Early next week, cold air may bleed into the eastern U.S. around the same time a storm system develops. The smart money is on midweek rain followed by chilly weather, but – for the first time – we have to consider the very small possibility of snow or mixed precipitation entering the picture.
Just yesterday, I cautioned against buying into individual model simulations a week or more into the future that simulate snow and questioned the merits of even showing them. They’re just not reliable.
Right on cue, this morning’s GFS model simulated significant snow accumulations in the region next Wednesday and Thursday. The European model, on the other hand, favors a modest amount of chilly rain that might mix with a bit of snow. Alternative simulations of these models known as ensemble members (these same model with their starting data tweaked) are all over the map (a sign of large uncertainty in the setup), but just a few are wintry, most are wet, and a few keep it dry.
Runs of the GFS model ensemble show a wide range of possibilities one week from now. (Penn State)
So in assessing long-range threats, it’s best to look at the whole universe of model runs, rather than picking just the one that shows snow and then running to the supermarket.
After considering all of the models, it’s then useful to think about what solutions makes most sense given the weather pattern and time of year.
Next week, the weather pattern isn’t ideal for a winter weather event, but not horrible either.
In short, it looks as if we’ll be transitioning from a hostile pattern for winter weather (negative PNA, positive NAO) to one which is neither hostile nor favorable (neutral PNA, neutral NAO).
A big question in this setup is whether enough cold air will drain southward by the time precipitation is beginning. In the winter, this question wouldn’t be so pivotal given more generous background cold.
In early-to-mid November, everything has to come together just right to get snow…hence it’s rare but not unheard of (recall the Veteran’s Day storm of 1987).
As we move ahead in the next few days, I’ll be paying attention to a couple key features that will be critical in determining whether wintry precipitation becomes a legitimate threat.
1) The strength and position of high pressure to the northwest: The high pressure system currently simulated by models is strong and impressive and, in those model runs showing wintry precipitation, it’s positioned to effectively funnel cold air into the D.C. area
2) The strength and position of an area of upper level low pressure: Ideally, we want to see a strong upper level low pressure system passing just to our south which would keep our low level winds from more of a cold, northerly direction while at the same time generating the lift required for significant precipitation.
Vets Day 1987 (benchmark for Nov snow locally) had 500mb low much further south than current GFS. pic.twitter.com/FRGWHuK1Zt
— Ian Livingston (@islivingston) November 6, 2013
CWG’s winter weather expert Wes Junker will share his thoughts on this set up tomorrow, provided it continues to show at least modest wintry potential.
Right now, I’d give a 10 percent chance of seeing snowflakes next week in the region. Better chances are farther north into Pennsylvania and New York State.
