Changing pattern could lead to multiple storm threats

We warned you that this was coming. Now, the pattern change is finally beginning to appear on medium and long range model guidance from late December into January. Thanks in part to changes in the stratosphere and retrograding mid and upper level atmospheric features, the pattern over the next week or two will become much more favorable for not only colder than normal air, but winter weather in our area as well. Contrary to what it may seem, the pattern currently is quite unfavorable for cold air and winter weather. Warmer than normal air has settled in to Southern and Central Canada over the past week or so, muting any potential shots of cold air with northwesterly winds.

But the notable feature around the Northern Hemisphere will begin retrograding over the next several days. It begins with the troughiness over the Gulf of Alaska and ridging in the Eastern Pacific. Both of this features will progressively retrograde westward over the next 7-10 days. The end result will bring a mean trough to the Aleutian Islands and a ridge to the West Coast of the United States and British Columbia (also known as a +PNA). Further securing the changing pattern will be the development of a +EPO ridge in Western Canada and the potential for ridging to develop over Eastern Canada, Greenland and the Western Atlantic.

What exactly does this mean for us? Not too much just yet. It means that features across the Northern Hemisphere which were previously unfavorable for cold or winter weather will be changing positions — putting us in a more vulnerable position for cold air and winter storms with each disturbance that moves through from essentially this coming weekend onward.

Speaking of this coming weekend, forecast models have begun to focus in on the potential for another coastal storm developing from Friday through Sunday. A disturbance which moves through the area during the middle of this coming week will strengthen into an upper level low over the Northeast US and Southeastern Canada by late this week, reinforcing a confluent flow over New England. Meanwhile, a Pacific disturbance will dig southward into the Four Corners region before ejecting northeastward from there.

GFS model showing the potential for a storm system late this week into this weekend. Map shows 500mb heights and vorticity.

GFS model showing the potential for a storm system late this week into this weekend. Map shows 500mb heights and vorticity.

While the antecedent airmass remains somewhat marginal (like we mentioned earlier, there isn’t much cold air around at all) the potential exists for this energy to eject through the Mississippi Valley, interact with additional northern stream energy, and force the development of a low pressure system somewhere off the Mid Atlantic coast. Should this occur, winter weather would be likely in parts of our area, especially away from the coastal plain. Ensemble guidance is in good agreement on the general track of the storm system — while there is obviously some variance in the details. The signal for the storm, however, is strong.

What remains to be seen at this point is exactly where the storm tracks, and how it evolves. The not-so-cold airmass in place will make it difficult for snow to fall near the coast, but the track of the storm system could assure that winter weather occurs in our area — especially north and west of the city. So while the details remain fuzzy at this point, we’re all keeping a close eye on the potential.

This storm, however, may very well be the first of many. The pattern changes quite dramatically in the days that follow. So much so, that the potential storm this weekend actually become a reinforcement of cold air. With increasing high latitude blocking, the pattern could become quite volatile. Cold air will be dislodged southward from Northern Canada, eventually, by the development of a -EPO. Historically, a -EPO in January has led to below normal temperatures throughout much of the United States.

GFS ensembles showing a high likelihood of a -EPO from Christmas onward.

GFS ensembles showing a high likelihood of a -EPO from Christmas onward.

Further substantiating the threat for cold is the forecast development of a west coast ridge or +PNA, and an increasing likelihood of a neutral or negative NAO by early January. Supported by Sea Surface Temperature anomalies, stratospheric warming progression, and the general global pattern evolution, the potential for colder than normal temperatures seems likely to increase greatly during the week of Christmas moving forward into January.

With an active Pacific jet stream, this almost certainly guarantees a heightened potential for winter storms as well — with the potential for a “big” event (or two).