Plain-language summary:
A cold upcoming week is shaping up with scattered snow showers northwest of the Appalachians through Tuesday before a widespread light to moderate snowfall late Wednesday into Thursday, least in the Champlain Valley and most in the Laurentians. It will turn much colder afterward for at least a few days. The cold pattern likely relaxes somewhat the following week, which could open the door for a bigger storm or even a potential thaw with a warm surge and rain, though that is quite uncertain for now.
Meteorological discussion:
It will be a fairly cold week by early December standards, with a persistent -EPO, +PNA, and to a lesser extent, -AO helping to keep a pattern of western North America ridging and arctic air pouring into eastern North America. Depending on how long this cold pattern lasts, it could be the coldest December since 2017 in our region. So far though, our region has not received the really cold air, as a persistent low-pressure system over James Bay has led to the cold air first entering the northern Plains before heading east, modifying over the warm Great Lakes, and then reaching our region in a west-southwesterly flow. This is never a good setup for extreme cold in our region and means that despite the cold air aloft, steep lapse rates ensure that surface temperatures are not all that cold, though still cold enough for snow. Lake-effect snow off Lake Ontario will continue to bury the western Adirondacks through Monday before the wind shifts to northwesterly behind a subtle disturbance, shifting the lake-effect snow band to the south, with the band also weakening with increasing low-level stability. The subtle disturbance/convergence zone will produce snow showers, heavy at times due to the low stability and relatively moist atmosphere, in eastern Ontario and southern Quebec during the day today, and then in northern Vermont tonight. The snow showers will fizzle out east of the Appalachians due to the downsloping flow. Another subtle disturbance will spark lighter snow showers over a similar area Monday evening and night.
 |
| Source: PivotalWeather |
 |
| Source: Climate Prediction Center |
As the winds turn more northerly behind the disturbances, they allow for a more direct shot of cold air unmodified by the Great Lakes into our region, leading to the coldest temperatures of the season thus far Tuesday night, and areas northwest of the Appalachians failing to get above freezing on Tuesday. This cold air still isn't extremely cold as it has already gotten stale and might be modified by the unfrozen Hudson Bay. The added moisture from Hudson Bay and all the unfrozen lakes in Ontario and Quebec means that this cold air mass won't be particularly "clean", with clouds and scattered snow showers, particularly northwest of the Appalachians. An Alberta clipper will dive into northeastern Ontario and then into central Quebec Wednesday into Thursday while slowly strengthening. This is farther north than ideal for a good snowfall in our region, though there do not appear to be any particularly strong cold or warm fronts, with a broad area of light to moderate snow encompassing northern New England and southeastern Canada instead. It could snow almost nonstop for 24+ hours in some areas, most likely in central Quebec, due to the broad nature of the system. Areas to the south of the low-pressure track will see southerly to southwesterly winds, slightly milder temperatures, and a terrain dependence to the snow amounts due to areas of upsloping and downsloping (enhanced in Laurentians and perhaps southern Adirondacks while reduced in the Champlain Valley). Still, there will likely be at least light snow accumulations for just about everywhere. Behind this, it will get much colder with a direct shot of cold air from the northwest aided by a cold Arctic surface high pressure system that will dive into the northeast U.S. on Friday, as well as fresh snow cover, even though strong winds with the strong pressure gradient will prevent ideal radiational cooling for most. Another weaker clipper is possible next weekend, though there is considerable model disagreement on this.
 |