As temperatures have moderated along the east coast, an abnormally strong cold airmass has entered the Western half of the United States. This cold airmass is thanks to a strong -EPO ridge over Alaska, which helps to dislodge Arctic cold into the United States. However, because of the +NAO, +AO, and -PNA, this cold is forced into the West, and will initially have a lot of resistance as it tries to head eastward. This pattern is illustrated very well by today’s 12z European model, valid for Thursday night, shown below.
The strong -EPO ridge is evident as there are high 500mb heights into Alaska, which is forcing an Arctic airmass into the Northern Plains. This airmass has the chance to break records, and will try to spill eastward with time; however, it has a lot of resistance thanks to the strong ridge in the Southeast. This very warm air in the Southeast and Arctic air in the Northern Plains will create an impressive temperature gradient in the northern Mississippi Valley and Ohio Valley region, and this gradient is where some storms will form. As these storms form, the ridge in the Southeast will force them to track to our west, leading to warm air and rain for Thursday night and Friday. However, the passage of each low pressure will try to gradually shift this gradient eastward due to counterclockwise flow behind them — it is a step-down process. The image below illustrates how strong this gradient really is, and it’s pretty mind-boggling.
One might be a bit confused as far as this whole gradient pattern is concerned, because generally speaking, there is always cold air to the north and warm air to the south. And for the most part, that is actually true — most patterns are inherently a gradient pattern by definition. But when there is much below average cold in the northern tier, and much above average warmth in the south, that will strengthen the gradient ten-fold. The image above shows surface temperature anomalies valid for Friday morning, and the red numbers on the top-right part of the image show the maximum anomalies in both the cold and warm direction. The model is showing an airmass that is nearly sixty degrees below average in the Northern Plains, and nearly thirty degrees above average in the Southeast. This would yield temperatures being from -20 to -40F in the Northern Plains, and potentially in the lower 80s in the Southeast. A 100-degree temperature gradient across the country is certainly nothing to sneeze at. This cold air will allow for lots of snow to fall in the West and the Plains, and accumulating snow will occur as far south as Texas, where winter-storm watches have already been issued.
Unfortunately for snow-lovers in the Northeast and our area, we are on the warm side of this temperature gradient. Considering that storms track right along the strongest part of the gradient, being on the warm side of the gradient guarantees being on the warm side of the storm track. This is why the disturbance on Thursday night and Friday will bring rain to our region.
Once that weak disturbance moves out of the region, it will try to drag some of the cold air behind it to the east, which will shift the gradient eastward. However, the disturbance on Thursday night and Friday appears to be quite weak, and the gradient will only shift eastward a tad. Another wave of low pressure will form along the newly-eastward-shifted gradient for Friday night and Saturday, but the gradient is still too far west for any snow to fall; and the airmass will still be unfavorable for snow due to that weak wave not being able to efficiently drag enough cold air eastward. Perhaps places well northwest of NYC will have a brief changeover to snow on Saturday morning. It is after this disturbance passes that the airmass will finally be cold enough to support snow in the northeast, as this second “step” in the step-down process will be enough to drag a piece of that cold air eastward. The core of the cold will remain to the west.
If one compares the above image to the first image, one can see that the southeast ridging has shifted well eastward, the heights in the Northeast are lowered, and there is a good cold air source in southeast Canada — thus the gradient has shifted to the southeast. High temperatures will only be in the upper 30s to around 40 on Saturday afternoon, and probably even colder on Sunday as we await the next storm system. The big question is whether the gradient will remain far enough to the south on Sunday afternoon and evening for our area to see snow.
The problem now is that there is no blocking in the Atlantic region to keep the cold air source in southeast Canada in place — this is where having a -NAO would have helped immensely. The fast flow in the Atlantic allows any cold air source to quickly move eastward. Additionally, there is another strong trough diving down into the western half of the country. Without any blocking, this strong trough will allow heights to rise again along the east coast. Thus, as this strong trough tries to move eastward, it will draw warm air out ahead of it back northward, and the gradient will thus shift back northward as well in a south-to-north fashion as opposed to a west-to-east fashion. This means that any storm track will still be unfavorable for snow.
That being said, the fact that the gradient will initially be to the south and still be pretty strong will allow some moisture to run out well ahead of the storm itself, due to the nature of the temperature gradient and the moisture drawn in from the Gulf of Mexico. If this moisture can arrive quickly enough, it will run into the pre-existing cold air and give the area a period of wintry precipitation on Sunday evening before turning over to rain.
Moving ahead to the storm system, there is a 1037mb high pressure system in Canada, which helps to initially supply cold, northerly winds, and moisture runs into it. The 32-degree line is south of the area as light precipitation approaches. However, due to the fast flow in the Atlantic, that high pressure is forced to slide eastward into the Atlantic, which will instead draw warmer easterly and southeasterly winds from the Atlantic Ocean, changing precipitation in our area to rain.
By Monday morning, that area of high pressure has moved into the Atlantic, drawing in warmer air from the Atlantic, and there is a 1001mb low in the Great Lakes, which helps to provide a southwesterly flow. That system is particularly strong in the mid and upper levels of the atmosphere. This means that winds at the surface will be dictated by the strong surface high pressure, but the winds aloft will be dictated by that strong upper-level low pressure system to the west. With winds shifting to the southeast at the surface, and being southwest aloft, it is hard to believe that the coast will be able to sustain a strong, dense layer of cold air in the atmosphere to support a long period of wintry precipitation.
Additionally, the magnitude of the upper-level disturbance well to the west means that although the surface winds will initially be northerly, they will start off southwesterly at the mid and upper levels. This means that temperatures will most likely reach above freezing aloft before they reach above freezing at the surface. This leads to the threat of sleet and freezing rain before any changeover to rain, and also potentially limits the window for snow at the onset.
Inland areas will likely have to deal with several hours of sleet or freezing rain, because the warm air at the surface from the Atlantic at the surface will not move over those regions as readily. This will have to be watched closely.
That being said, we do believe that all precipitation should start as snow — even at the coast. And of course, the event is several days away, so uncertainty remains, and nothing is set in stone yet. If the wave of low pressure on Friday night into Saturday wave can become a little bit stronger, it may be able to force more cold air into the region and help the initial gradient be further to the south. This would allow cold air to be a bit more stubborn at the onset of the storm, and allow for initial snowfall to accumulate, even at the coast. However, given the general pattern with the fast flow in the Atlantic, stubborn southeast ridging, and a strong trough diving into the western half of the country, this gradient will inevitably become more meridional and shift northward with time, and prevent a significant snowfall event.
After this storm system moves out, there may be another wave of low pressure behind it for Tuesday which will also have to be watched. This will be addressed in a later post. That disturbance may finally be able to drag a more significant piece of that record-breaking cold air in the Plains eastward, especially considering it was already closer to our region to begin with. It may take a few days, but once the cold arrives, the results could be quite impressive.
Today’s European model shows low temperatures in the lower single digits in the NYC area, and temperatures below zero north and west of the city! That would be absolutely unprecedented, as NYC has not had low temperatures below four degrees in the December 1-15 period since 1876. Also, considering this is over a week away, the odds of the European model being correct with this extreme scenario are quite low. But considering how cold the last dump of cold air was in late November, a similar cold air dump in December could yield high temperatures in the low 20s and lows in the lower teens.