Plain-language summary:
Today will be very active across our region, with rather cool temperatures and heavy rainfall causing flooding in some areas from western/southern Quebec to northern Maine, while areas to the south see a one-day surge in warmth and humidity accompanied by severe thunderstorms, with even an isolated tornado possible. It will be cloudy and cool tomorrow, especially along and northwest of the Appalachians, but turns moderately hot, dry, and sunny for Tuesday. It will still be moderately hot on Wednesday south of the U.S./Canada border, but a cold front will trigger clouds and some thunderstorms, though likely few if any of them will become severe. It will turn cooler, drier, and sunnier for Thursday and Friday, and Thursday night will be the coolest since last weekend. It likely turns moderately hot next weekend, but the heat will likely be short-lived, in contrary to earlier expectations, with near to slight below average temperatures likely for the beginning of July.
Meteorological discussion:
It turns out that only 3 days this week were really hot, with a weak cold front bringing daytime temperatures back to near or below average for Friday and Saturday, though with the cold front being rather weak and stalling in our region, warm nights and high humidity still lingered except for far northern areas. Today is a very active day weather-wise across the region, with varying impacts depending on location. An
elongated low pressure is tracking through western/southern Quebec and will move east into Maine tonight along an unseasonably strong temperature gradient, with cooler, drier air to the north and oppressively hot and humid air to the south. To the east of the low pressure, a warm front advanced northward into southern Quebec but almost stalled out. This stalling out is due to a combination of the low pressure just tracking mostly along the warm front but also due to the low-level cool air
channeling down the St. Lawrence valley that is resisting any northward
movement of the front in that area. Similarly, low-level cool air dammed east of the Appalachians in Maine and eastern New Hampshire is resisting the progression of the warm front there. The storm is combining winter-like frontogenesis and
synoptic/mesoscale/terrain forcing for ascent (warm saturated juicy air
forced to lift above the front) with summertime moisture and low
stability (or even instability) aloft! It's an unusual combination, and
some places from southern Quebec to northern Maine could get >100 mm of rain, or a month’s worth of rain, by tomorrow, leading to a flood threat.
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| Source: PivotalWeather |
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| Source: Storm Prediction Center, TropicalTidbits, and PivotalWeather |
To the south of the front, warm, moist southwesterly winds will dominate, with a few peeks of sun allowing for daytime instability. With the rather strong dynamics for summer (significant both speed and directional wind shear, with veering winds with height) combined with summertime heat and humidity, severe thunderstorms will initiate, with some of them possibly being supercells capable of producing tornadoes. The Storm Prediction Center has a 10% tornado chance (within 25 miles of a given location) area in parts of Vermont and New Hampshire, which is rarely seen in our region, along with an enhanced risk of severe thunderstorms in general. As is always the case with these storms, it is not possible to tell exactly who will get hit until the storm is about to occur. The storms and clouds will prevent temperatures from climbing above 86F (30C) in our region, unlike the high heat to the south.
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| Source: TropicalTidbits |
As the elongated low pressure moves eastward tonight, its cold front will slowly push south and then southeastward through the region, bringing in cooler air tomorrow. With the western extension of the low-pressure system still over our region and lingering low-level moisture behind the front, showers and low-level clouds will hang on especially in the upslope areas of the northwestern slopes of the Appalachians, leading to being especially cool there. However, surface high pressure and upper-level ridging briefly build in afterwards, leading to a hotter, sunnier, and dry Tuesday. By Wednesday, the next low pressure and associated cold front already arrives, though if the front and associated clouds and storms is a little slower, it could reach 86F (30C) in some low-elevation areas, especially south of the U.S./Canada border. Some thunderstorms will accompany the cold front, as usual for summer, but the dynamics do not look as impressive as for today, and severe thunderstorms appear to be isolated at most.
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| Source: TropicalTidbits |
A
low-pressure system developing just to our southeast along the cold
front could bring some chilly rain late Wednesday night into Thursday
morning. Then, a potent cool and dry air mass by summer standards will move in late Thursday and Friday associated with a cool Canadian high pressure and direct northwesterly flow bringing the cool air mass in with little moderation. With clearing skies, low temperatures below 50F (10C) likely for most Thursday night, the coolest since last weekend, in stark contrast to the constant warm nights and humidity this past week. However, as the surface high pressure slides southeastward, the cool air mass will quickly modify with the June sun angle, and winds will shift to southwesterly, bringing in warmer and more humid air. However, despite the warm advection, any hot weather will be tempered by increasing clouds, though there could still be a couple of hot days. This is the potential "heat wave" that was advertised for the end of June as early as last week, but it now appears unlikely to last more than a couple of days, as whatever upper-level ridge that builds in the eastern U.S. will quickly get knocked down to the southwest by a trough quickly moving eastward from the northern Rockies into eastern Canada, bringing a cold front that knocks temperatures back down to near or even below normal, a marked change from earlier projections. It appears that the slightly positive PNA favoring some ridging in western North America, ridging mostly suppressed just to the southwest, as well as the repeated clouds and rain locally, are making it difficult to sustain any heat in our region through the beginning of July. Instead, the pattern looks rather changeable, with alternating cool and warm air masses and rain and thunderstorms associated with cold fronts.
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| Source: Climate Prediction Center |
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| Source: TropicalTidbits |
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