Plain-language summary:
It will be warm and humid today, with cloud cover preventing it from getting particularly hot, especially east of the Appalachians. Scattered showers will occur especially northwest of the St. Lawrence River. More showers and thunderstorms are expected on Monday and Monday night before it turns unseasonably cool, the coolest since mid-June, for Tuesday and Wednesday. It will still be rather cloudy with a few showers on Tuesday and Wednesday clearing out with a warming trend afterward. Dry and mostly sunny weather could then persist into next weekend and even the following week, in stark contrast to so far this summer, with it turning hotter and possibly leading to one last heat surge close to the end of August before it inevitably turns cooler in September.
Meteorological discussion:
A slow-moving, almost vertically stacked and cut-off low is located over southern Ontario today. Our region is on system's eastern side, leading to southerly flow of warmth and humidity, though cloud cover will prevent it from getting particularly hot. Low clouds and a slight bit of cool air damming will keep areas east of the Appalachians cooler. Scattered showers are also possible, especially northwest of the St. Lawrence River and along the Green Mountains on east. As the low-pressure moves overhead on Monday, showers and thunderstorms will become more widespread, especially along the southeastwrad-moving cold front and in the afternoon and evening, with most of the rain occurring Monday night in Maine. The cloud cover will likely prevent much daytime instability from building, limiting any severe thunderstorm chances. A second upper-level low with a much cooler air mass will cut off and move in on Tuesday, leading to highs below 70F (21C) for most for the first time since mid-June, and it will also be rather cloudy. As the upper-level low gets cut off, it will stay almost stationary over our region through Wednesday, leading to Wednesday also being chilly and cloudy. This is a big change from previous model runs that had it moving out much more quickly with a warming trend and sunshine returning by Wednesday. This is occurring despite -PNA (slightly), +NAO, and +AO indices, but apparently, the -PNA is centered too far west for upper-level ridging in our region given the high-amplitude nature of the weather pattern. Either way, broad Canadian surface high pressure will eventually move in and clear the skies.
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| Source: TropicalTidbits |
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| Source: TropicalTidbits |
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| Source: TropicalTidbits |
However, summer weather is not over yet. The Canadian surface high pressure will move off the U.S. East Coast, the cool air mass will become stale and weaken in the sunshine, and the northerly flow will change to a westerly or southwesterly flow. A trough moving into the western U.S. (negative PNA) will evoke an upper-level ridging response in the eastern U.S. and Canada. This ridge looks quite persistent starting next weekend and even into the following week, and if the surface high pressure can remain to our south, we will get a persistent westerly to southwesterly flow advecting warmer and warmer (but still relatively dry) air day after day from the Rockies and Plains, while also not getting too much of a southerly flow to advect tropical moisture and clouds that would produce rain and lower temperatures (especially daytime temperatures) again. The dry and mostly sunny weather with low dew points will also promote drying of the ground, in stark contrast to most of this summer, making it easier to get hot, though the lower sun angle of late August will make it difficult for it to get extremely hot. Still, there is a growing signal for one last surge of heat around the end of August before a cold front with a cool Canadian air mass (stronger in September) inevitably comes through. Exactly how hot it gets depends on whether sneaky cooler Canadian high pressure systems can come in from time to time that would turn the flow to northerly and at this late in the summer, really cool things off and disrupt the flow of hot and dry air from the Rockies. There looks to be one likely interruption to the warmup next weekend with a subtle shortwave trough over eastern Canada, but it is uncertain how much that disrupts the longer-term warming trend.
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