Coldest December night since 2020 for most, then somewhat warmer with widespread snow Monday night into Tuesday; chilly Wednesday and Thursday before gradually warming up but dry for a while before potential storm at end of month, perhaps with mixed precipitation and thaw; gradually turns colder for early January

Plain-language summary:

 

After a very cold day today, tonight will be the coldest December night since 2020 for most. It will warm up somewhat before a widespread light to moderate snow falls Monday night into Tuesday, up to 4-6" (10-15 cm) from eastern Ontario to central Maine. It then dries out for several days but remains seasonably chilly, though gradually warming up after Thursday. A storm might reach our region for the very end of December, perhaps with mixed precipitation and a brief real thaw, before colder air gradually returns for early January, though it is too early to tell if it will be cold and dry or if the cold will be accompanied by big snowstorms.

Meteorological discussion:

Cold night tonight

 

The coldest air of the season is upon us, with an upper-level ridge in western North America (+PNA) at the right spot to promote an arctic air mass coming directly from the north or northwest with little or no modification from the Great Lakes or still partially unfrozen Hudson Bay. In addition, the cold high-pressure will be right over our region this evening, leading to clear skies, light winds, and good radiational cooling, especially with the fresh snow cover that fell for most on Friday and Saturday. Temperatures away from oceans, larger lakes, and urban areas will reach at or below 0F (-18C) tonight, easily the coldest since last January and the coldest December temperatures since 2020 for most.

 

Source: HYSPLIT and TropicalTidbits

Snow Monday night into Tuesday

 

The high pressure will move to the east on Monday with southerly flow leading to moderation, but a low-pressure system will already be approaching our region from the Great Lakes. The low-pressure will slowly weaken as it moves eastward into our region by Tuesday, but with the moderating arctic air mass in place, the marginally warm and more moist air will overrun the cold air in place, leading to a widespread snow with temperatures plenty cold enough for all snow. The region of heaviest snow will depend on the exact track of the low-pressure system, but it appears to be from eastern Ontario to far southern Quebec and then into northern New England, where locally 4-6" (10-15 cm) will fall. The cold temperatures, substantial ascent in the dendritic growth zone aloft, and lack of strong winds to fracture the snowflakes as they fall will likely lead to rather high snow to liquid ratios (perhaps ~15:1) and therefore higher snow accumulations than shown below.

Source: TropicalTidbits and PivotalWeather

Gradually turns milder later next week through New Year's Day, dry then uncertain storminess

 

The low-pressure system will move to the east on Wednesday, with a weak high-pressure system over central Quebec taking over. At this point, the extended and poleward-shifted Pacific jet (associated with a positive EPO), helped by a positive NAO, will have routed the arctic air out of most of North America, with incredible warm anomalies over most of Canada. However, that will also lead to a weaker temperature gradient, a weaker jet stream, and slower-moving systems, but the surface high-pressure over central Quebec will lead to a persistent low-level northeasterly or easterly flow advecting colder air, making it very difficult to completely scour out the low-level cold air that is being established now. As such, New England and southeastern Canada will be among the last places in North America to lose the cold air, and may not really even warm up noticeably until after Thursday, when the surface high-pressure moves to the southeast, and the low-level flow turns to southeasterly. There are indications that there will never be a strong surge of warm air that fully makes it into the region even through New Year's Day, though it will certainly turn milder as the low-level cold air becomes stale, warm air moves in aloft, and low-level winds turn to the southeast off the still warm Atlantic Ocean. The high-pressure over Quebec could also blocked storms over the southern and central U.S. from reaching our region for a while, or perhaps could advect enough cooler air to lead to mixed precipitation (instead of all rain) with any storm, perhaps arriving at the very end of December, despite the widespread warmth across most of North America. However, with the slow-moving and somewhat blocked weather pattern, the timing and intensity of any storm system is highly uncertain.

 

Source: TropicalTidbits

 

 

Source: TropicalTidbits

 

Source: Climate Prediction Center

 

Colder early January

 

After New Year's Day, the Pacific jet will retract, and upper-level ridging will build again over Alaska and the Yukon (associated with the EPO turning negative), with the NAO turning neutral and AO turning negative, opening the door for colder air to arrive from the northwest again. Around this time, it could briefly really thaw if the initial push of colder air comes in well to the west over the Rockies and forces a strong low-pressure system to move into the Great Lakes and then into Quebec, north of our region, leading to a strong southerly push of warm, moist air. However, although there is some model disagreement on this as it is almost 2 weeks out, it appears likely that a positive PNA (upper-level ridging in western North America) will develop in early January, which will promote the cold air eventually reaching central and eastern North America. Given how warm Canada will be for the last week of December, it will take a while before the cold air from Canada will actually be really cold, but given that it is January, any cold air outbreaks will still feel like winter and be cold enough to snow. It remains to be seen if the upper-level trough is so deep and far east that it just suppresses any storminess to the south, leaving our region cold and dry (like most of the cold air outbreaks this season so far), or if it can sometimes loosen and dig far enough west to lead to a strengthening storm tracking from the southern U.S. to and up the U.S. East Coast (e.g. a real Nor'easter) that would produce a widespread heavy snow.

Source: TropicalTidbits