Tuesday, May 17, 2011

After morning heavy rain, severe storms are possible this afternoon and evening

Heavy rains this morning produced flash flooding in west-central parts of Virginia as a line of showers and storms tracked over the same locations again and again, basically along I-64 from the Richmond Metro up into Albemarle County. Here is a snapshot of some of the rainfall totals through 10AM:

You can see in red on the map below the Flash Flood Warned areas, where at least 1"-2" of rain fell within a few hours this morning:

All the other counties in green remain under a Flash Flood Watch until late tonight. More showers and thunderstorms are expected to develop in central Virginia and track northwest into the Blue Ridge this afternoon and evening, and can produce heavy downpours again. With these two rounds of rain, first this morning and then later today, flooding will continue to be a problem in this part of the state.

Some of the storms that form this afternoon and evening may become severe, capable of damaging wind gusts in excess of 60 mph. A brief, isolated tornado cannot be ruled out, but that chance looks low.

The reason why we have an enhanced threat for severe thunderstorms today is because the center of the upper low-pressure system that has been spinning over the eastern U.S. the past few days is going to drift northeast over the Commonwealth today and tonight, bringing additional lift and dynamics for storms to potentially rotate. As it does so, it is pulling in rich Atlantic moisture from the east and southeast into central Virginia to serve as ample fuel for rain and storm development. Also, we can get topographical lift from a flow like this as the elevation rises from the coast to the mountains. This process is what aided heavy rain and storm development Tuesday morning.

As we get breaks in the clouds mid-day and this afternoon, the atmosphere should warm enough to become unstable with storm initiation occurring. Here's a snapshot of the upper low before sunset today, with scattered showers and thunderstorms ongoing in central Virginia:

I expect storms to rapidly weaken a few hours after sunset, with any severe threat overwith by Midnight. We should remain dry in central Virginia through Wednesday morning, but as the upper low begins tracking just to our northeast, we could receive more showers and a few thunderstorms on the southwest side of the low Wednesday afternoon. Until this low completely exits our region at the end of the week, at least some showers and storms will be possible in central Virginia, which is why we are keeping rain chances in the forecast through Friday. By this weekend, our closed upper low will be replaced with a ridge of high pressure, allowing us a few days to dry out after much-needed rainfall.
--Meteorologist Carrie Rose
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