Late-season Nor'easter to bring heavy mountain snows and a mix of rain and wet snow to the valleys; likely to be the last measurable snow in lower elevations

Plain-language summary:

A late-season Nor'easter will bring heavy mountain snows in the Adirondacks and Laurentians (up to 12" or 30 cm) and a mix of rain and wet snow to the Champlain and St. Lawrence valleys. This will occur late Monday night through Tuesday. Precipitation will be steady across northwestern New York, eastern Ontario, and the Laurentians, while pronounced drying will occur farther east after a few hours of heavy precipitation. The heaviest precipitation will fall over southern Quebec, northern New York, and western New England. Farther east, the Eastern Townships of Quebec and northeast kingdom of Vermont will get much less precipitation, and in New Hampshire and Maine, a brief period of heavy rain (except snow in the higher elevations and northern Maine) will transition to spotty rain showers. This will likely be the last measurable snowfall in lower elevations.

Meteorological discussion:

In my last post, I said that given the pattern all winter, I was skeptical of a big coastal storm and was leaning towards the GFS model which showed little if anything. However, the GFS has caved to the other models and there is very good agreement now on a noteworthy, strengthening Nor'easter tucked right along the coast, something we did not see all winter. However, somewhat surprisingly, the storm is on track to move farther west than almost all model guidance just a couple of days ago, and will be too far west for a big snowstorm in New England.

Although the initial system will be over the Great Lakes region on Monday, typical of what we have seen all winter, a southern disturbance will reach the Carolina coastline around that time and quickly strengthen there and move north-northeastward, near or just northwest of the Gulf Stream gradient, on the western edge of a classic Nor'easter track, somewhat similar to the January 17, 2022 snowstorm. There will be a classic precipitation structure with heavy warm-advection precipitation ahead of it, affecting New England Monday night, mostly as rain in southern New England and snow in northern New England, and a band of heavy snow to the northwest in the colder air over the Catskills and Adirondacks, even at lower elevations. Although temperatures are marginal, the snow will likely come down hard enough in the heavy band to accumulate on grass, regardless or day or night or elevation. Due to the April sun angle warming up the low-levels ahead of the storm, lower elevations will be warm enough for rain initially but wet-bulb cooling and strong upward motion will cause a changeover to snow, with temperatures dropping to near freezing.

Source: PivotalWeather

On Tuesday, the low pressure center will move into eastern Vermont and eastern Townships of Quebec, with heavy warm-advection snow moving off well to the north, a more persistent band of heavy snow to the northwest over northwestern New York, eastern Ontario, and the Laurentians, and a enlargening dry slot in New England and eastern Townships of Quebec. With the storm tracking so far west, the dry slot will be enhanced by strong southeasterly flow downsloping off the White Mountains. Due to the southerly flow on the eastern side of the storm, temperatures will spike on Tuesday for a few hours along and east of the low pressure track, changing what little precipitation remaining to rain, before colder and drier air wraps in behind from the west Tuesday night. A brief period of ice pellets/sleet before a change to rain is possible over northern Maine as the warm air aloft rushes in. One exception could occur along and immediately to the west of the low pressure, when there could be a narrow and brief band of moderate to heavy snow on the back side in the middle of the day as temperatures drop back to near freezing. This is most likely in northern New York, southwestern Quebec, and western Vermont. Since the storm will not cut-off and occlude like the blizzard in the northern Plains earlier in the week, it will be fast-moving, and precipitation will be over by Tuesday night aside from mountain snow showers.

The fast movement and marginal temperatures will limit accumulations, and snow amounts are highly uncertain. As usual for April, snow will be heavily elevation-dependent, with a best guess of 1-3" (2-8 cm) in the Champlain Valley, 2-5" (5-13 cm) in the St. Lawrence Valley from Montreal to Kingston, Ontario, and a coating to 2" (5 cm) in the St. Lawrence Valley northeast of Montreal. The higher terrain of the Green and White Mountains could receive 2-4" (5-10 cm) in a few hours before the dry slot moves in. The jackpot will be in the Adirondacks, Catskills, and Laurentians just northwest of Montreal, which are in the perfect spot to receive the heavy frontogenetic snow band for the longest and are in higher elevations, where 6-12" (15-30 cm) is likely. Except in the highest elevations, temperatures will be at or above freezing when it snows, so the snow will only accumulate when it comes down hard. Snow could change to rain with any lighter pockets of precipitation. Note that the 10:1 ratio shown below will be greatly overestimated, especially in lower elevations. In lower elevations, with no snow in the foreseeable future afterwards, this will likely be the last measurable snowfall.

Source: PivotalWeather