Plain-language summary:
After a cool morning, a sharp warm up will begin this afternoon and peak tomorrow, especially south of the U.S./Canada border, before a weakening cold front triggers scattered and possibly slow-moving showers and thunderstorms Thursday afternoon into Friday morning. Heat and humidity returns for the weekend before a period of possible heavy rain and localized flooding occurs early next week, followed by a bigger cool down.
Meteorological discussion:
Although it was rather cool this morning, warm advection with light southwesterly flow is beginning this afternoon. Very hot and humid air is located to the southwest over the Midwest U.S., which led to a line of severe thunderstorms developing there this afternoon. Interestingly, it is not really associated with the cold front, which is located well off to the northwest. Nonetheless, this convection and associated cloud cover will create a subtle boundary on its own and likely moderate the air mass somewhat, meaning that northernmost New England and especially southeastern Canada will not experience the full force of the hot air.
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| Source: Aviation Weather Center and TropicalTidbits |
As has been the case all summer long, latitude will be important in dictating tomorrow's temperatures. With the cold front close by to the northwest and cloud cover extending southeastward from that, the hottest temperatures will be in southern Vermont through southern Maine, where downsloping westerly winds, combined with the least cloud cover and warmest 850 mb temperatures, will likely lead to 95F+ (35C+) temperatures in lower elevations. Farther to the northwest, in eastern Ontario, northern New York, southern Quebec, and northern Maine, it will likely not reach 86F (30C) with more cloud cover closer to the front and cooler 850 mb temperatures, though it will still be very humid.
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| Source: TropicalTidbits and PivotalWeather |
A once potent cold front over northern Ontario will weaken and slow down as it pushes into southern Quebec tomorrow afternoon and northern New England tomorrow night. This is due to resistance from the mid-level ridge centered off the U.S. East Coast. Although it will weaken, it along with the uneven topography in the region will still provide triggers for showers and thunderstorms, though they will probably not be as widespread as they were in the Midwest U.S. today with the mid-level ridge and less synoptic-scale forcing. Nonetheless, with the front stalling, any storms that do form could train over the same area and produce isolated flash flooding, given the large CAPE and very moist air mass. The biggest threat of this is over Vermont and New Hampshire Thursday evening into Friday. With the slightly cooler air mass, showers, and thunderstorms, Friday will be noticeably cooler than Thursday though still very humid.
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| Source: TropicalTidbits and PivotalWeather |
By Friday night, the cold front will have completely disappeared with the mid-level ridge strengthening just off the U.S. East Coast. Temperatures will heat back up for the weekend with continued high humidity. The high humidity will lead to unusually warm nights, with lows of 68F+ (20C+) for lower elevations. On Saturday, the heat will be rather uniform, extending deep into Canada. A disturbance moving off to the north Saturday night, riding over the ridge, will provide a cold front and break down the ridge slightly, but the front will not advance south of the U.S./Canada border due to the ridge. As such, Sunday will be hot again, perhaps even hotter than Saturday, especially south of the U.S./Canada border. Depending on how much moisture and cloud cover is left from the preceding front, it could reach 95F+ (35C+) again in lower elevations. Despite the ridge, the heat and humidity could still lead to scattered terrain-induced showers and thunderstorms. North of there, it could be cooler depending on how far south the front gets.
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| Source: TropicalTidbits |
Eventually, another disturbance will ride over the ridge Sunday night into Monday, likely farther south than its predecessor but with some uncertainty. The front could slice through southeastern Canada and stall out in northern New England, providing a very strong temperature gradient with hot and mostly sunny weather to the south and cool and rainy weather to the north on Monday. Once again, with the slow movement of the front and very moist air mass, flash flooding will be a concern, the location of which will be dependent on where the front sets up. The risk of heavy rain and flooding appears higher this time with multiple disturbances moving along the slow-moving front producing widespread thunderstorms or heavier stratiform rainfall, but exact timing and location remain to be determined. The front and associated stormy weather should pass off to the southeast and break late down the ridge by Tuesday or Wednesday, leaving us with substantially cooler weather in its wake.
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| Source: TropicalTidbits and PivotalWeather |
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