Plain-language summary:
A sharp cold front is bringing some showers and much cooler air into the region today, marking the end of summer weather for the region for the most part. It will be rather cool Monday through Thursday, with tonight being especially cool, and it being dry except some showers with unseasonably cool daytime temperatures on Wednesday. It briefly turns warm to hot Friday into Saturday before a cold front with showers and possibly thunderstorms Saturday night into Sunday brings in another unseasonably cool air mass for several days, with more dry weather.
Meteorological discussion:
An unusually strong and sharp cold front by summer standards is plowing southward through our region today, with quickly falling temperatures even in the middle of the day. While southern Vermont to southern Maine is seeing temperatures approaching 90F (32C) with mostly sunny skies, behind the cold front, it is only around 60F (16C) in southern Quebec and northern Vermont! Only 90 miles (145 km) separates 89F (32C) from 57F (14C) in Vermont this afternoon. A line of showers marks the front, but the low-level cold air will come in so quickly that it will generally not allow for particularly strong thunderstorms, especially with the front arriving in the morning in most spots.
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| Source: Aviation Weather Center |
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| Source: TropicalTidbits |
The -NAO associated with the strong Greenland ridge, along with a strengthening broad upper-level ridge in the western and central U.S., is promoting rather pronounced upper-level troughing in Quebec and Atlantic Canada early this week, leading to a surge of cool and dry air directly from the north into our region. Some places could approach 41F (5C) tonight, while almost everyone along and northwest of the Appalachians will struggle to get past 70F (21C) on Monday, even with almost full sunshine. Given the Greenland blocking, the continental-scale weather pattern won't shift a whole lot for several days, as the boundary between hot and cool gets stuck just to our south. It will be somewhat warmer on Tuesday with the cool air mass modifying, but then a weak disturbance will move east-southeastward along the boundary, reaching our region Wednesday with widespread cloudiness and light rain. Though the weak dynamics will mean relatively little rain for most places, the widespread thick clouds will mean especially cool daytime temperatures, below 68F (20C) for most. Sunshine and dry and somewhat warmer weather will return on Thursday.
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| Source: TropicalTidbits |
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| Source: Climate Prediction Center |
A piece of the central U.S. ridge will push east into our region Friday and Saturday, leading to briefly hotter temperatures, but not nearly as hot as earlier in August. By Saturday night or Sunday, another reinforcing trough and unseasonably cool air mass will move into our region, with some showers and possibly thunderstorms along the preceding cold front. All this troughing will cause Erin to make a sharp right turn just off the North Carolina Outer Banks mid-week, with the trough not digging far enough south and/or amplified enough to promote southerlies to pull Erin northward into our region; instead, the westerlies on the southern edge of the trough will simply steer Erin to the east, well south of our region.
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| Source: TropicalTidbits |
By the following week, a big cool surface high pressure will have set in from the northern U.S. Plains to eastern Canada, similar to at the end of July, drying things out again, only likely cooler given the more robust cool air masses in late August coming from the northwest. As the surface high pressure slides eastward, it could warm back up somewhat while staying mostly dry, with the -NAO easing a bit, but a bit of upper-level ridging maintaining in the western U.S. will make it difficult to sustain more than a day or two of true hot weather before another upper-level trough and cold front beats the heat back down again. Although occasional hot days are still possible through mid-September, I think the truly hot part of summer is over.
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| Source: TropicalTidbits |
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