Late-season heat wave through Thursday, peaking on Wednesday for most; sunny through Wednesday, then scattered thunderstorms on Thursday and Friday and gradually turning cooler, with the most cooling to the north; temperatures likely to return closer to average for the following week

Plain-language summary:

 

After a cool, cloudy, and rainy August with no real heat, a heat wave will dominate from Monday to Thursday, with Wednesday being the hottest for most. Winds will be rather light and humidity will be high during the heat wave. It gradually cools down starting Thursday with increasing showers and thunderstorms, though it will likely still be quite humid through at least Saturday as a cold front possibly stalls over our region. It will likely turn cooler and drier by early the following week, with sustained heat unlikely to return for a while afterwards.

 

Meteorological discussion:

 

After having no heat or upper-level ridging whatsoever in August, a broad upper-level ridge has overtaken the eastern U.S. and is nosing into Ontario and Quebec. This is associated with a pattern change in which there is a trough in western North America (-PNA), promoting a ridge in eastern North America, and a lack of high-latitude blocking (+NAO), preventing any slow-moving cut-off lows in the eastern U.S. Underneath the ridge, it is mostly sunny, hot, and humid. Warm air aloft is moving in from the west, and combined with the albeit weaker September sunshine, will lead to increasing temperatures through Wednesday. Already today, a few spots will approach 86F (30C) in lower elevations, but they could rise to 93F (34C) by Wednesday. Light westerly winds today and tomorrow will weaken to almost calm by Tuesday and Wednesday for many areas. This is due to the ridge nosing so far north, pushing the low-pressure track way up in northern Quebec, and trapping a weak upper-level low, the extratropical remnants of Idalia, to the southeast. This will actually prevent the hottest air from continuing to pump in from the west, especially from Vermont southeastward, and likely will prevent record highs from being broken in most places. It will also lead to cooler temperatures in Maine and eastern Quebec, which will never see the warmest air from the west, and the afternoon sea breeze will also keep it cooler close to the coast. Still, 850-hPa temperatures of 20-22C in western areas is >99^th percentile for this time of year, and it will be the hottest since early July or even before that for most, especially from Vermont westward. Western areas, like eastern Ontario and neighboring areas of western Quebec, will be the hottest, since they are farthest away from the upper-level low in the northwestern Atlantic, and could see subtle downsloping with a light south to southwesterly wind still prevailing there. 

 

Source: TropicalTidbits

 

Source: Tomer Burg

 

Given the moist ground due to the excessive rainfall earlier in the summer, and transpiration from trees, it will be very humid. The warm air aloft and weaker September sun means that the boundary layer will not mix out as high as it usually does on sunny days, which makes the high temperature forecast tricky. Sometimes, on days with shallow boundary layers, it mixes out a bit more than models show, leading to hotter temperatures. This is a possibility given that the ground will have finally dried out some, with no rain since last Wednesday. However, even with the heat and humidity, the upper-level ridge will be so strong and the air aloft will be dry enough that there will be rather few clouds, except some afternoon cumulus clouds, with synoptic-scale descent and warm air aloft preventing any towering thunderstorms. The clear skies and calm winds will also lead to good radiational cooling at night through Tuesday night, leading to not as warm nights and patchy valley fog away from urban areas given the moist air mass. Nights could be warmer Wednesday and Thursday nights due to more clouds.

 

Source: PivotalWeather

 

The upper-level ridge slowly breaks down by Thursday, with the low-pressure track sinking south into the Laurentians northwest of the St. Lawrence River. The heat and humidity will still be present, but with the ridge weakening, there will be less convective inhibition, and scattered showers and thunderstorms will likely develop, with perhaps more widespread storms along the cold front that will slowly sag southward, though there is model disagreement on how well-defined the front will be. The front could also stall in our region and become more diffuse, leading to a continuation of high humidity and a more prolonged period of showers and thunderstorms that extends into the weekend, though the clouds and rain will keep temperatures lower. Areas north of the U.S./Canada border are more likely to experience a substantial cool down. Eventually, the front will pass through the whole area, perhaps by the following Monday, and it will turn cooler and drier behind it. However, the air mass behind it does not appear to be particularly cool for September. 

 

Source: TropicalTidbits

 

Beyond this weekend, the heat gradually retreats back to Texas and the southwestern U.S., where it has usually been since late June. As this happens, the PNA and NAO are both expected to turn to neutral. This allows more troughs and cooler air masses over our area, though it is unlikely to be particularly cool overall for September, since the neutral PNA will likely allow for some warm surges into our area. However, given the rapidly falling average temperatures, we should see the coolest air mass since early June or even May by then, and we will see tastes of fall. It does not appear that September will be a truly summer month, at least after Friday or next weekend.

 

Source: TropicalTidbits

 

Source: TropicalTidbits

 

Source: Climate Prediction Center