Foot's Forecast | Weather Intelligence

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.

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)

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.

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.)

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.