Plain-language summary:
After a rather warm and mostly sunny day today in most of the region, except for coastal areas, rain and scattered thunderstorms are expected in the region tomorrow into tomorrow night, with more rain in the Laurentians and from southern Vermont to southern Maine and less in the Champlain Valley and southern St. Lawrence Valley. It will then turn much cooler through Friday, with mostly cloudy skies and scattered showers for Tuesday and Wednesday and likely drying and clearing for the rest of the week into next weekend, though the timing of this is uncertain due to the uncertain speed of next week's slowly-moving storm. It will then warmup somewhat, but generally seasonable temperatures with no big heat waves are expected going forward into the first week of June.
Meteorological discussion:
With broad, weak high pressure dominating the region, today will be rather warm and mostly sunny with low humidity across most of the region, though southeasterly flow off the cool Atlantic waters will keep coastal areas cooler. However, a low-pressure system currently in the Midwest U.S. will slowly strengthen as it tracks northeastward into northern Ontario tomorrow evening and then central/northern Quebec by Tuesday morning. This will lead to a southerly flow of warmth and moisture advection, leading to clouds and rain first over eastern Ontario and northwestern New York by tomorrow morning, and then the rest of the region by evening or early tomorrow night. Given some daytime warming for areas in southern Quebec and Vermont on eastward before the rain arrives, some instability will develop, with scattered thunderstorms developing, though instability won't be nearly large enough for anything severe. There will also likely be a weak broken squall line traversing our region tomorrow afternoon adn evening from west to east along a subtle surface trough with near moist adiabatic vertical profiles. Due to the southerly flow, the Laurentians and areas along and just southeast of the Appalachians from southern Vermont to southern Maine will receive more rain, whereas the downsloped areas of the Champlain Valley and southern St. Lawrence Valley will receive less rain.
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| Source: TropicalTidbits |
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| Source: TropicalTidbits |
By Tuesday, the low-pressure system over central/northern Quebec will start to slow down and weaken, with us in a weak cool advection regime in cool air aloft that will lead to some daytime instability, given the May sun angle, and showers, especially since the westerly low-level flow will lead to moderation of any cool, stable low-level air mass. A secondary cold front will arrive on Wednesday and be the impetus for shower or weak thunderstorm activity as it plows southeastward from southern Quebec to Maine. Behind this, winds turn to northerly, bringing a more direct flow of cooler air, with the coolest temperatures in a couple of weeks. From Wednesday to Friday, many areas in lower elevations along and northwest of the Appalachians will have daytime highs of only 59-66F (15-19C), with even cooler temperatures at higher elevations.
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| Source: TropicalTidbits |
After that, the low pressure over central Quebec becomes slightly cut off and will continue to move slowly, with some models even suggesting it move south and then back west into our region by next weekend, though this appears to be relatively unlikely scenario, as models tend to show this kind of extreme solution more often than it actually happens. More likely, the cut-off low will move just enough east by next weekend that our region will be slowly warming up with more sunshine by then, perhaps except Maine. Afterward, it is rather uncertain, but a building ridge in western North America (+PNA) for likely the following 5-7 days will tend to favor slight troughing in the central and eastern U.S., with a slightly negative NAO perhaps contributing to ridging over central and eastern Canada overtop the troughing, leading to a lack of both cold and hot air as well as weather systems moving slower than usual. Normally, such troughing over our region will lead to cooler than average temperatures, but with the lack of any real cold air (like has been the case since last winter), it will generally still be near to slightly warmer than average on most days except for really cloudy and rainy days.
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| Source: TropicalTidbits |
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| Source: Climate Prediction Center |
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| Source: TropicalTidbits |