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Weather Blog: Rain continues through Thursday, flash flooding expected tonight

Tuesday - 4/29/2014, 1:22pm  ET

WASHINGTON - A wet and raw Tuesday is in store as this weather pattern won't break anytime soon.

A slow-moving, but strong area of low pressure will crawl to the northeast, out of Southern Iowa, heading toward the Great Lakes by Wednesday. An area of high pressure hangs off the New England coast shooting an easterly breeze into the region Tuesday. A front stalled to our south with creep north Wednesday as a warm front, edges toward the D.C. area by the afternoon. It's an attending cold front, still west of the region today, that will cross through the area on Thursday. A secondary low will develop today over the southern tier of that cold front and drift northeast toward the Ohio Valley.

So there are all the players that we are working with over the next few days. A lot, I know.

Here is a look at the 9 a.m. Tuesday visible satellite (and surface analysis) showing that pesky slow moving low over Southern Iowa:

And here is the bigger picture from a forecast surface map on Tuesday afternoon showing the stalled frontal system to the south of the D.C. area, dissecting North Carolina, and a cold front to the west shooting down from Wisconsin through Louisiana.

Warm air aloft is overrunning the cold air at the surface, which results in rain. Expect drizzle and light rain intermittently (with some pockets of more moderate rain) around the region Tuesday, with showers picking up in intensity through the evening. There is a slight risk for a chance of severe weather for areas south of the D.C. metro region today.

Flash Flood Watch

As the warm front lifts north, there is a better chance for thunderstorms to kick off south during the evening and overnight hours hours. And we will certainly watch leftover storms rolling out of North Carolina, where there is a better chance for severe weather. Moisture in the atmosphere will increase around our area during the late afternoon and into the evening and overnight hours. Therefore, as we continue to see the mechanism to produce rain across the region, rainfall rates will pick up in intensity as there will be more available moisture. This is why the National Weather Service has put us in a Flash Flood Watch tonight.

River Flooding

We will also have to monitor our area rivers and their tributaries as they approach flood stage (and some are already forecast to flood).

There is also a flood watch tonight for the following rivers:

  • Potomac River at Point of Rocks affecting Frederick and Loudoun counties

  • Shenandoah River at Millville affecting Clarke, Warren and Jefferson counties

  • Seneca Creek at Dawsonville affecting Montgomery County

  • Rappahannock River at Remington affecting Culpeper and Fauquier counties

  • Opequon Creek near Martinsburg, W.Va., affecting Berkeley and Jefferson counties

  • Monocacy River near Frederick affecting Frederick County

Severe Weather Wednesday

By Wednesday, with an increased spread of warm, moist Gulf air streaming into the region accentuates the heavy rain and storm threat through the day. Temperatures will shooting to around 70 degrees on Wednesday with a southerly breeze rolling at about 10 mph to 20 mph. This will increase our instability around the area and will trigger some thunderstorms across the listening area (most likely after the lunch hour but there could be a few in the morning hours). The Storm Prediction Center has most of our area, south of Baltimore and Annapolis, in a slight risk for severe storms on Wednesday with the biggest threat being damaging winds and a possible tornado.

The region will see a total rain accumulation of several inches by early Thursday morning, according to our in-house model.

There could just be a few showers early in the morning on Thursday but we should at least see partial clearing by the afternoon. Temperatures will shoot into the mid 70s. Friday looks to be the driest day of the week with high pressure building into the region. Daytime highs will top out around 70 with a mix of sun and clouds.

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