Plain-language summary:
The first thunderstorms of the year rumbled in the region, mostly along a warm front west of the Green Mountains, yesterday evening. A brief warmup will follow the warm frontal passage late this morning into this afternoon west of the Green Mountains and south of the U.S./Canada border. Some thunderstorms are likely just ahead of the cold front this afternoon west of the Green Mountains and south of the U.S./Canada border before weakening as they head to the east this evening.
It will still be mild with a mix of sun and clouds tomorrow (along with a few scattered showers west of the Appalachians), but it turns cooler with valley rain and higher-elevation snow on Saturday, with snow in northern Maine Saturday night. Outside Maine, snow showers are likely Saturday night into Sunday, especially in northwestern facing slopes, with accumulations likely in higher elevations and a dusting possible at lower elevations. After a dry Monday, another storm will bring some rain and at least higher-elevation snow on Tuesday, but there is quite a bit of uncertainty, and for now, some wet snow is still possible even at lower elevations.
Meteorological discussion:
Yesterday evening, the first thunderstorms occurred for many, mostly west of the Green Mountains. They occurred in western Vermont, northern New York, eastern Ontario, and southern Quebec just north of a dynamic warm front. Surface temperatures were below 50F (10C) where the thunderstorms occurred, which is unusually cool for them to occur, but they were elevated thunderstorms, fueled by warm, moist air aloft surging from the southwest. (South of the surface warm front, temperatures and dew points were pushing 77F (25C) and 59F (15C) respectively).
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| Source: College of DuPage |
Late this morning into the early afternoon, the warm front will lift north of the region, mostly west of the Green Mountains and south of the U.S./Canada border, and clouds will break. In this region, both of these will lead to temperatures warming considerably. As is common in spring, low-level cool air will remain dammed east of the Green Mountains and clouds will have a hard time breaking there. There will also be cool air channeling down the St. Lawrence Valley from the northeast, limiting warming from Montreal on north. A cold front approaching from the west will spark more scattered showers and thunderstorms this afternoon, first over eastern Ontario and northwestern New York and then moving into southern Quebec and western Vermont before weakening as they move into the cooler, more stable air east of the Green Mountains.
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| Source: PivotalWeather |
The cold front extends southeastward from a powerful, occluded storm over Lake Superior (which brought the blizzard over North Dakota and Manitoba), with cold air wrapping eastward on the southern side behind the cold front. In this setup, the cold air moves southward into the Plains first, and then gets modified as it moves eastward, so the cold air will be not so cold when it arrives in our region. It will still be mild with a mix of sun and clouds (along with a few scattered showers west of the Appalachians) on Friday ahead of a weak shortwave rotating around the slowly-moving, vertically stacked low pressure that will still be over northern Ontario. The shortwave will bring valley rain and mountain snow along with chilly temperatures on Saturday, mostly south of the U.S./Canada border. Snow will likely fall even at lower elevations in northern Maine Saturday night. (Note that the ECMWF model shown is often too warm during precipitation, so snow could be more widespread than shown).
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| Source: PivotalWeather |
A secondary shot of cold air will arrive behind the shortwave. This air mass will be colder with a more direct flow of cold air out of the northwest. As cold air moves in aloft above the warmer low-levels, instability-driven snow showers will develop. Many models are also pointing to a period of enhanced snow showers Saturday night into Sunday morning with the upper-level trough axis passing overhead, even at lower elevations, especially in the favored northwestern facing slopes over the Adirondacks, Green and White Mountains, and Eastern Townships into western Maine. This is at the coolest time of day, before the strong April sun has its effect, so accumulations could occur with a dusting possible even in lower elevations. By afternoon, the daytime heating will lead to greater instability and perhaps heavier but more scattered snow showers, but accumulations will be minimal or none except at high elevations due to the April sun angle. (Note that I am not a fan of the NAM and RGEM models beyond 48 hours, but I am showing them just to give an idea of the precipitation structure.)
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| Source: TropicalTidBits |
Beyond that, it dries out Sunday night into Monday, but another storm will approach from the west by Tuesday. A primary low pressure will cut to the west over the Great Lakes region, but there will probably be a strengthening coastal low pressure that will weaken the southerly flow and keep cooler air over the region. Models still disagree on this, with the operational GFS showing no coastal low pressure and almost all over models showing it. (Though even the GFS ensemble has many members showing a coastal low pressure). However, there hasn't been much good coastal development throughout the whole cool season, so I am inclined to lean towards the GFS solution for now, which would imply a brief period (6-12 hours) of light to moderate rain and snow only at high elevations. However, if the coastal low develops and slows down, rain would change to wet snow on the western side and last for a good 12-18 hours, perhaps even in the lower elevations, with heavy accumulations in the higher elevations. There is also some indication of another system around April 23 that could be the last chance for snow (except for very high elevations) before the weather pattern turns warmer.
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| Sources: PivotalWeather, Tomer Burg, and UQAM Weather Centre |
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