Plain-language summary:
Some snow showers are expected for Maine and northwestern slopes elsewhere today and tonight with it turning briefly colder and then milder by Thursday. A significant but not exceptionally strong Nor'easter will bring moderate to heavy snow for interior locations, especially at higher elevations and on eastern facing slopes. Areas close to the coast will receive mainly rain and a little mixing with rain is possible in the Champlain and Connecticut River valleys. As the storm slowly moves northeast and starts to weaken, snow showers will continue through the weekend except from southern Vermont into southern Maine, where it will probably dry out completely. It turns a little colder behind the storm but not very cold. A much stronger surge of Arctic air into the Plains next week could spark another storm in our area for mid-to-late next week.
Meteorological discussion:
While December did not start out wintry, it is sure beginning to feel and look like winter this week. Before the Nor'easter arrives at the end of the week, a small piece of the polar vortex is moving southwestward into eastern Quebec today and then into Maine tonight. This unusual track is due to the Greenland block preventing systems from quickly moving eastward. This feature will provide snow showers to Maine as well as the Eastern Townships of Quebec and northern/eastern Vermont, especially northwestern facing slopes which will get some upslope enhancement with the northwesterly flow. With the ascent mainly in the dendritic growth zone, high snow-to-liquid ratios are expected with some snow showers potentially being heavy briefly, and moderate winds will easily blow around the fluffy snow. Still, accumulations will be no more than 1-3" (2-8 cm) due to the brief period of snow and lack of moisture.
That piece of the polar vortex is interacting with a strengthening storm near Nova Scotia today and pull the storm northwestward into the Gulf of St. Lawrence and slow down. Though the polar vortex will briefly bring colder conditions, the Gulf of St. Lawrence storm will wrap warm, Atlantic air around itself and eventually entrain it into New England, leading to above freezing daytime temperatures on Thursday except at high elevations.
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| Source: TropicalTidbits |
Then, we move on to the well-advertised Nor'easter starting on Friday and lasting until Saturday. Models have been going all over the place initially, ranging from a big thaw to a suppressed storm way to the south. They have finally come into reasonably good agreement, though the GFS is currently a weak and offshore outlier compared to the other models. The lack of cold air ahead of the Nor'easter will limit snowfall amounts, especially closer to the coast. This will not be a classic, rapidly strengthening Nor'easter with a big area of heavy snow. Instead, an initial strong storm over the central U.S. Plains will weaken and slowly move east into the Midwest U.S., while a secondary storm will develop along the coast, riding along the semi-permanent baroclinic zone near the East Coast caused by the temperature difference between the cold land and the warm Gulf Stream. The strong ridge over northeastern Canada will slow the system and block either storm from pulling straight north and bringing a true warm surge. The leftover initial vertically stacked storm will be a little too far west to add much energy into the coastal storm, preventing rapid intensification. Still, modest intensification will take place as the storm rides along the coast.
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| Source: PivotalWeather |
As the coastal storm initially approaches, the leftover cold air mass from the piece of the polar vortex will dam against the eastern slopes of the central Appalachians underneath warm air aloft, leading to freezing rain and ice pellets southwest of our area. However, due to the unusual wrapping of maritime air, characterized by warmer low-level air, from the northeast, this temperature inversion and icy area will disappear as the storm heads north. With temperatures near freezing but monotonically decreasing with height in interior New England, snowfall will be elevation dependent. With strong easterly flow north of the storm, at least during the first half of the storm, snowfall will also be dependent on which side of a mountain range a place is located. Eastern slopes and summits will receive heavier precipitation and be cooler due to upslope flow, while western slopes and valleys will receive lighter precipitation and be warmer due to downslope flow. There will also be a dry slot punching through central New England. As usual, the heavy snow will occur far enough northwest of the center due not get dry slotted, likely from eastern Ontario and southern Quebec over to the Adirondacks, Green and White Mountains, and into the northern half of Maine. These areas are also far enough inland to not get too much of the warm Atlantic air to change it over to rain. Areas closer to the coast will receive mostly rain due to the easterly flow of warm Atlantic air, though farther north in coastal Maine should see some snow on the front and back ends of the storm. If the storm tracks a little farther northwest, the Laurentians will also get heavy snowfall due to the upslope easterly flow. Meanwhile, the Connecticut River and Champlain Valleys will receive less snow due to the downslope flow and possible dry slot and might mix with rain for a short time when precipitation is lighter, especially if the farther northwest track verifies, which would bring a more prolonged downsloping easterly flow and dry slot.
As the storm slowly moves into Maine and New Brunswick on Saturday, winds will weaken and flip to north or northwesterly, which would flip the upslope and downslope areas. It would also end the advection of warm Atlantic air and cause everyone to change back to all snow, with western slopes receiving some upslope enhancement. Areas that get dry slotted could get a brief period of moderate to heavy snow on the back side of the storm. Precipitation will lighten as the surface low starts to weaken and move farther east, but snow showers will continue as the upper-level low associated with the leftover Midwest storm approaches, especially over the western slopes and north of the U.S./Canada border. Southern Vermont through southern Maine will dry out completely with stronger westerly flow if the more northern track verifies. If the upper-level low moves a little to the south, we would have more widespread snow showers continuing through the weekend. In the end, it is a complicated forecast and the details are subject to change, but I would preliminarily expect a total of 8-16" (20-40 cm) over the higher terrain of the Adirondacks, Northern New England, and maybe eastern Ontario and the Laurentians, with 4-8" (10-20 cm) in the lower elevations elsewhere away from the coast.
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| Source: PivotalWeather |
After the Nor'easter is over, modified cold air and still scattered snow showers will dominate for the following few days. A much stronger shot of Arctic air will push southward into the U.S. Plains next week. On the leading edge of it, the injection of polar upper-level energy could promote a northeastward moving storm from the southern Plains into New England, producing more heavy snow and/or mixed precipitation, but it is too far away for certainty on anything yet. It does look likely that most of the really cold air will be aimed to our west due to the continued (although weakened) Greenland blocking and its location, but the air should still be plenty cold for snow for the vast majority of the time.
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| Source: PivotalWeather and MétéoCentre |
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