Narrow axis of heavy, flooding rain tonight through Tuesday; moderately hot and very humid with frequent chances of thunderstorms for the rest of the week and perhaps beyond, with hottest temperatures east of the Appalachians

Plain-language summary:

The heat and especially humidity that has dominated for the past several days will continue today, though it will be slightly cooler with increasing clouds and thunderstorms, especially south of the U.S./Canada border. A narrow axis of heavy, flooding rain is expected between the Adirondacks or Green Mountains to central Quebec tonight through Tuesday, with it being drier and hotter to the west. A stagnant large-scale upper-level pattern with broad southwest flow will maintain the hot and humid pattern for the rest of the week southeast of the Appalachians, while it will be stormier, cloudier, and cooler to the northwest. Though the pattern will likely shift somewhat and perhaps become less stagnant, it will likely remain hot, humid, and somewhat stormy next weekend and beyond.

 

Meteorological discussion:

 

It has been quite hot and especially humid for the past several days, and that will continue to a slightly lesser extent today, especially southeast of the St. Lawrence Valley where the drier air from Friday’s cold front never really arrived. Increasing clouds ahead of a storm developing along the U.S. East Coast will lead to slightly cooler temperatures south of the U.S./Canada border. Scattered thunderstorms are also likely south of the U.S./Canada border, especially over the favored higher terrain areas. A subtle trough and low-level convergence zone extending north from the storm to central Quebec will trigger scattered heavy showers and thunderstorms through tomorrow as it feeds into the humid and relatively unstable air. As the storm itself then strengthens and passes over eastern New England tomorrow night into Tuesday morning, a band of heavy, possibly torrential, rain with embedded thunderstorms will occur just to the west. A zone of 3-5” (75-125 mm) with locally 6-8” (150-200 mm) is likely to occur, with the zone over the Adirondacks or Green Mountains northward to central Quebec. Given the repeated rounds of thunderstorms and heavy rains over many areas over the past week, flooding is a big threat, though the risk is lower for areas that have missed the bulk of it, like the northern Champlain Valley. High temperatures will be below average tomorrow (70s F or 21-25C) for most of northern New England and southern and eastern Quebec due to the rain, but humidity and nighttime temperatures will remain very high. To the west, especially over central Ontario, it will be drier and hotter due to more sunshine. The evolution of this storm has been quite uncertain until now, when there is finally more agreement among models for a slower solution, with the storm not leaving until later Tuesday.

 

Source: PivotalWeather

Source: TropicalTidbits

The elongated storm will slowly move east to Maine and weaken by Tuesday evening, with drying and warming to the west with increasing sunshine. However, as a strong upper-level low sets up and remains stagnant over Hudson Bay with persistent upper-level ridges over the West Coast and Atlantic Canada in response to the somewhat amplified and blocked high-latitude pattern with the -NAO, disturbances will rotate around the upper-level low and move northeastward from the Midwest U.S. into Ontario and Quebec. These disturbances will lead to repeated rounds of showers and thunderstorms, perhaps more notable along and northwest of the Appalachians, with more clouds and cooler temperatures there. Southeast of there, it will continue to be hot and quite humid with more sunshine but also more instability and still potential scattered afternoon and evening thunderstorms. The disturbances may bring weak cold fronts, but the upper-level low will remain almost stationary and allow southwesterly flow to maintain the flow of hot and humid air southeast of the Appalachians, with somewhat cooler and at times vaguely drier air to the northwest. The upper-level ridge over the West Coast is too far west to reflect as a +PNA and push cool air into our region.

 

Source: TropicalTidbits

Source: Climate Prediction Center

This hot, humid, and stormy pattern may shift slightly next weekend and beyond as the upper-level low retreats to the northeast and the upper-level pattern starts moving faster with the -NAO blocking pattern weakening somewhat, but the southwesterly flow of hot, humid air could continue to allow for frequent chances of thunderstorms, with no real ridging expected to suppress them. With the high humidity and storminess, nighttime lows are expected to continue to be more above average than the daytime highs, with no extreme daytime heat likely despite frequent moderately hot daytime temperatures.

 

Source: TropicalTidbits