Plain-language summary:
An insanely mild week-long period will be ending later this week, with a slow-moving storm bringing wintry mix tonight into tomorrow (more rain and freezing rain to the south and snow and ice pellets to the north) and then scattered rain and snow showers for Thursday night through Saturday. After turning somewhat colder and drier for the rest of the weekend, a couple of Nor'easter opportunities are showing up for late next week, though it still does not look especially cold.
Meteorological discussion:
The past week has just been insanely warm for the middle of the winter, with all the snow southeast of the St. Lawrence Valley being completely gone except for a few high elevation locations and northern Maine. Montreal's 5-day consecutive stretch above freezing and Burlington, VT's low of 51F (11C) on December 31, both almost unheard of at this time of year, are just a couple of illustrative examples.
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| Source: Northeast Regional Climate Center |
However, a cold Canadian high pressure is attempting to nose southward into southern Quebec, with low-level cold air sneaking southward tonight to Maine and to the U.S./Canada border farther west. The advance of the cold air will be stopped by a slow moving storm moving from the Midwest U.S. with the leftover incredible warmth and humidity. That warmth will still be present aloft initially, leading to ice pellets and freezing rain where surface temperatures are below freezing, except in northern Maine and locations at similar latitudes in Ontario and Quebec, where the cold air will be deeper and more established. The warm air aloft overrunning the cold low-level air will also lead to a several-hour period of moderate to heavier precipitation, given all the moisture in place. The heaviest icing will occur in the St. Lawrence Valley of northwestern New York and far southwestern Quebec, where the valley channels the cold low-level air from the northeast and warmer air will remain aloft.
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| Source: TropicalTidbits |
The evolution has changed a lot over the past few days from the initial solution of a sharp and more quickly-moving cold front to a stationary and weakening front with the storm riding along it, due to the swapped longitudes of the low and high pressure systems.
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| Source: TropicalTidbits |
The main low pressure is already occluding though, and by tomorrow night, the warmest air will have been pinched off to the southeast despite the main low pressure still being over Lake Huron. This will lead to a collapse of the heavy precipitation, leaving light rain and snow showers over the region. Colder air will move in aloft due to the approaching upper-level low, but the low-level cold air mass will grow stale and slowly retreat to the St. Lawrence Valley on north. With the temperature inversion destroyed and not much low-level cold air around the upper-level low, precipitation will be rain showers at lower elevations and snow showers at higher elevations. Winds will be rather weak during this period as the main low pressure weakens over western Quebec and another weak low pressure forms off the East Coast.
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| Source: TropicalTidbits |
By late Friday night, another shortwave approaching from the north will bring a somewhat colder air mass, shift the winds to the northwest, and change any rain showers to snow showers by Saturday. There will likely be a brief period of enhanced showery activity with a possible burst of snow, though accumulations appear to be light. With a colder weekend to follow, there will finally be a semblance of winter!
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| Source: TropicalTidbits |
A weak storm could bring a little snow from Vermont to southern Maine on Monday, though most models show it bring suppressed to the south for now, keeping our region dry. Ensembles are showing two opportunities for a Nor'easter later next week, with a relatively cold high pressure over northern Ontario combined with an upper-level ridge over the Rockies and repeated storms coming in from the West Coast tunneling underneath into the southern U.S. The very active Pacific jet stream causing these storms is also flooding most of the contiguous U.S. and southern Canada with mild air, and it will still not be especially cold. This configuration, more typical of an El Niño than a La Niña (which we are currently in), appears to favor more coastal areas for snow, with mostly cold and dry conditions to the northwest. The relatively low amplitude nature of the pattern also does not appear to favor a very strong storm tracking straight northward, so it is questionable how far inland the snow can reach. However, this year, storms have had a tendency to track farther northwest due to a stronger East Coast ridge than modeled a week in advance. The anomalously warm waters off the East Coast will also give a boost to any storm tracking there. So it is too early to write these opportunities off or have any specific details.
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| Source: TropicalTidbits |
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