As promised, this is part two of today's discussion - the first part is below. This is to follow up with thoughts on the potential for a storm early next week.
It's important to understand the difference between a forecast for mass consumption, and what really goes on with the forecast. That is, on TV today I mentioned that there "may be a storm at the beginning of next week, and it may favor snow for New England, but it's still so early that so much can change, so we'll see what happens."
Of course, from a meteorological perspective, the best forecasts often come when the most information is available for pattern recognition from well in advance to right before the event. The longwave pattern will be the first and most important part of the equation to nail down, and the general trend is to watch today's storm deepen dramatically, becoming stacked beneath an intensifying upper low that pulls northward to the mouth of the St. Lawrence River. Another low will deepen south of the Aleutian Islands in the coming days, and this encourages ridge development over the Western U.S. Of interest by Monday is the orientation of the upper low north of New England, the confluent flow over the Northeastern U.S., and the upper level flow dropping southeast out of Canada and into the Eastern U.S. trough. Perhaps the most important player in this is the unloading of arctic energy from the polar vortex by the north pole, dislodged when the upper low over the St. Lawrence expands and retrogrades westward, increasing the northerly flow and sending a strong chunk of vorticity and cold air southward. This drops into the longwave trough axis over the Great Lakes at the same time a strong southern stream shortwave (disturbance) rounds the base of the trough over the Southeastern United States, tapping the Gulf of Mexico for ample moisture. Think about the amazing recipe that's coming together in even this most basic description - air from the North Pole and the polar vortex (the heart of cold air this side of the Northern Hemisphere) dives south at the same time deep Gulf of Mexico moisture and warmth streams north. The merger could be amazing, and though the differences in projections are large, almost all guidance has good agreement on this storm absolutely bombing and becoming a mega-storm. The problem becomes how to handle the merger which is essential to development of the storm - timing, track and intensity - and the best we have is a combination of guidance and our own pattern recognition. Let's start with the pattern recognition.
I'd like to see the upper low retrograding quicker across Eastern Canada and to Hudson Bay, because this would assist in nudging the longwave trough a bit farther west. At this point, the forecasted position of the upper low keeps the trough axis just far enough east that the storm would really take off under a jet streak that would be moving over the Mid-Atlantic coast, bombing the storm just a bit too far southeast to affect New England, though under this scenario the storm would begin to undergo intensification as it moves across the Mid-Atlantic coast and that would deliver a decent snowstorm by early spring standards to the Northern Mid-Atlantic. The ECMWF is the farther west in its solution - but also has a longwave pattern that is the closest in agreement to the Ensemble solutions of the GFS and Canadian - and allows the storm to explode quickly off the coastline owing to strong baroclinicity. This is an important facet that probably is being handled better by the ECMWF than the remainder of the guidance given the confluence of airmasses taking place, and I think it's important to acknowledge the potential for dramatic strengthening not far off the coastline.
So, right now a bomb is likely, probability of a bomb to affect the Mid-Atlantic with snow is high, with a pounding nor'easter is moderate, but with a storm as far north as New England is low probability, in my opinion, with what we have right now. That said, that forecast is based upon the consensus in model guidance and we should watch for westward shifts in the upper low over Hudson Bay as an indicator that the longwave trof axis will come west and the storm may develop closer in and farther north.
Matt
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