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

30 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

17 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.


9 comments:
DECEMBER IS FINALLY HERE!
WILL IT BRING GOOD CHEER?
(for powderhounds, that is..)


I know many of you have been waiting patiently (or otherwise) for this site to resume. All I can say is thanks for hanging in there, and the rule for this winter will be: "Good things come to those who wait." Our inaugural storm this weekend will be "subprime" at best for powderhounds in the southern Mid-Atlantic (Maryland, Virginia), a teaser for those Keystoners (in Pennsylvania and New Jersey) and an Acetaminophen Alert for dem Yankees (New York and New England). Our western and mid-western kin are getting the prime rib portions this time around, at least for those who enjoy a good honking winter storm. The whole business will have flushed out by Monday, replaced by a traditional cold December air mass for much of the week ahead. Combine that with a clipper system arriving Wednesday into Thursday and you have the best first chance to enjoy measureable snowfall for much of the mid-Atlantic before week's end. Later today will feature more detailed post on the current and next storm.