Warm, muggy, and stormy through mid-week, then perhaps slightly drier late week into weekend; warmth, humidity, and thunderstorms likely to continue beyond but perhaps no real heat waves yet

Plain-language summary:

After a cloudy and rainy day for many yesterday, it will be somewhat drier and hotter today but still with isolated thunderstorms, with hazy skies due to wildfire smoke near and north of the U.S./Canada border. More widespread showers and thunderstorms with continued high humidity will dominate Monday to Wednesday, with wildfire smoke pushing off to our north. Severe thunderstorms are unlikely, but heavy rain with isolated flash flooding is likely, especially in southeastern facing slopes of the Green, White, and Laurentian Mountains. It will slowly dry out later in the week, possibly with a day or two of slightly lower temperatures and humidity, before warming back up. Warmth and humidity will likely dominate afterward, but continued frequent chances of showers and thunderstorms along with the wet ground may prevent any true heat waves from occurring.

 

Meteorological discussion:

 

In stark contrast to earlier in the summer, deep southerly flow is transporting subtropical humidity into our region, with widespread dew points of  >65F (18C), and even >70F (21C) in areas with very recent rainfall. After a storm brought widespread clouds and rain yesterday, there will be a break for most today ahead of another slowly-moving and almost vertically stacked low pressure system approaching from the Great Lakes. With a very subtle cold front bringing in slightly drier air, especially north of the U.S./Canada border, promoting fewer clouds and more sunshine, it will be hotter than yesterday for most, with a few low-elevation locations possibly reaching 86F (30C). However, scattered thunderstorms could still form given the heat and some lingering humidity, especially over the higher terrain. Given the lack of synoptic-scale forcing for storms, the storms are unlikely to be widespread. Also, thick smoke near and north of the U.S./Canada border will keep temperatures cooler than modeled, given that the models do not take the smoke into account in its temperature forecasts, which combined with the drier air, will mostly prevent convection. 

 

Source: TropicalTidbits

Source: Penn State Meteorology

 

 

Synoptic-scale forcing for storms will be higher from Monday to Wednesday as the low pressure system over the Great Lakes slowly moves into our area. Southerly flow will increase ahead of the storm and push the wildfire smoke north of our region. Also, it is not just humid near the surface, but throughout almost the whole atmospheric column. The atmospheric profile is generally nearly moist adiabatic and with high relative humidity (saturated at times), with weak instability during daytime due to modest daytime heating. With mostly cloudy skies and wet ground, daytime heating will be limited but not completely eliminated, and will be just enough to spark rather widespread afternoon showers and thunderstorms. The rather weak instability will limit any chances for severe thunderstorms, with limited updrafts for hail generation, deep warm layer for any hailstones to melt in before reaching the ground, and lack of dry air aloft to promote strong cold pools that would produce strong gust fronts. However, the deep warm, moist layer will lead to a deep layer of efficient warm rain generation and limited evaporation of rain before reaching the ground. Therefore, any storm will be able to easily produce very heavy rainfall rates, heavier than radar reflectivity might suggest given the small raindrops, and isolated flash flooding could occur, especially since these storms are going to occur for 3 days in a row and make the ground increasingly wet. However, given the hit-and-miss nature of the storms, there will be substantial spatial variation in rainfall amounts. There will be also terrain modulation in rain amounts given the southerly flow, with the southern or southeastern slopes of the Laurentians, Green Mountains, and White Mountains likely to receive especially heavy rain (locally up to 6" or 150 mm total), while downsloped areas like the Champlain Valley and the eastern St. Lawrence River valley will receive less (perhaps as little as 1" or 25 mm total) and will also have fewer clouds and be hotter.

 

Source: TropicalTidbits

Source: PivotalWeather

As the low-pressure system weakens and moves off to the east, it will mostly dry out Friday and into the weekend, though scattered terrain-induced thunderstorms could still occur given the lack of a strong Canadian cold front to transport drier air. It will likely turn subtly cooler and drier for a day or two with brief northerly flow behind the storm before turning warmer. Though there is obviously uncertainty in the weather pattern evolution afterward, ensembles agree on a general ridge forming near the North American West Coast and another one in Quebec or Atlantic Canada. This will lead to a continuation of the incredibly hot, fiery, and smoky summer over Quebec, but the deep southwesterly flow will lead to more warmth and humidity dominating in our area with the wildfire smoke being blown to the northeast mostly away from our area. This could also be a rather stormy pattern with disturbances from the southwest running into the warmth and humidity in our area, though there will likely also be days of ridging with quieter weather. The frequency of disturbances and wet ground could prevent true heat waves from occurring despite the warmth and humidity.

 

Source: TropicalTidbits