Lower humidity tomorrow, then heat and humidity quickly returns early next week before strong cold front brings storms and first real taste of fall. Perhaps turns hot again later?

Plain-language summary:

After some showers and thunderstorms today, it will clear out with lower humidity tomorrow. However, heat and humidity will quickly surge back in for Monday and Tuesday. A strong cold front will likely bring strong to severe thunderstorms on Tuesday, followed by the first true taste of fall, with the chill being the most pronounced farther northwest with the core of it in northern Ontario. A warm up likely follows for a few days afterwards, with possibly hot weather returning by September standards.

Meteorological discussion:

A weak low pressure in southern Quebec that is bringing showers and thunderstorms to northern New England today will drag a weak cold front southward this evening. The front will stall in southern New England, but there will be clearing and a noticeable drop in humidity to the north. A month ago, a weakening front like this probably wouldn’t do much, but now cold air masses are growing stronger with the lower sun angle. Many areas outside urban areas will approach 50F (10C) in the broader valleys and below that in the mountainous cold hollows Saturday night.

 

Source: TropicalTidbits and PivotalWeather

However, ridging will quickly build across eastern Canada, the flow will turn to the southwest, and heat and humidity will rebuild. A couple of very weak disturbances aloft could throw some clouds and complicate temperature forecasts, but most low-elevation areas will reach 86F (30C) or higher on Monday, and from southern Quebec eastward, on Tuesday as well. A low pressure will strengthen and move northeastward into Hudson Bay and then northern Quebec on Tuesday, leading to breezy southerly or southwesterly winds, especially where wind can channel, such as the St. Lawrence and Champlain Valleys. The low pressure will also push a strong cold front eastward, and depending on the orientation and timing of the front, could lead to strong to severe thunderstorms given the heat and humidity.

 

Source: TropicalTidbits

The much cooler air mass will only slowly move in, facing resistance from the East Coast ridge and the incredibly warm ocean temperatures off the East Coast. Combined with the upper-level trough positioned to the west and more westerly instead of northerly flow, the cool air mass will modify as it moves eastward, and northern/western Ontario will likely get the core of the cool air. Still, it will be the beginning of September then, and it will provide the first true taste of fall, with the most pronounced cooling over Ontario. 

 

Source: TropicalTidbits

Source: NOAA

 

However, there are signs that there will be another warm up to possibly hot temperatures a few days afterward, but by then, it becomes increasingly difficult to sustain hot weather. Also, the mean ridge could be over central North America, which tends to lead to “sneaky” cold fronts bringing cooler air from the north especially over eastern New England, though this could be counteracted by potential hurricane in the southwestern Atlantic pumping heat northward. However, this is still quite far out.

 

Source: TropicalTidbits