Thursday, 11:30 a.m.
Flash flooding took center stage around New York City yesterday with over 3 inches of rain pelting Central Park. There were other areas with flash flooding back in Pennsylvania, too, as the very slow-moving upper-level low over Virginia pulled tropical moisture inland from off the Atlantic and dumped heavy rain on parts of the region.
That upper-level low is finally starting to pick up some forward momentum. The 12z 9 May NAM 500mb forecast for this afternoon moves it more or less right over New York City:
So, after early tonight, most of the rain with this storm will finally exit the playing field. The problem is that it will be replaced by not one but two additional storms that will have their share of rain. The first of these is coming through the Midwest now with some rain and thunderstorms. In general, the rain will not be heavy enough to produce widespread flooding, but do remember that flood waters across parts of Illinois are still receding, so it won't take a lot to renew flooding there.
The second area with a growing risk for heavier rain and strong thunderstorms is in the southern Plains as an upper-level disturbance is spit out from around the large upper-level low over the central and southern Rockies. When you combine these two, there is a decent-sized area at risk of strong, gusty thunderstorms:
Both of these systems will be trouble makers into tomorrow, as the one tracks across the Great Lakes and pulls a cold front into a warm and pretty moist environment from upstate New York back into the Ohio Valley. Meanwhile, the latter feature will have an even richer supply of tropical moisture to work with over the western Gulf Coast region and lower Mississippi Valley.
Even on Saturday, the weather is simply going to be stormy from the Northeast to the Gulf Coast. Look at the latest forecast for Saturday evening from the 12z 9 May NAM model, showing the precipitation during the preceding six-hour period:
When you add it all up, just from now through Saturday night, there's quite a bit of precipitation!
Granted, that forecast won't work out as spelled out by the model, but it gives you the sense of how vastly different the pattern is from a week ago, when it was bone dry in the Northeast.
The stormy pattern ends with the passage of a strong cold front off the East Coast Sunday. It will struggle to clear the central and eastern Gulf Coast at first, but even there drier air will win out by Sunday night, setting the stage for a very cool start to next week. In fact, this air mass is exceptionally cool and dry, and with high pressure building from the Midwest into the Ohio Valley, then the mid-Atlantic states, there is a pretty high danger of frost with this setup:
Wet to chill. Eventually it will all just flip and get summery, but that's not happening any time soon!
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