Plain-language summary:
A storm will bring lower elevation rain and higher elevation snow, this afternoon into Monday, heaviest southeast of the St. Lawrence Valley, with even cooler air following behind it, accompanied by breezy conditions and scattered rain or snow showers depending on elevation, through Thursday. Skies clear out with calming winds Thursday night into Friday, leading to a widespread frost or freeze outside urban areas and near larger lakes and the Atlantic Ocean, before turning warmer and remaining mostly sunny through next weekend. Weather will likely be more variable with fluctuating temperatures and occasional storms afterward, though perhaps biased slightly warmer than average.
Meteorological discussion:
An approaching storm will be passing initially to the south this evening and then tracking northeastward into Maine and New Brunswick, putting our region on the cold northern side of the storm. The storm will tap into an unseasonably cold air mass to the north aided by a highly amplified weather pattern with a big ridge in western North America and trough in eastern North America (+PNA and -NAO combined). If this were the middle of winter, there would be a widespread snowstorm, but since it is October, it will be just rain in lower elevations, but snow will fall over higher elevations of the Adirondacks, Green and White Mountains, Eastern Townships of Quebec, and Maine for the first time this season tonight into tomorrow, with light wet snow accumulations possible or likely. Areas to the northwest of the St. Lawrence Valley will receive less precipitation, farther away from the storm and its best dynamics.
 |
| Source: PivotalWeather |
After the storm leaves Monday night, the broad upper-level trough in eastern Canada will cut off and amplify and then move only slowly eastward through Thursday, again caused by the amplified weather pattern. This will lead to a brisk northwesterly flow of cold air into our region through Thursday, with cloudiness and scattered showers typical of these cool, moist air masses that pick up moisture from the unfrozen Hudson Bay and numerous smaller lakes and rivers in eastern Canada in the fall. The higher terrain will be cold enough for the showers to fall as snow. However, with the upper-level trough axis and cut-off low remaining to the east, there will be general descent and lack of widespread precipitation in our region. As surface high pressure approaches our region Thursday, clouds will clear and winds will lighten, leading to better radiational cooling and a widespread frost and freeze outside of urban areas and near larger lakes and the Atlantic Ocean on Wednesday night and/or Thursday night.
 |