Plain-language summary:
Showers and possibly scattered thunderstorms are likely today into Monday, though like it always has been for the past few weeks, rainfall amounts for most will be rather unimpressive. It then turns cool and dry Tuesday through early Thursday, then cool and showery late Thursday through possibly Saturday. Afterward, it gradually warms up but stays dry, worsening the drought.
Meteorological discussion:
Scattered showers and thunderstorms today into Monday with unimpressive rain, then cool and dry Tuesday and Wednesday
A broad upper-level low is pushing east across James Bay today, with disturbances rotating around it. The first disturbance is currently over the region, generating clouds and showers, a rarity recently. A second disturbance associated with the main cold front will trigger more showers and possibly thunderstorms Monday mainly from Vermont eastward. Both today and Monday, the majority, but not all, of showers and thunderstorms will occur in the afternoon and evening when daytime heating and instability is maximized. Even then, as is typical for the past few weeks, there isn't a good surge of tropical moisture, and the air aloft is still relatively dry, limiting the intensity and coverage of showers. As such, rainfall will be unimpressive for most, with only isolated locations receiving >1" (25 mm). Behind the cold front, it will be cool and dry on Tuesday and Wednesday as another broad surface high-pressure system moves into eastern North America.
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| Source: Aviation Weather Center and TropicalTidbits |
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| Source: TropicalTidbits |
Cool and unsettled with slow-moving disturbance late Thursday through Saturday
Another upper-level disturbance/cold front will arrive Thursday into Friday, with a brief slight warmup ahead of it. Unlike previous model runs, most of today's models, except the GFS, show upper-level ridging building overtop the disturbance quickly enough to make the disturbance almost cut off over our region, making it move much slower than otherwise. In this scenario, cool, cloudy, and showery weather would extend from Thursday afternoon all the way through Saturday before finally moving off to the east by Sunday as a broad, cool, Canadian surface high pressure moves in. There is always considerable uncertainty in timing with these cut-off systems though. Despite the clouds and showers, with no deep, rich moisture return ahead of the system once again, rainfall will not be impressive. This stretch of cool weather is powered by a classic +PNA pattern with upper-level ridging and hot weather near the West Coast of North America and upper-level troughing and cool air in eastern North America. The slightly negative -NAO and associated high-latitude blocking is also helping to push the mid-latitude jet stream southward, allowing the cool Canadian air to push southward deep into the U.S., though the slightly positive +AO might be preventing the coldest Arctic air from reaching our region.
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| Source: TropicalTidbits |
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| Source: Climate Prediction Center |
Dry weather returns Sunday and beyond with gradually warming temperatures and worsening drought
After the cool, unsettled weather later next week, models generally agree that the upper-level ridge in western North America will weaken as a strong, possibly cut-off upper-level low approaches the Northwest U.S. coast. This will promote a downstream upper-level ridge to build in central and likely also eastern North America, especially over Canada, and the amplified or even cut-off nature of the pattern means that the continental-scale pattern will likely move quite slowly. The pattern appears to be configured to bring yet more big and dry surface high-pressure systems across eastern North America, with no storms or any flow of moisture from the western Atlantic or Gulf of Mexico. It will also likely turn gradually warmer as we lose the shots of cold air and it stays rather sunny, though not as hot as earlier in August given the lower sun angle in September, and nights will likely remain cool with longer nights in September and relatively dry air. There is also no strong wind to advect hot air either, and it is possible that most of the warm weather stays to the west while backdoor cold fronts straight from the north or even northeast keep our region cooler. Actually, the upper-level low approaching the northwest U.S. coast fizzles, and we are just left with a broad upper-level ridge over most of Canada with a near-neutral PNA and a neutral NAO (losing the Greenland blocking leading to the amplified troughs in the near-future). This is not a favorable configuration for big heat and humidity, but favors well below average rainfall in our region for a while still, making the drought just worsen after what is among the driest Augusts on record for some. Of course, long-range precipitation forecasts are always iffy, and models are not unanimous on an extremely dry forecast for now at least.
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| Source: TropicalTidbits |
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