Northern New England & Southeastern Canada Weather Blog: Cooler and clearing out today; somewhat hotter tomorrow, then more humid with showers and thunderstorms Tuesday through Thursday; drying out Friday, then heat wave likely starting next Sunday perhaps through early August

Plain-language summary:

Some clouds and a few showers are expected this morning in northern New England before clearing out by afternoon with cooler temperatures and lower humidity than most recent days. It will turn somewhat hotter tomorrow, and then cloudier, more humid, and rainier from Tuesday and Thursday, with seasonably warm high temperatures and much warmer nights. After temporarily cooler and less humid Friday, a heat wave is likely starting next Sunday, with the remainder of the month likely being quite hot.

Meteorological discussion:

A weak reinforcing cold front is passing through our region this morning, bringing a band of clouds and scattered showers in Northern New England, before it will wash out completely in the mid-Atlantic U.S. by tomorrow. Behind it, it is markedly cooler with low humidity (contrary to most of this summer!) due to the low-level flow being directly from the north, failing to get above 77F (25C) in most places even at lower elevations and where the sun returns this afternoon. However, the northerly flow will be weak and disappear by tomorrow, allowing the July sun to modify the cooler air mass, and it will be somewhat hotter again tomorrow but still dry and mostly sunny.

Source: TropicalTidbits

A subtle slow-moving weak cut off low in the Midwest U.S. will back the low to mid-level flow to southwesterly, advecting subtropical moisture into our region on Tuesday, along with some upper-level vorticity that will generate clouds and rain. This will lower daytime temperatures, especially in eastern New England where the rain will be the most persistent and there could be a very slight cold air damming effect east of the Appalachians, but the increased clouds and humidity will raise nighttime temperatures. The upper-level ridge with light winds there preventing much movement of the smoke and the southwesterly flow in our region will also prevent most of the expansive area of smoke in central Canada from reaching our region this week. More showers and thunderstorms are likely Wednesday and Thursday as the cut-off low slowly moves into our area and slowly weakens. With relatively weak synoptic forcing, severe thunderstorms are expected to be isolated at most. However, a slightly stronger trough diving out of northern Ontario will provide a cold front that will push through the region on Thursday, enhancing showers and thunderstorms. It will briefly turn cooler and less humid with southwesterly flow switching to northerly to northwesterly flow for Friday and Saturday.

Source: TropicalTidbits

Source: NOAA

Upper-level ridging, essentially the central Canadian ridge having pushed eastward, and westerly flow then quickly builds in for next Sunday, leading to the start of a likely heat wave with clearer skies and much hotter temperatures, perhaps widespread 90F (32C) heat in lower elevations early the following week. The central Canadian ridge will get replaced by a trough with much stronger westerly winds aloft next weekend, so maybe that will really push the smoke into our region, if it doesn't all settle out by then. The smoke could lower high temperatures if it is thick enough, but we just won't know how thick it will be for at least another several days, especially since it is not possible to know how many more fires will ignite in western North America until then. Though a cold front or two is possible near the very end of the month, widespread ridging in eastern Canada promoted by the PNA turning slightly negative with western North American troughing for a time, combined with a positive NAO preventing troughs from diving very far south, will likely keep our area rather hot. However, record heat appears unlikely due to the initial injection of cool air on Friday, as well as the rains this upcoming week and earlier in the summer keeping the ground a bit moist. However, aside from Tuesday through Thursday, the weather pattern actually looks drier than usual through the end of the month, with generally hot weather likely to continue into early August.