Plain-language summary:
The somewhat impressive heat wave south of the U.S./Canada border is coming to an end tonight into tomorrow morning with a cold front sparking showers and thunderstorms, a few of which could be severe. Cooler, but not particularly cool and less humid air will follow for later Monday into Tuesday, before a modest warm up on Wednesday and Thursday and then another modest cooldown next weekend. Temperatures will not be overly cool, but rather close to seasonal averages. Heat could return for the following week, but as always, there is uncertainty that far out.
Meteorological discussion:
Today marks the last day of the heat wave that has mainly affected areas south of the U.S./Canada border. The dominant, deep westerly flow along with an unusually strong (for summer) north to south temperature gradient means that areas in southern Vermont, southern New Hampshire, and southern Maine have been especially hot with downsloping westerly flow and sunshine giving a boost. For today, clouds will keep temperatures (but not humidity) lower, generally under 86F (30C) in northwestern New York, eastern Ontario, and southwestern Quebec. To the southeast, mostly sunny skies will cause temperatures to approach 95F (35C) in the favored hot spots.
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| Source: TropicalTidbits |
A cold front arriving from the west will trigger showers and thunderstorms in the hot, humid air. However, a bunch of mid-level clouds are moving in well ahead of the actual front, which will limit instability for the storms to feed on. In addition, the front will pass the area overnight tonight, further reducing instability. Still, with relatively strong winds aloft and some directional wind shear, some scattered severe thunderstorms cannot be ruled out, and the Storm Prediction Center has northern New York and northern Vermont in a slight risk of severe thunderstorms.
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| Source: PivotalWeather and SPC |
Behind the front, the flow turns more west-northwesterly, which is cooler but still not directly out of the north which is more favorable for transporting a truly cool air mass. Combined with sunshine on Monday afternoon and all day on Tuesday, high temperatures will still be close to, if not slightly above, seasonal averages for most areas. Lower humidity will lead to cooler nights.
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| Source: TropicalTidbits |
As high pressure builds in for Wednesday, and the coolest air remaining to the north, it will warm up again with some spots reaching 86F (30C), but without the high humidity. A weak low pressure heading to the northwest will cause winds to turn to a weak southerly or southwesterly flow, leading to a modestly hot day on Thursday with scattered afternoon thunderstorms possible along a weak, diffuse front with slightly higher humidity. The Bermuda ridge to the southeast will slow the front's movement and another system could ride along the front, bringing more showers and thunderstorms for Friday, but that is still somewhat uncertain.
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| Source: TropicalTidbits |
High pressure builds in for the weekend behind the front, with northwesterly flow initially leading to cooler temperatures. However, abundant sunshine will quickly modify any cool air mass, and with the high pressure and associated upper-level ridge expected to stay for a few days after, temperatures will likely heat up again, though by how much is still questionable. There is some disagreement on this, but a front could swing by the middle of the following week, but at the same time, there could be a relatively deep trough moving into the Pacific Northwest that would end the heat wave there and push the hot air out of the Rockies and eventually into the U.S. Plains and Northeast U.S., leading to more heat but perhaps lower humidity and fewer thunderstorms. That is still a long ways out though, so stay tuned for changes.
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