Warm end to March for most, but first day of April to bring snow showers to many

Plain-language summary:

After a warm last day of March, scattered showers and thunderstorms will affect the area, from eastern Ontario this afternoon to Maine tonight. Tomorrow will be considerably cooler except in Maine with scattered to widespread showers expected, but nothing severe. Rain showers will mix with and change to snow in Ontario and northern New York tomorrow afternoon and Quebec, Vermont, and New Hampshire in the evening. Little if any accumulation is expected in the St. Lawrence and Champlain Valleys, but higher elevations will receive a dusting to 3" (8 cm), especially on northwestern facing slopes.

Meteorological discussion

After a bit of wintry mix last night, temperatures have warmed up a lot today under strong southerly flow, especially over northern New York and eastern Ontario. However, as has been the case many times this winter, the St. Lawrence Valley northeasterly wind channeling is keeping low-level cooler air locked in from Massena, New York to Montreal and to Quebec City. As is often the case in the spring with southerly flow aloft, cooler air has also dammed in northern New England east of the Green and White Mountains.

Source: PivotalWeather

 

Scattered showers and thunderstorms will affect the area from eastern Ontario to northern New York this evening. A few storms could bring a strong gust of wind, but the winds will weaken tonight as the storms move east into more stable air in northern New England and Quebec. These storms are associated with a loosely-defined cold front or pre-frontal trough. 

Source: Environment Canada

However, as the wind switches to westerly and cold advection ensues behind the showers, it will mix out the cold air damming and St. Lawrence Valley cold pool, with temperatures actually warming up in the affected areas while the rest of the region cools down.

Source: PivotalWeather

 

Meanwhile, an upper-level trough will approach from the west and will induce instability, so it won't dry out like it often does behind cold fronts. Instead, rain and snow showers will form over eastern Ontario and northern New York tomorrow morning and strengthen with a little daytime heating as they move into Quebec and Vermont in the afternoon. 

Source: PivotalWeather

 

With the deep northwesterly flow in place by afternoon, precipitation will be locally enhanced in the northwestern slopes of the Adirondacks and Green Mountains and Eastern Townships of Quebec. As weak cold advection continues and the sun goes down in the evening, all precipitation will change to snow, even in the St. Lawrence and Champlain valleys, but will also become more confined to the higher terrain. With marginal temperatures, especially in the valleys, snow accumulation will largely be above ~800 ft (~250 m) elevation except in particularly intense snow showers. Northwestern facing slopes will continue to receive light upslope snow through tomorrow night with up to 3" (8 cm) over the higher terrain before the column dries out. As is typical for northwesterly flow setups, southeastern Vermont to the eastern half of Maine will be downsloped and see delayed cooling with no snow. It will also be mainly dry there, except some rain showers in eastern Maine tomorrow afternoon due to being close to a strengthening surface low in New Brunswick. The HRRR is often a little too warm during precipitation, so it might be underestimating snowfall total, but I think it handles the spatial pattern well.

Source: PivotalWeather