Summerlike heat with moderate humidity engulfs the region through Wednesday or Thursday, with thunderstorms possible Tuesday and Thursday; brief modest cool down afterward, then likely warm again and possibly stormier; perhaps another modest cool down afterward

Plain-language summary:

After a spring with a remarkable lack of cold or hot weather, it will be quite warm today and turning hot Monday through Wednesday with moderate humidity. Some exceptions to the heat and humidity are immediate coastal areas on all days and northern areas on Tuesday. Some showers and thunderstorms will mark the cold front and end of the hot spell Wednesday night into Thursday, though their coverage and intensity are still uncertain. A modest cool down will follow but will likely be brief before another warm up occurs, though it appears less intense than the one over the next few days, and it is uncertain how much clouds and thunderstorms accompany it. It will likely turn cooler again later in the following week, but any cool down is unlikely to be particularly intense or long-lasting.

 

Meteorological discussion:

After a very temperate spring thus far with an unusual lack of temperature swings between cold and hot weather, now an upper-level trough/ridge pattern is building in the western/eastern U.S. respectively, associated with a negative PNA and weakening of high-latitude blocking. This warmup was not well-predicted by long-range models, with most suggesting a continuation of the cool or temperate pattern for a little longer. It will already be quite warm today except in Maine where low clouds and maritime moderation will prevail. Then, widespread high temperatures in the 80s F (27-31C) are expected at lower elevations from Monday through Wednesday, with Wednesday likely being the hottest in most areas. Coastal areas will be a little cooler due to the still chilly western Atlantic water, though downsloping and offshore wind on Monday and Tuesday could mitigate any maritime moderation. Areas in central Quebec to northern Maine will likely be cooler on Tuesday as a weak disturbance and surface low pressure system moves into that area or just to the north, leading to more clouds and rain. There will also likely be thunderstorms given the heat and humidity feeding into the front, especially just to the south over eastern Ontario, southern Quebec, and far northern New England, where some of the storms could turn severe for the first time this year with instability and low-level wind shear. It will also be more humid than typical for May, with dew points in the low to mid 60s (16-19C) for most.

 

Source: TropicalTidbits

 

Source: TropicalTidbits

Source: TropicalTidbits

 

A rather strong storm will move into northern Ontario and then central Quebec later Wednesday into Thursday, bringing a cold front into our region. If the front is a bit slower, it could still be quite hot and humid on Thursday from Vermont on south and east. The cold front will have weakened then, and the flow will be more westerly or even west-southwesterly rather than northerly behind it, leading to only a gradual cool down to near or slightly below average temperatures. The front will likely be accompanied by some showers and thunderstorms, given the heat and humidity ahead of it, but their intensity and coverage is uncertain, depending on mesoscale details as well as timing relative to the diurnal cycle, which we won't know well for another couple of days. Since there will still be a general upper-level trough in the western U.S., the upper-level trough and cool down moving into our region will likely be short-lived and be replaced by another warmup next weekend or early the following week, though exactly how hot it gets and how much clouds and thunderstorm activity will be accompanied by the warmup remains to be seen. For now, it appears unlikely to get as hot as the next few days.

Source: TropicalTidbits

Later the following week, the upper-level trough in the western U.S. will likely be replaced by an upper-level ridge, although that may be the strongest over western Canada and may not be very strong. This will likely force a cool down over the eastern U.S. and Canada, including our region. However, just like during the past cold season, the lack of true cold air will limit how chilly it gets, and temperatures may only drop to near climatological averages except for an odd day that is very cloudy and rainy. With the PNA not expected to turn truly positive, I doubt the cool down will last very long.

 

Source: TropicalTidbits

Source: Climate Prediction Center