Plain-language summary:
Today will be the last summerlike day for a while or perhaps until next summer southeast of the St. Lawrence Valley, with it already being cooler to the northwest. The coolest air mass of the season will arrive tonight, with Monday night being the coolest night of the season thus far. It will stay dry and gradually warm up throughout the week, but never becoming hot and humid again. It could turn rainy next weekend, though the timing is uncertain due to the slow-moving nature of the storm, and the amount is also highly uncertain depending on the interaction between different low-pressure systems. It likely dries out again afterward with it initially cooling down, then potentially being generally near to modestly warmer than average toward mid-September, though clear skies will promote cool nights and cool shots can still sneak in from the north depending on the exact configuration of weather systems. Also, a potential tropical cyclone could approach the Gulf Coast around 10 days from now, complicating the weather pattern in ways not foreseen by models right now.
Meteorological discussion:
Today will be warm to hot in most of our region with a rather warm air mass right ahead of a strong cold front moving northwest to southeast, warmest from Vermont eastward where it will be rather sunny with downsloping southwesterly flow, and the cold front will not arrive until evening. Northwest of the St. Lawrence Valley, it will be markedly cooler with the front arriving by noon. The strong cold front will be accompanied by a line of showers and possibly thunderstorms which will produce gusty winds, but it is unlikely to be hot or humid enough for truly severe thunderstorms. Right on cue for the start of meteorological fall, the coolest air mass of the season will descend into our region straight from the northwest tonight into tomorrow with a broad, cool Canadian high pressure moving into the Great Lakes. This is associated with a classic +PNA pattern (upper-level ridge in western North America, upper-level trough in eastern North America.) The cool air mass will be rather dry, so will be accompanied by mostly clear skies and the coolest nighttime lows of the season thus far (40s F or 5-9C for most on Monday night).
The high pressure will gradually move east through the week, leading to the flow becoming weak and then out of the south, causing the cool air mass to become stale and modify. Broad surface high pressure and upper-level ridging building in will also limit any cloudiness and promote a rather sunny week, in contrast to most of this summer. However, the surface high pressure never gets far enough south to promote a strong westerly flow from the still hot Rockies region, and combined with the lower sun angle, will limit how much it warms up. The dry air mass will also take time to moisten, and the lower surface pressure and storminess along the Gulf and Southeast U.S. coasts will block the moist tropical air mass from reaching our region, so it will not be humid either. A return to true summertime heat and humidity is therefore not expected.
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| Source: TropicalTidbits |
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| Source: Climate Prediction Center |
By Friday, a trough will dig and move into the Midwest U.S., but as often happened this summer, it appears that upper-level ridging quickly building over top the trough will cause the trough to become a cut-off upper-level low that moves very slowly eastward across the Great Lakes and perhaps eventually into our region. As is always the case with cut-off lows, the exact timing is highly uncertain due to the slow speed. While the cut-off low is likely west of our region on Friday, it will likely still be moderately warm and dry in our region, with cooler, cloudier, and rainier weather arriving when the cut-off low moves over our region perhaps next weekend. The cut-off low could interact with and phase with a storm spawned by the subtropical jet stream along the Gulf Coast, in which our region would get a widespread soaking rain. However, if the two features remain somewhat separate, only showers and light rain would result in our region.
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| Source: TropicalTidbits |
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| Source: TropicalTidbits |
Model ensembles indicate that it dries out early the following week with an initially cooler air mass. How quickly it dries out depends on how slow the cut-off low is in moving out of our region. Then, upper-level ridging likely gradually builds in through mid-September, though like was often the case in August, strongest over the Great Lakes and northern Ontario associated with surface high pressure over our region rather than in the southern U.S. as is more usually the case, which will lead to light winds, clear skies, and cool nights and a lack of strong westerly wind to bring in hotter air. The ridging could deflect any cold air masses to the northeast and promote generally modestly above average temperatures, especially during the daytime. The warm days and cool nights would be the opposite of most of this summer. However, if the +PNA becomes too strong and the ridge retrogrades to the west, upper-level troughs and colder air masses will easily be able to sneak into New England straight from the north, leading to cooler and showery weather, even as areas to the west stay warm and dry. The near neutral NAO and AO indices don't offer any strong signals. Also, a currently rather dry tropical wave in the central Atlantic will move westward into the Caribbean, possibly develop into the tropical cyclone, and then move into the Gulf of Mexico and make landfall along the U.S. Gulf Coast around 10 days from now. Of course, it is way too early to have confidence in this solution, but it is a rather plausible one, in which case the tropical cyclone would affect the weather pattern in ways that models do not show now.
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