Plain-language summary:
A long-duration warm and dry spell will be ending by Wednesday as rain returns for western areas on Monday and the rest of the region for Wednesday and Thursday, though the rain does not appear to be particularly heavy. It will cool down and dry out afterward for next weekend. Another warm and relatively dry period is likely afterward, with no real cold air in the foreseeable future, though this period may not be as long-lasting as the one we just had.
Meteorological discussion:
A remarkably long-duration warm and dry spell (even if there was no record heat) is ending as a surface high pressure over Labrador is causing easterly to northeasterly flow of somewhat cooler air into our region, especially in the low-levels. Due to a very strong upper-level high pressure over eastern Nunavut and Greenland associated with a very negative NAO (having shifted from being stationary over southeastern Canada, causing the prolonged warm and dry spell), systems are still moving more slowly than usual, but the shift of the blocking ridge location means that the relatively warm, sunny weather won't be around for much longer. A storm currently over the Great Lakes is slowly moving eastward and will first bring rain to eastern Ontario and northwestern New York on Monday before dissipating as it heads eastward, with increased clouds for the rest of our region. The storm and clouds will have mostly dissipated by early Tuesday before another storm brings increased clouds later Tuesday and then eventually rain on Wednesday and Thursday for the entire region, the first widespread rain in over two weeks! It will become breezier ahead of the storm with increasing southerly winds, though given the lack of temperature gradient, it will just become a little more humid with Atlantic and Gulf of Mexico moisture finally entering the region in some form. Still, it doesn't appear to be a particularly strong storm with a particularly moisture-rich air mass, precluding heavy rainfall in most locations.
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| Source: TropicalTidbits |
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| Source: Climate Prediction Center |
With the slowly-moving weather pattern, the storm and associated rain will also move through the region rather slowly, though it should be off to the east by Friday, with cooler air behind it. However, the evolution and timing of features around this time becomes quite uncertain. Part of the storm's associated upper-level trough will cut off in the U.S. Plains, a symptom of the blocking pattern, by the middle of the week and will steer future Tropical Storm or Hurricane Helene northward into the southeastern U.S. likely by Friday. How the trough cuts off, and how deep the trough is (both of which are uncertain due to the difficulty of accurately simulating such trough splits) will determine if how far northward it can pull Helene. The vast majority of model guidance suggests that Helene and the cut-off low will combine and just stall and weaken in the southern or mid-Atlantic U.S., leaving our region relatively dry and gradually turning warmer again, being blocked by another developing blocking ridge over the northern U.S. Plains, not dissimilar to the blockage of Francine's movement northeastward less than 2 weeks ago.
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| Source: TropicalTidbits |
After Helene moves inland, it will quickly weaken. Another trough entering the western U.S., combined with the blocking ridge still over northeastern Canada, will lead to another warmup with possibly another rather long period of dry weather, perhaps not as long as the one we just went through. The blocking ridge location to our north is not ideal for record warmth in our region (which would require a strong deep-layer westerly flow), but it will still be warmer than average most of the time, at times even much warmer than average, with the broad blocking ridge cut off cold Arctic air from our region for the foreseeable future, even with the ridge extending westward to western North America with a +PNA, which often promotes cold spells in our region.
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| Source: TropicalTidbits |
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