Plain-language summary:
Except for southern Vermont to southern Maine still being modestly hot and humid tomorrow, today will be the last hot and humid day in our region for at least a week. For today, clearing skies through early afternoon, then scattered thunderstorms in the afternoon and evening are expected. Some scattered showers and thunderstorms are expected late tonight into tomorrow morning in Quebec and eastern Ontario, pushing farther south into northern New England late morning into the afternoon. Much cooler, drier, and sunnier weather is expected for most of our region from Tuesday into Friday, though a bit of rain is possible in southern areas on Tuesday. Next weekend's weather is highly dependent on the uncertain future location of Tropical Storm Debby, with it most likely going too far south to bring much if any rain to our region except perhaps far southern areas, but with still the outside possibility that it brings a widespread heavy rain south of the U.S./Canada border late Friday into Saturday. Whether Debby impacts our area or not, it will likely clear out by the following Monday, with temperatures expected to return back to near or slightly above average, though no heat waves are expected in the foreseeable future.
Meteorological discussion:
Today will be the last hot and humid day for at least a week in our region, except tomorrow still being modestly hot and humid from southern Vermont to southern Maine right ahead of a potent cold front. A already weak disturbance that brought scattered thunderstorms last night is heading east of our area, with subtle sinking motion leading to clearing skies from west to east through early afternoon. However, the sunshine and still very high humidity will lead to instability, triggering scattered thunderstorms again this afternoon and evening. A robust cold front approaches from the north tonight into tomorrow, bringing showers and a few thunderstorms first to parts of Quebec and eastern Ontario late tonight, then slowly pushing into northern New England tomorrow. The clouds and rain will keep temperatures cooler, except for southern Vermont to southern Maine as mentioned earlier, where there will still be decent sunshine before the storms arrive in the afternoon. The cold front will plow to the south of our entire region tonight, leaving us with much cooler and drier air for the rest of the week as broad high pressure from the upper Midwest U.S. to Quebec keeps northerly flow in our area. This persistently cooler pattern is due to a potent ridge building over northwestern Canada with a positive PNA and negative AO, forcing a trough to bring in sub-arctic or arctic air into eastern Canada and New England.
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| Source: TropicalTidbits |
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| Source: TropicalTidbits |
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| Source: Climate Prediction Center |
A storm passing to our south and quickly moving from west to east will bring a little rain to southern Vermont and southern New Hampshire on Tuesday, but it will be dry elsewhere with some mid to high level clouds in northern New England and very few clouds north of there. As the surface high pressure crests overhead in southern Quebec Wednesday into Thursday, the flow of cool air will stop, and the air mass will modify in the still strong August sun, leading to somewhat warmer highs on Thursday, though the low humidity and mostly clear sky will still lead to cool nighttime lows, below 50F (10C) outside the broad St. Lawrence and Champlain Valleys. The forecast gets a bit complicated afterward depending on Tropical Storm Debby's track and Debby's interaction with a mostly stalled out front to our south. A few model runs showed just enough of an upper-level trough to the west to pull Debby northwestward, interact with both the stalled out front and the trough to the west, to create heavy rain for most of the northeast U.S. on Friday and Saturday. This would create yet another flood risk for our region after the localized torrential rains over the past week, though most of the region has not received particularly much rain. However, most other model runs show there not being enough of a trough to the west to pull Debby that far north, instead trapping Debby in the southeast U.S. in weak steering flow, and with any of Debby's moisture interacting with the stalled out front making it no farther north than southern Vermont and southern New Hampshire, though there could be high clouds extending farther north. With storms in weak steering flow, whose motion could be in opposite directions depending on relatively small differences in position and synoptic-scale configuration, there is tremendous uncertainty in Debby's long range position, with ensemble members being up to 1200 miles (2000 km) apart by next Saturday. In either case, the mid-latitude jet stream will likely whisk Debby or its remnants to the southeast of our region by the following Monday.
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