Severe thunderstorm watch through 8 p.m. tonight west of I-95 in D.C. area

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After a brief hiatus yesterday, heat and humidity have returned to the D.C. area which could fuel some nasty thunderstorms late this afternoon and evening.  The National Weather Service Storm Prediction Center (SPC) has issued a severe thunderstorm watch for areas just west of I-95 in the Washington, D.C. area. .

It writes:

A LINE OF ONGOING THUNDERSTORMS WILL INTENSIFY THIS AFTERNOON WHILE SHIFTING EASTWARD THROUGH THE PIEDMONT. OTHER STORMS MAY FORM AHEAD OF THE SQUALL LINE ACROSS THE PIEDMONT AND COASTAL PLAINS AS THE ENVIRONMENT BECOMES MODERATELY UNSTABLE. WIND SHEAR PROFILES WILL BE SUPPORTIVE OF EMBEDDED BOWING STRUCTURES AND SUPERCELLS CAPABLE OF DAMAGING WIND GUSTS AND PERHAPS A TORNADO OR TWO.

These storms are forecast to develop out of ahead of the main wave of heavy rain expected tonight associated with an advancing cold front.  A flash flood watch is in effect due to the potential for over 2 inches of rain in a short time period. We will have an update on the flood risk mid-afternoon.

The thunderstorm watch extends south through central and southern Virginia into east central North Carolina, where the risk of damaging storms is a bit more likely. The main threat with any storms that come through are damaging winds. Due to some turning of the winds with altitude, an isolated tornado or two is not out of the question.

The table below expresses the likelihood of different severe weather types over the watch area:

(National Weather Service Storm Prediction Center)

(National Weather Service Storm Prediction Center)

The most likely time frame for storms is between 5 and 9 p.m. this evening (50-60 percent chance west of I-95, 40 percent chance to the east), although isolated pop-up storms, which could be strong, are possible prior (20-30 percent chance).

Here’s an simulated radar image from the HRRR model, showing activity move into the area in the 6-8 p.m. window.

Radar simulation at 6 and 8 p.m. tonight from HRRR model (WeatherBell.com)

Radar simulation at 6 and 8 p.m. tonight from HRRR model (WeatherBell.com)

Capital Weather Gang’s severe storms expert Jeff Halverson shares his views on the situation:

Early breaks in the overcast today have allowed for decent solar heating and rapid destabilization of the atmosphere.  Additionally, low-mid level winds from the south are rapidly intensifying over our region, increasing the wind shear, and very moist air is also arriving.  These three factors have built the severe thunderstorm potential across parts of our region.

The main threat will be one or more lines of strong to possibly severe thunderstorms this afternoon and evening.  These storms will produce heavy rain, which may be prolonged.  Downdrafts in these storm cells may transport high winds aloft to the surface.  Isolated, weak tornadoes may spin up.  The Storm Prediction Center says 5% possibility of tornadoes, 15% possibility of damaging wind gusts.  This translates into a relatively low-end severe thunderstorm threat.

It’s important to NOT let the possibility of severe storms distract from the flash flood hazard, which will be concurrent, and flash flooding is a very real possibility through the overnight hours.

The threat for tornadoes may become more widespread this evening and tonight, as a new low pressure center forms to our south, increasing the wind shear.  The tornado threat may be larger to the south and east of the D.C. region, closer to the low pressure, but we will need to carefully monitor for any northward shift.

We will have more updates as needed throughout this afternoon.

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