Tuesday, 11:20 a.m.
Dew points of 70 to 75 degrees helped to fuel strong thunderstorms that erupted overnight and brought severe weather and flooding through the mid-Atlantic states earlier today. The morning surface pressure analysis reveals part of the problem:
Now superimpose that with the late-morning dew point temperatures:
Note carefully those high-octane dew points over much of Virginia east of the Blue Ridge over to the Delmarva Peninsula! When you inject that kind of tropical moisture into a nice little mesoscale area of low pressure ahead of a cold front, you have the makings of some violent weather!
The good news is that this is accelerating off to the east-northeast, and the heavy rains now moving off the New Jersey coast and through New England will depart quickly. Then the cold front pretty well marked out by the two images above will sweep off the Northeast and mid-Atlantic coasts tonight, ushering a much drier air mass into the region.
After today's rather dramatic exclamation point to this run of humidity, a large area of high pressure will build into the Midwest tonight:
This will promote clear skies with light winds, allowing temperatures to slip into the 40s in some communities. That same high will end up over the Ohio Valley and central Appalachians tomorrow and tomorrow night, with similar results - temperatures at night well down into the 50s, with some 40s showing up away from any urban areas and any water. By Thursday and Thursday night, the high will cover the mid-Atlantic and New England, and while there will be some moderation of the air mass by then, it will still be noticeably cooler and drier than normal.
With time, though, it being only the middle of August, this cool and dry air mass will moderate. And the dry air mass will slowly but surely moisten up. It will come about from two sources. The first is simply the return, onshore flow from off the tepid waters of the western Atlantic, where water temperatures are in the 70s to lower 80s, depending upon the location.
The other source will be more from the pure tropics, as the upper-level flow will quickly back into the southwest this weekend:
The initial front that gets stuck across the Southeast and the Deep South will more or less corral moisture around it, and as the southwest flow aloft becomes established, and then the surface high gets out of the way, it's only a matter of time before all of that moisture comes north. It is setting the table for a soggy period over the central and especially eastern Gulf Coast region the second half of the week into the weekend. It also means the Southeast will just get wetter with time.
And then, all of that dry, pleasant, comfortable weather farther north in the mid-Atlantic and Northeast will slowly but surely give way to an increasingly tropical air mass over time this weekend and especially early next week. It's not as certain as to how wet it will get, at least initially, but the humidity won't be denied. Summer is not dead yet.
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