Hot and mostly dry through Wednesday; stormy Thursday, then turns cooler for a while but coolest air likely staying to the west, then likely warming back up for mid-June

Plain-language summary:

It will be mostly sunny and increasingly hot with relatively low humidity, light winds, and relatively cool nights through Wednesday before higher humidity, showers and thunderstorms, and stronger winds arrive on Thursday. It turns much cooler for at least several days afterward with scattered showers, especially in the afternoon, though the coolest air will likely stay to our west. It will likely warm back up by mid-June.

Meteorological discussion:

Upper-level ridging is building into eastern Canada behind an almost stationary cut-off upper-level low just south of Nova Scotia. The air mass is dry, given the lack of moisture return after the cool, dry Canadian high pressure moved over the region earlier in the week. This and the clear skies are leading to a large diurnal temperature range, with below average nighttime lows but near to above average daytime highs recently. With the strong June sun and weak temperature advections, it will continue to warm up through Wednesday, with a few spots in the Champlain and St. Lawrence valleys possibly reaching 86F (30C) on Wednesday. Eastern New England will be cooler due to southeasterly flow off the chilly western Atlantic water. Slightly higher humidity could lead to some instability and scattered thunderstorms over the higher terrain on Wednesday, but the lack of synoptic-scale forcing will preclude thunderstorm activity over the lower elevations.

Source: TropicalTidbits

A broad low-pressure system will slowly move eastward through northern Ontario Wednesday into Thursday, bringing an eastward-moving cold front draping south into the mid-Atlantic U.S. Increasing clouds, southerly winds, and moisture ahead of the front will cause Wednesday night to be warmer and Thursday to be cooler than the previous day. The cold front will be associated with showers and thunderstorms. It is too early to say exactly when the front and thunderstorms will arrive and if there will be severe thunderstorms, though the lack of truly high humidity and therefore strong instability suggests that severe thunderstorms are rather unlikely. 

 

Source: TropicalTidbits

A ridge developing in western North America associated with the PNA turning positive will force a broad upper-level trough to move into the Midwest U.S. starting on Thursday. The trough will eventually cut-off and move rather slowly and slowly weaken, as a high-latitude blocking pattern marked by a negative NAO takes hold at the same time. As such, an area of unseasonably cool air will move into the Midwest U.S. from central Canada and then move eastward into our region behind the cold front on Thursday. However, since the cooler air is not coming directly from the north, we will not see the full magnitude of the cooler air mass, which will stay to the west. With rather low upper-level heights but still relatively warm low-levels, daytime instability could lead to scattered showers and weak thunderstorms each day through next weekend. Given how slowly-moving the trough will be, it is very uncertain exactly when it will finally east or just decay. However, most indications are that as the initial push of cool air mostly depletes the cool air mass source over central Canada, the cool air mass will get cut off and gradually modify with the strong June sun, though there may be a secondary reinforcement a few days later when the trough or cut-off low is more directly over our region. Also, the West ridge/East trough pattern will likely break down somewhat by mid-June. Both of these factors favor a warmup back to near or above average temperatures then.

 

Source: Climate Prediction Center

 

 

Source: TropicalTidbits