A multi-wave winter weather event is increasingly likely to unfold during the early to middle part of next week across a large majority of the Northern 1/3 of the United States, initially in the Great Lakes and then northeastward into New England. The exact area where this winter weather potential sets up obviously remains highly uncertain. Forecast models have been extremely inconsistent in handling mid and upper level features which will eventually have major impacts on the sensible weather experienced by many. But in this case, there are clues in the meteorological pattern that point to the answers to some of our questions. Instead of looking specifically at forecast models and their trends, we can begin to look for answers to our questions in the hemispheric pattern.
The winter weather threat traces its roots back to the total degradation of the West Coast ridge and positive PNA on along the West Coast of the United States into British Columbia. Inherently, this is a bad sign for snow lovers on the East Coast Any time we degrade a PNA ridge there is a positive mid and upper level height field response to the east of that over the Southeastern United States. In fact, the collapse of a PNA ridge can bring a response all the way into the Western Atlantic. As the amplified pattern out west collapses, the mid level jet collapses east into the Untied States and allows for the flow to enhance further to the east. So not only does the Southeast Ridge suddenly gain amplitude, but the pattern becomes more progressive over the Great Lakes and Southeast Canada — working against snow chances in the East.
But while the PNA collapse is imminent, a very stout -EPO ridge in the Pacific Ocean continues to rebuild on all forecast models and ensembles. This helps to continue to dislodge polar air farther south. The northern stream jet will remain very active as multiple mid and upper level troughs push southward through Canada around a vast Polar Vortex. This will aid in the development of surface high pressure in that region with multiple reinforcing shots of low level cold air across the Northern 1/3 of the United States. As a result, an enhanced baroclinic zone and thermal gradient will develop from the Ohio Valley into the Northeast
Forecast models agree that Sunday afternoon, a broad upper level trough will be positioned over the Southwestern United States. Divergence increases after this point regarding the amplitude, strength, and organization of individual perturbations ejecting northeastward along the aforementioned baroclinic zone. Multiple mid level troughs with ample shortwave energy will dive southward with impressive speed and force into the Southwest US through the middle part of next week.
The initial winter weather threat for the Northeast United States looks likely to occur as early as Monday. As a mid level trough drives southeastward through Central Canada and eventually toward Southeast Canada and New England, precipitation will develop along the baroclinic zone from the Ohio Valley into the Northeast United States. This is a very odd and unusual winter weather setup for our area, with the surface low hundreds of miles to our north and west. However, this may work in the favor of wintry precipitation as we will not have to deal with a strong southerly flow and mid/upper level warming. Getting snow out of this setup won’t be easy — but a light winter weather event appears increasingly likely if this mid level trough does amplify as far south as models say it will today. This event likely will feature more wintry precipitation inland, where the boundary layer will remain more favorable.
But the amplitude of this mid level trough is more important for the second winter weather potential — during the middle part of the week. Forecast models agree that a more consolidated piece of the mid level trough over the Southwestern United States will eject northeastward along the baroclinic zone during the middle part of next week — in about 7 days time. As this occurs, a juxtaposition of impressive moisture return and a thermal gradient enhancing lift for precipitation will occur. This is a classic “southwest flow event” by nature and origin. Timing, therefore, becomes important. The amplitude of the mid level trough on Thursday will become essential.
If the upper level trough on Thursday swinging southward amplifies far enough southeast, and swings north of New England, a strong surface high will develop (Thank you, -EPO) over Southeastern Canada and New England. This surface high, if timed correctly with the approaching mid level energy, will act to enhance the baroclinic zone and keep mid and low level cold air entrenched in the entire forecast area. If this mid level trough on Thursday does not amplify enough, or remains progressive, the mid level energy will track much too far northwest for our area. The 18z GFS, pictured below, shows what may happen if the timing is incorrect and the mid level flow is not confluent enough to our north.
At the present time, it becomes important to look at the components of the mid and upper level atmospheric pattern. This setup, while able to bring us prolonged winter weather should the chips fall on the table correctly, almost always favors New England for the brunt of the snow. Additionally, forecast models currently are in good agreement that the mid level vorticity — and mid level low centers — will track well to our north and west. This is a classic model signal for mid level warming and precipitation type changeover in our area.
Currently, confidence is highest that this period will feature heightened potential for a moderate winter weather over New England. That being said, the pattern can very easily become conducive for a significant winter weather event in and around New York City if the incoming surface high and ejecting shortwave from the Southwest US can juxtapose. We still have a lot of time to go with this one — so we will have to monitor forecast models over the next several days.