize and get a sense of overnight temps, I'll post a final call for schools Monday morning.

Thursday, December 13, 2007

13 comments:
THE WEEKEND KAHUNA:
UPDATED AND COMPLICATED

FORMATTING NOTE: In deference to readers from school systems, instead of images I have embedded links in the text if your servers have an "all-image block" feature for security.

SYNOPSIS
A highly complex and evolving winter storm is projected to impact a large area of the country from Texas to Maine in the period Saturday through Monday. The Mid-Atlantic may see heavy wet snow along and west of the Appalachians, with a wide variety of precipitation east of the mountains, including sleet, freezing rain and yes, possibly even hail where severe storms develop along and east of the I-95 corridor from Richmond to Philadelphia. Parts of interior Pennsylvania, New York and New England may receive a near crushing blow of heavy snow, strong winds and intermittent freezing rain Sunday into Monday, on top of accumulations expected Thursday into Friday. Sunday night features the arrival of a strong northerly flow from a second Arctic High that will produce a hard freeze of any standing water or snowpack, which may seriously affect commuters and students on Monday the 17th.

STORM EVOLUTION
A large area of surface low pressure in the southwest on Thursday will develop in the southern plains Friday and move into the Lower Mississippi Valley on Saturday. The regime of paralyzing ice events in the Mid-West will give way to mostly snow as High Pressure finally begins encroaching on the eastern two-thirds of the country. This will begin reducing the warm moist air mass covering the southern Mid-Atlantic since Monday that sent overnight temperatures Tuesday into the 70's in southwest PA, and in the upper 70's to 80 in southeast Virginia on Wednesday.

Concurrently, Tropical Storm/Depression Olga near southeast Cuba on Wednesday may move to the southern Gulf of Mexico by Saturday. It is becoming increasingly likely that tropical moisture will become incorporated into the trailing cold front and developing low pressure as it moves across the Southeast. If Olga restrengthens or is able to access more tropical moisture before absorption into the Low, this could make for an even more significant event.


ANALYSIS AND DISCUSSION
On-going difficulty of computer models in projecting the timing of Arctic air intrusion into the Northeast on Saturday and Sunday may result in a complex and rapidly changing forecast which may confound meteorologists and the public alike. Eventually, a clash of extremely cold air with very warm, moist air over a short period of time may even result in what I am calling a "convective winter weather event" Saturday night into Sunday morning in the Mid-Atlantic. Tropical moisture arriving with cold air could spawn explosive convection in Eastern North Carolina, Eastern Virginia, and the Del-Mar-Va for a few hours. Once the secondary low establishes and moves northeast, it will carry the remaining tropical moisture with it and possibly create an extremely wet heavy snowfall in interior southern New England that con converts to a drier more powdery snow as Arctic air takes over. Given that cold air may not be firmly in place at onset of the storm in states such as Virginia, Maryland, West Virginia..if precipitation becomes all snow, it may have a very low liquid to snow ratio of 8:1 or even 6:1.


One factor that sours my outlook for major snows along I-95 south of Philly is a poor location of the Canadian High. Olga also presents a unique complexity in that some areas in Eastern North Carolina, Southeast Virginia and the DelMarVa could see almost summer-like explosion of thunderstorms in the middle stages of this storm late Sat into Sunday. If tropical moisture gets incorporated into the developing Low pressure at the same time that cold Canadian air is being pulled in from the north, the combination of these two could spawn some highly unusual "convective winter weather" ... in that I mean thundersnow or even thundersleet. The problem for powderhounds is that the track of the primary low (into Ohio) and secondary low (Carolinas to New England) will not be conducive for an all-snow-event. Indications of the potential "kitchen sink" type storm are evident in this graph of precip-type and timing for BWI airport.

I have dubbed this event a "Tropical Snowstorm," because the infusion of this moisture at upper levels over extensive surface cold air just when a secondary low pressure is deepening off the Virginina / Carolina coast may create extremely heavy snows in Eastern New York and Interior New England and in Pennsylvania... north and west of I-81. in PA, VA. The unfortunate part for snow lovers is that areas along I-95 are likely to see more mixed precipitation, due to close proximity of the secondary surface low, especially if it track close to the coast. As you have experienced in previous storms, this setup creates a multi-precip type event that may cut down considerably on any one type of precip.
Despite many differences among computer models, it is likely that following the storm, strong winds and very cold air up to 10 degrees below normal will pervade the Northeast region from Sunday through Tuesday, causing additional power outages problems.

ARRIVAL TIME (MID-ATLANTIC REGION ONLY)

Current indications are moisture will be arriving in the Ohio and Tennessee Valley on Saturday, reaching coastal population centers by late evening. I expect this to be a rain changing to snow situation, unless boundary layer temperatures on Saturday end up colder than expected. (Washington, Sat High 35 - Baltimore Sat High 36.. rain and snow currently forecasted by NWS, where as yesterday, their call was for all rain, no mention of snow.)

PRECIPITATION TYPE
Based on the dynamic nature of this potential "forcing" event, on Sunday I would not be surprised to see thundersnow or thundersleet
in areas east of I-95 and even tornadoes in Eastern Virginia or Del-Mar-Va, as well as the possibility of hail. Anyone can agree that when Arctic air and Tropical moisture meet abruptly in a very short period of time over a relatively small distance (confined to the Eastern Mid-Atlantic), there is bound to be a variety of extreme weather and precipitation types. The precipitation type analysis created by the GFS and found at this site:
continues to shows a very dynamic event.. totaling near 1.5" of liquid equivalent over the 2-day period for just the Eastern Mid-Atlantic.

Regarding areas to see all snow: I expect this to be along and west of a line from West Chester County, NY to Harrisburg, PA to Frederick, MD... total storm amounts could approach 12" given overrunning snow Saturday that is added onto by wraparound snow Sunday and then lake effect on Monday (for Western PA and Western MD, WV).

Overall, I cannot yet project specific point by point snowfall/ice amounts, but will at least keep you updated on the liquid equivalents and potential for changes in the precipitation types. For the Mid-Atlantic, I expect to provide my "storm grade amounts" Friday or Saturday. For New England, I will likely post some snowfall projections on Saturday night for Sunday.

CLIMATIC INDICATORS
The North Atlantic Oscillation, currently slightly positive, is forecasted to trend neutral to slightly negative. AccuWeather forecasters have already discussed extensively, this indicates that a large push of cold air may occur right at the storm's onset (in MD and VA) Saturday night. This tells me that even though major cities and their suburbs from Washington to Central New Jersey will see more rain than snow, it also indicates cold air is going to play a major role in the final track and outcome of this storm.

FACTORS NOT CONSIDERED IN ANALYSIS AS OF YET

-What effect will Gulf Stream temperatures will have on development of the secondary Low?
-Would a recent and refreshed snowpack in PA and NY be able to strengthen and reinforce the approaching High to a degree that more cold air filters in on Saturday, enhancing overrunning snowfall Saturday night?
-Would earlier entrainment of tropical moisture lead to more rapid development of secondary Low, thereby sapping energy from primary Low and allowing for cold air to invade more quickly along the Appalachians?

WHAT ABOUT SCHOOL AND WORK MONDAY?

Since this situation is so fluid and will be such until it ends, it may be hard to project exactly the impact to your Monday morning schedule. It does bear watching that some computer models show a period of freezing rain Sunday, followed by very cold overnight temperatures and strong winds. Were that to play out as shown right now, many schools in the "wetter/southerly" part of the storm (Central and Northern MD, Eastern and Northern VA) could face delays. What I've been telling people who ask (or happen to be unlucky enough to see me in the hall..) is: "Whatever falls Saturday or Sunday is going to freeze over hard Sunday night, lots of standing water with overnight temps around 20 in the major cities and teens in the suburbs.

As for the Heat Miser vs. Snow Miser contest... By Christmas Day, all this is distant memory south of Philly, and temperatures could reach 60 F at BWI by 12/25. So I believe Heat Miser wins Christmas this year. Sorry powderhounds.


Saturday, December 8, 2007

18 comments:
HEAT MISER VS. SNOW MISER:
WHO WILL WIN THIS CHRISTMAS?

The Best Weather Duel(t) In History

Check back later this weekend for a preview of our upcoming holiday weather forecast. The 2 week period leading up to Christmas is going to feature a major battle between the 2 most famous siblings of Mother Nature's family: Heat Miser and Snow Miser. The battlelines have already been drawn by a stationary front emblazoned on the weather map from the Mid-Atlantic back to Missouri. All this week, the front and it's 2 opposing air masses on either side will create an on-going series of disputes between the misers. South of Baltimore, it appears that Heat Miser is going to win out for a while and limit Snow Miser's incursions to the Mason-Dixon line on north. However moving into the week before Christmas, it appears increasingly likely that Snow Miser will marshal his minions and make a charge into Heat Miser's territory. The final clash could very well result in one more snow event for the Mid-Atlantic and Northeast in the 12/17-18 period that may even whiten the ground all the way to "Southtown, U.S.A."


To find out where that really is, or for a trip down memory lane about the story behind the 2 "misers" and their appearance in the vintage 1970's holiday classic The Year Without A Santa Claus, enjoy this excerpt from the movie on Youtube. You remember the tune... "I'm Mr. White Christmas, I'm Mr. Snow..."

Thursday, December 6, 2007

1 comment:
THE WEATHER HAS BIG PLANS FOR YOU


The problem is...those plans are fraught with peril over the next 2 weeks. But since this is a site dedicated to basic weather for busy people, let's get right to it, shall we.

EVENT # 1- FRIDAY 12-7: LOOKEY WHAT WE HAVE HERE.
Go ahead, take a look on the radar, you'll like it. While you're at it, check out current surface temperatures across the Mid-Atlantic, bitter me timbers on a Friday morning. Where oh where have I seen this before? No, not the clipper we just had... but January 4, 2003. A very similar setup tomorrow and I have to go out on a meteorological limb here and tell you this could turn out to be the second surprise snowfall in 1 week. Four years ago, on Sunday the 4th, we went to church in the Baltimore area under some light snow courtesy of an upper level low, the NWS forecast was for perhaps 1/2 inch. It was only expected to snow a few hours, and snow it did.. leaving 5 INCHES between 9AM and 5 PM. The event caught everyone so off guard that school was closed on January 5th.

Now fast forward to 2007. Unlike that storm, we already have a fresh snowpack driving down overnight temperatures, and another system with enough moisture to produce 1-3" snows in central Ohio. What do I think will happen? Upper level lows have a way of "activating" precipitation at the surface given the right conditions. If we take a 1:20 liquid to snow ratio, and multiply by current projections of .03" liquid for this event, it translates into over 1/2 inch of snow. That's assuming the system does not incorporate moisture from the bay, which I believe the clipper did, and this one will too. Read the Baltimore-DC NWS current forecast discussion to read between the lines and see what I see. Perhaps the radar is just showing virga (when the precip is falling but not reaching the ground), or is there a chance that evaporative cooling turns a "rain snow mix" into an all snow event?

EVENT # 2 - THE LONG ICE STORM COMETH? Intriquing animation of precip type based on a merger of analysis from the GFS and NAM computer models, definitely check this out.

I know your local forecasts are printing out the chance of a deluge for days starting Saturday and continuing into next Thursday. At this time of the year, when there's a lot of battle going on between cold and warm..do you really think it's just going rain for 6 days straight? Let me put it in simple terms... a 1033 mb High in Eastern Canada pressing down on a frontal boundary draped across the US from New Jersey to Missouri is a forecast fraught with peril. From Saturday to the middle of next week, (8th to 12th) computer models are already projecting a significant to extreme ice event from Illinois to New York, including most of Pennsylvania. This could end as a surprise snowstorm o the New England coast toward end of the week. Potential ice accretion of 1/2" over many many metropolitan areas stretching back 1000+ miles. In Maryland, if that cold air can penetrate the stationary front, a multi-day rain event could turn into a multi Tylenol ice event in no time flat. What does it mean for you?... might want to have extra batteries on hand now, I'm totally serious. There will be more on this event posted over the weekend.. but closely monitor your NWS forecast site for changes, as I think this is a dangerously evolving situation.


EVENT # 3 - THE PRE-SOLSTICE SNOWSTORM?

Dec 07 Storm 3

A major US government computer forecast model, called the GFS (for Global Forecast System) has been for several days now advertising an extremely significant coastal storm in the 17th-18th time frame. Liquid potential is 1 - 2 inches depending on your location, (closer to 1" liquid in the Mid-Atlantic, and 2" in Northeast and New England). As forecasters at Accuweather have already pointed out, climate indices are aligning to provide a nearly ideal setup (a positive PNA and negative NAO) which I will explain in more detail over the weekend. It also appears that extremely cold air will make itself available for a portion of the storm, making ratio calculations (considering possible 20:1) right now a bit premature, but at least fun.


That's all for now. Over the weekend, please consider posting in the comments what your local TV forecasters are reporting, because we have one heck of a week coming up if you like surprises.

Wednesday, December 5, 2007

16 comments:
WELL, THAT WAS A BIT WAY OFF

The eldest Foot girl helping Daddy realize there's a bit more than 1.5" of snow.


There's a lot of snow Daddy!


But for 2 little girls...the amount did not matter, just the fact that it snowed!
Our first snow pic of the winter


From reading the NWS Public Information Statements on snowfall reports from this storm, you would think we're back to the good ole' days of weather forecasting. (Actually our TV and Weather Service Meteorologists did a fine job, it was a tough storm to nail down throughout). On a lighter note, this event reminds me of a story from yesteryear I saw once in the humor section of Reader's Digest. The story was about a telephone call that an elderly grandmother in the midwest had placed to the TV station one wintery day. She wanted the meteorologist in charge to know she was particularly displeased with his forecast for the overnight hours. The woman is reported to have told the operator:(insert authentic midwestern twang accent here) "I'd like your weatherman to please come down to my house and shovel away the 6 inches of 'party cloudy' from my driveway.") I'm guessing there's someone in Glen Burnie, MD tonight who might be able to relate to grandma here, since they got a whopping 6.2 inches and were only expecting 2-3!

SO WHAT THE HECK HAPPENED?

While I'm not a licensed meteorologist, I have solid forecast training from the Penn State Weather Center, interned at the National Weather Service in Philadelphia, and have been "doing this kind of stuff" now going on 22 years. I'm also certified by Pennsylvania and Maryland to teach Earth Science, with an degree in that field to boot. So with all that background and training, you'd think that I and others in the forecasting arena would have caught on to what turned out to be the simplest observation of all... Oh the wind.


I realized around noon that surface and low-level winds across Central and Eastern Maryland were nothing but southerly all morning. There were other factors at upper levels that I know played a major role in the late day enhancement and evening banding which we all witnessed from 7 to 9 PM. However, my students and I were noticing on the radar a simple observation that could not be ignored. Yes, Virginia, there were breaks in the precip on west side of the mountains, but we started to wonder why the precip was redeveloping so quickly and even more heavily on east side of the Blue Ridge. My answer: low-level transport of moisture from the bay and ocean directly into the clipper's path, due in part to clockwise motion of a weak high pressure off the Carolina coast. Both the high and low were somewhat weak, so pressure gradient was not too intense and even though both systems moved out into the Atlantic, they did so in tandem for a few hours. The high then continued to provide a source of moisture on the backside even as a very minor secondary low was developing.

Overall, I believe southerly surface and low-level winds I believe are what did the trick in the end. The snow started at 9AM in Dundalk, and never looked back, and even at the onset, I realized something was up. As the flakes were very very numerous and extremely small.. leading me to consider that moisture content was going to be higher than we expected, and the liquid to snow ratio possible higher, say 1:13 versus 1:10 or 1:12. That's my theory, I hope it makes sense to you, and appropriately fits the facts of the case.

WHAT ABOUT SCHOOL?
You just had to ask, can't take an early dismissal and be satisfied with that now, can you? Never mind the benchmarks and short cycles that are looming, or that observation that got rescheduled, or the sports games that were canceled, or the meetings you wanted to get out of the way... no just throw all caution to the wind (in this case, a southerly one, right) and go way out on a limb and say what you really feel...

Because you know you want "it" but you just can't see how it might happen given the circumstances on the ground. What is it? I know what you want... forget a delay, a 2-point conversion or a 50 yard field goal... you want the Hail Mary Triple Overtime Game Ending Touchdown. You want the whole day off, don't you? I can even sense some of our loyal administrator readers out there are angling in their mind.. "Well, I could go in half day, get some paperwork done, then make some calls, clear a bunch of email..."

FOOT'S FORECAST FOR THURSDAY 12-6 DELAYS AND CLOSINGS IN CENTRAL MARYLAND: "THE MORNING ROUNDUP": this is based on impact that the final 3 hour band of heavy snow had on the area Wednesday night, dropping an unexpected additional 2-3 inches)


Cecil County: 2-Hour Delay
Harford County: 2-Hour Delay or Closed
Baltimore County: 2-Hour Delay or Closed (stranger things have happened...)
Baltimore City: 1-Hour Delay
Howard County: Opening 2 hours early for mandatory professional development
Anne Arundel: Already pulled the plug, 2 hour Delay
Carroll and Frederick County: 2-Hour Delay


I'll be expecting to hear from many of you, oh about 5:15 tomorrow morning when we see who is going to dance and who will be sitting out this one. Please continue to post your observations in the comments below.
18 comments:
YOU'RE NOT GETTING OUT EARLY
(oh, I meant to say that was only for Howard and
Anne Arundel Counties in Maryland! Sorry I didn't clarify sooner ;-)

WED 12-5 EVENING POST GAME COMMENTS AND ANALYSIS IN BLUE. Text in black was written at 5:30 AM on 12-5. As a fast moving Alberta clipper races east across the Mid-Atlantic, students and teachers in West Virginia, Northern Virginia and Central Maryland will be noticing the light flurries at lunchtime increase to light snow throughout the day. WHAT REALLY HAPPENED: 8:00 am - 9:00 am start time across the Baltimore Region. By last period or mod, with snow becoming steady, teachers will no doubt be pestered with the same question over and over and over and... you get the point. I don't even have to post the question. Students started up with "the question" even before the snow started.. prior to homeroom no less.

By mid-afternoon Wednesday, enough snow will have fallen across central and northern Maryland, and Northern Virginia to coat the grass, untreated sideroads and sidewalks, but not enough to disrupt traffic, main roads or secondaries. Reports started coming to me by late morning that secondaries were becoming snow-covered in northern and western parts of Baltimore County. The daytime arrival of the snow is such that roads will get a little slick, but movement of vehicles will warm surfaces enough to prevent a glaze from developing. Highway and school grounds crews working have ampletime to salt or clear roads / sidewalks and keep up with any changes in snow intensity throughout the day. Although there is plenty of cold air in place the approaching clipper system has less than 1/2inch of liquid equivalent moisture with which to generate snow. As my Earth Science classes calculated on Tuesday analyzing the latest computer models, our projections came to this:


Available moisture: .30 " x a liquid to snow ratio of 1:12 given colder air = 1.56 inches across the Baltimore metro region. Well, that was, like, a tad way off. I measured 4" in my yard, which seems slightly higher than 1.5" That's the amount I'm pegging for accumulation at BWI airport, and we will grade the clipper on that number. Ouch this is going to hurt, gonna be an ugly grade, possibly the worst ever, because 4" on a call of 1.5 is a 62% deviation from the forecast, which nets me an accuracy rating of 38% = E-----
I believe lesser amounts the farther north you go toward Pennsylvania. (Wrong: Bands of snow streaking across the PA-MD border had dropped up to 4" by mid day.) Slightly higher totals of 2-3 inches are likely on the Eastern shore as aptly pointed out by one of our loyal readers, Mr. Justin Berk of ABC2 News. Justin also made important mention in some emails we exchanged yesterday about the possibility of several bursts of snow in the evening hours before ending. His point is well to be considered that this could leave wet roads following the evening rush with the potential to refreeze overnight. Given that idea, there is the outside chance of some school delays Thursday. Update: Anne Arundel County already submitted a 2 hour delay as of 9:12 PM.

As for today, it seems prudent to expect that schools WILL NOT dismiss early (Wrong again..only Baltimore City Schools, Howard County and Anne Arundel were the holdouts.) but WILL LIKELY CANCEL AFTERNOON AND EVENING ACTIVITIES considering the snow should be increasing from afternoon into the evening. (That was easiest call of them all overall. Okay enough rhyming.) The biggest problems will occur in the evening traffic period due to slick roads and reduced visibility.

The only possibility I can see for an early dismissal is if snow intensity starts earlier and stronger than expected, and by 12 pm it is clear roads are becoming snow covered... prompting a 1 hour early call. (Well, that's exactly what happened, I just didn't think it had that good of a chance of really panning out the way it did.) But I think that's still highly unlikely given the timing of the heaviest snow to be arriving when elementary students should already be home. (Good thing most counties called a 1-2 hour dismissal, helped reduce potential for accidents given that moderate to heavy was occurring while I was driving at 3:40...about the time some elementaries would have normally dismissed.)


Previous December 5th's have delivered better snow season kickoffs, so the best part to take away from this year's event is that it will look nice on your holiday decorations, assuming you took advantage ofthe dry weather in previous weeks to get it done. You could take some pretty pictures for your screen saver or greeting cards, and prove to your distant relatives that it really does snow in Maryland, occasionally. (Justin Berk of ABC2 News pointed out that snow has fallen in the area at least 4 of the past 6 years on this date, including Dec 5'ths in 2002, 03, 04, 05 and now 07.)

Saturday, December 1, 2007

19 comments:

A SUBPRIME STORM FOR SOME

SUNDAY, DECEMBER 2 UPDATES IN RED. (Note: this was first composed Thursday afternoon, and revised today) A rule that many Accuweather Forecasters use in their storm prediction has been “Predict the High and you predict the storm.” This first winter weather event for the I-95 corridor looks to be “subprime” at best, meaning that you will be sorely disappointed if you place all your confidence in less than solid data with the hope it will all work out. My analogy of course points to the ongoing turmoil in the US housing market, due largely to overconfidence in borrowers with blemished credit records, upon which a lot of hopes were placed.

In the case of our first storm, the blemish will be a subprime high pressure system this weekend that looks to be decaying as it moves east. (ACTUALLY...IT WAS THE MOISTURE SUPPLY THAT ENDED UP BEING SUBPRIME. DRY AIR IN MID LEVELS HAS PREVENTED MUCH OF THE PRECIP FROM REACHING THE GROUND.) That leaves behind a layer of cold air at the surface, which Sunday may be enough to bring a few hours of sleet along 95 and a few hours of snow north and west of the cities. However, hopes for a solid kickoff will be dashed as the brief snow and sleet change to rain by mid morning. Surface cold air is scoured out by warm air supplied by a broad area of low pressure moving from the Southern Plains to the Ohio Valley. A secondary low may develop along the New England coast, (SUN AM: More likely the Jersey/Delaware Coast) but it will do so too late to bring any snow to those hoping for it in the southern I-95 corridor from Philly on south. Following both storms is a sweeping cold front that will usher in strong gusty winds by Monday.

Thus, a wind-swept cold Monday is in store for the Mid-Atlantic and Northeast. For cold and snow lovers, the good news is that following this messiness will be a more “prime” high pressure. It is upon this second push of cold air I can more confidently place my hopes on a traditional snow event currently projected for the Thu-Fri time frame of next week.

CONSIDERATIONS TO KEEP IN MIND WHEN ANALYZING THIS STORM

Large High pressure extending out into the Atlantic several days ahead of the storm will allow some cold air to become entrenched in interior sections and allow areas north and west of the major cities to cool appreciably but not to an extreme or record-breaking level of cold. As of Sat PM 12-1, strength and breadth of this high appears to have increased as the day progressed, as the pink readings in the top right corner here show a pressure of 1038 mb. Nearly 1,000 miles away, in Baltimore, that pressure has only decreased to 1034 mb.

Sea Level Pressure

Lack of “blocking” from other systems in the North Atlantic means whatever arrives on the coast will keep on moving out, limiting precip amounts in the Mid-Atlantic States whether it is frozen or liquid, and directing the heaviest East Coast snowfall into New England.

TIMING AND PRECIPITATION ISSUES

As with previous storms that affected the I-95 corridor, timing had a lot to do with how upper-level forces affected what was observed on the ground. In this setup, I can see several factors throwing a monkey wrench into the outcome:

A. Radiational cooling. If the weekend high ends up being stronger and departs more slowly, that puts in place a weak “cold air damming” regime along the Mid-Atlantic, especially in micro-climates areas that are not along the coast. (Micro-climate to mean how weather in small towns away from the city can vary greatly from one locale to the next). SAT PM update: While cloud cover has shrouded the East Coast, fresh cold air continues to filter in on light north winds. Overnight temps are not destined to go extreme, but will be low enough that when precip arrives, it is going to start frozen and stay frozen into the morning hours. (SUN AM UPDATE: Baltimore and DC metro are likely out of the woods for frozen precip, but can still have brief periods of rain/sleet mixed..though light enough not to disrupt traffic)

Northeast Surface Temperatures

B. Energy Transfer or Lack Thereof. With a weak area of low pressure moving toward the Great Lakes, there is always the potential for a secondary low to develop along the Virginia capes. I have seen many times where myself and other forecasters pin too much hope on this secondary revving up quickly enough to become the snow producer. This time that is not going to happen. I expect the primary low from the midwest to stay stronger and longer than expected, and will be the main producer of any snow that comes out of this system for the I-95 cities. In fact, on Sunday for nearly all of the Northeast, any precip you receive will be due mainly to the over-running warm air set in motion by that Low Pressure way way out in Colorado as of Saturday night. There'll be no organized system on the East Coast responsible for anything you observe in the Mid-Atlantic..with exception of Eastern PA and New Jersey plus New Englanders and the eventual snow they'll receive from the secondary Low Sunday into Monday.

Satellite/Radar/Surface Pressure


C. Changeover Issues. An over-running situation (warm air aloft running or "advecting' over a cold air mass at the surface) also wreaks havoc with precip forecasts. Everyone's initial expectation is snow, but that all depends on thickness of the cold layer, and state of surface temperatures. When a system such as this arrives, it usually takes a few hours for the atmosphere to moisten and raise dewpoints to where precip that looks like it's falling according to your local radar actually reaches the ground. See the image below from Sat 12-1 evening radar...looks like snow everywhere right? Since dewpoints are low and the air still dry, that precip is evaporating before it reaches the ground, a term known as "virga." SUN AM REVIEW: Looks as though last night's radar was another infamous "virga blizzard" in that plenty of snow aloft, but never got to the ground, except at DCA airport for perhaps 1 hour.)

Current Radar

Although there is a strong high in southeast Canada, my projection for those of you below Pennsylvania is that the air mass won’t be that cold to start with. For example, weekend lows in the mid to upper 30’s in the Baltimore region will result in warm air intruding from the southwest and being able to scour out the cold more quickly… instead of snow changing to sleet and freezing rain, I expect to see only a brief period of snow, changing to sleet and then to rain in the afternoon hours of Sunday.

BUT, ALL YOU WANT TO KNOW IS:

1. Is there going to be snow? If so, how much and when?
2. Do you have to prepare for ice and traffic/school disruption?
3. What probability is there this turns into a bigger storm than expected?

MY ANSWERS:

1. Snow? The farther south you are, the less snow you will see, if any. Lets take the Mason-Dixon Line as our boundary. South of that line, and East of I-81, the maximum anyone will get from this is 2” before a changeover to sleet then rain. (SUN AM: That was not to imply many of you would be seeing snow, just wanted to set an upper limit on the potential for any quick advection snow bursts) North of that line, from PA into Northern New Jersey and Ney York, sunlight becomes a factor in which case you could end up with a long period of freezing rain and sleet as the clouds will warm considerably as Sunday progresses…however boundary layers are likely to stay cold enough for freezing on contact. In fact the air could be 35 F, but the ground is 31 F.

2. From Wilmington, DE south on 95 to Richmond, there is LITTLE CHANCE of traffic disruption Sunday morning through mid-day. A changeover to rain will have occurred almost everywhere by noon, washing away any ice buildup. Overnight into Monday morning could create a MINOR “black ice” issue as the cold front sweeps east, ushering in subfreezing temperatures and turning standing water into hidden ice slicks. Given this kind of scenario, I could envision school delays on Monday, but I believe it very unlikely this situation results in schools being closed… at least for the Metro Regions of DC, Baltimore, Philly, etc. Away from the cities, the “next county over” effect (such as Carroll Co. Md, Chester Co PA, etc.) especially up against some of the more hilly areas, I could envision enough cold air remaining in place that you end up with pockets of freezing rain and sleet that hang on longer than expected. The overall result is a patchwork of different decisions by school districts.

3. School? I highly doubt this turns into the surprise rout of say, Feb 13 where many Baltimore metro schools ended up closing 3 hours early due to the snow arriving 2 hours ahead of expectations. (A rout in that no one expects school to be closed, and then surprise they are all closed come Monday). If the Low coming out of the southern Plains cuts up into the Great Lakes, the best that the Mid-Atlantic States will see is the heavy snow confined to mountainous and interior sections… until sadly, it all changes over to rain ahead of the cold front.

The only real potential for this to surprise you is a slower than expected erosion of the surface cold air on Sunday. This would lead to sleet and freezing rain persisting longer, delaying a changeover to rain, and it is possible that in areas of Central and Northeast PA, what looks like just rain and wet roads is actually freezing rain. This is most easily discovered by bounding out of the house thinking there's no ice, and BLAM! Next thing you know, your crash landing has already been posted to Youtube by the neighbor before you're even in the car.

At 10:20 PM I see the radar is still a blanket of white, but air is still too dry for precip to reach the ground. I am starting to think perhaps evaporative cooling is coming into play here, and possibly the cooling effect of snow evaporating as it falls is helping to reinforce the localized "branches" of the main high pressure in Canada. Could that mean we see more of a snow event Sunday than sleet? In about 8 hours, when my 4-year old comes to tell me "the clock is on 6 daddy, time to get up" then I'll have the answer to that question.

SUN AM ANSWER: Two or three pellets of sleet is not enough to convince a 4-year old, or any of you, that there's much of anything up there.