Warm and mostly sunny Wednesday; steady, soaking rainfall on Thursday with snow in the Laurentians; valley rain showers and mountain snow showers for Friday and Saturday

Plain-language summary:

After a warm and mostly sunny Wednesday (though cooler north of St. Lawrence Valley and northern Maine), a storm will bring a steady, soaking rainfall on Thursday with up to 4" (10 cm) of wet snow expected in the Laurentians just north of the St. Lawrence Valley. Scattered valley rain showers and mountain snow showers are expected for Friday and Saturday before drying for Sunday, with no snow accumulation expected below 1000' (300 m) and only minor if any accumulation above that, except for the mountain summits.

Meteorological discussion:

Despite a "chilly" morning, mostly sunny skies and a warmer air mass will lead to quick warming today, especially with the April sun angle and the lack of foliage to block the sun. This is especially true west of the Green and Adirondack Mountains, where the southeasterly flow will downslope. It will be the warmest day of the year so far for many, with temperatures well into the 50s F (10-15 C) to the north and even 60-65 F (16-18 C) to the south. The exception will be northern Maine (where deep snow cover and slight northerly flow prevails) and areas well to the northeast of Montreal (where the usual cold air channeling down the St. Lawrence Valley will cool it off). Cloudier skies will lower temperatures in the far south, from the southern Adirondacks to southern New Hampshire, as those regions are closer to an offshore storm.

Source: PivotalWeather

 

 

 

The dry, sunny weather will end tomorrow as a storm approaches from the west. If this were the middle of winter, this would be a good setup for a front-end snowstorm, especially on the southeastern facing slopes where the southeasterly flow will upslope. Since it's April and there is no arctic air mass ahead of the storm, precipitation will be just rain at lower elevations, with a widespread and steady, soaking rain of 1-2" (25-50 mm) expected, though with a bit less in the Champlain Valley and Northeast Kingdom due to downsloping off the Green and White Mountains respectively and in Maine due to the drier easterly flow attacking the storm.

Source: PivotalWeather

The Laurentians north of the St. Lawrence valley will actually see rain change to heavy, wet snow with up to a few inches (10 cm) possible locally. This is aided by the formation of a secondary, coastal low that will keep the low-level flow easterly that will upslope and prevent the really warm air from coming in. This continues the "rich get richer" pattern from the winter. This might occur in the higher terrain in western Maine and the Eastern Townships of Quebec, though warmer air aloft may keep precipitation as rain or sleet/ice pellets there.

Source: PivotalWeather

After the main storm leaves, there is a deep upper-level trough that will produce scattered valley rain showers and mountain snow showers Friday and Saturday. Showers will be more convective and numerous in the afternoon due to some daytime heating and cold air aloft creating instability, although it will be mostly cloudy. No snow accumulation is expected under 1000' (300 m) elevation. Even above that, other than the mountain summits, the snow will have a hard time sticking with marginal surface temperature and April sun angle, so any accumulation will be minor and limited to nighttime. It will dry out by Sunday as the trough leaves and ridging builds in.

Source: PivotalWeather