Strong line of storms likely this evening

sim-radar

Simulated radar from this evening (4km hires WRF ARW model)

An energetic spring cold front sweeps into the metro region late this afternoon and evening accompanied by a line of strong showers and thunderstorms.  Heavy rain is likely with a lesser threat of damaging winds.

Timing

Models are pretty consistent in simulating the following arrival times for the primary line of showers and storms:

3-4 p.m. Loudoun, Frederick and Fauquier counties
4-5 p.m. Stafford, Prince William, western Fairfax and western Montgomery counties
5-6 p.m. Inside the Beltway, the District, Howard, Prince George’s and Charles counties
6-7 p.m. Anne Arundel, Calvert, and St. Mary’s counties

Models tend to simulate the heaviest rain between around 8 and 11 p.m.

Rain should end between 1 and 3 a.m. from west to east across the region.

This timing may require tweaking, but shouldn’t change dramatically.

Note: a few rogue showers and storms could develop ahead of this main line in the mid-to-late afternoon hours (20-30 percent chance)

Impacts

The National Weather Service Storm Prediction Center indicates there is a “slight risk” of severe thunderstorms in our area.  Its assessment notes a 15 percent chance of damaging winds of at least 58 mph within 25 miles of any point in our region.  It also indicates there is a 5 percent chance of a tornado.

severe-storm-dc-0419

Here is what I see as the most and least likely storm impacts:

storms_01High likelihood: Heavy rain
Medium-high likelihood: Gusty winds
Medium-high likelihood: Thunder and lightning
Medium likelihood: Localized flash flooding
Low likelihood: Damaging winds and/or small hail
Very low likelihood: Small tornadoes and/or large hail

Of all the impacts to be concerned about, heavy rain and flash flooding (mainly in low lying areas prone to flooding) rank highest on the list.  One to 2 inches of rain in a short amount of time is possible.

Technical discussion

Our local National Weather Service office in Sterling, Va. provides a nice overview on the ingredients available for heavy rain and strong winds:

Simulated winds at 5,000 feet (in knots) at 5 p.m.

Simulated winds at 5,000 feet (in knots) at 5 p.m.

On heavy rain…

PRECIPITABLE WATERS WILL BE WELL OVER TWO STANDARD DEVIATIONS ABOVE NORMAL AHEAD OF THE BOUNDARY. THE COMBINATION OF THE DEEP MOISTURE…STRONG LOW-LEVEL JET AND DYNAMICAL FORCING WILL RESULT IN LOCALLY HEAVY RAINFALL. RAINFALL AMOUNTS AROUND ONE TO TWO INCHES ARE EXPECTED WITH LOCALLY HIGHER AMOUNTS…AND MOST OF THAT MAY FALL IN A ONE TO THREE HOUR STRETCH.

On strong winds

THUNDERSTORMS WILL ALSO HAVE THE POTENTIAL TO PRODUCE LOCALLY
DAMAGING WIND GUSTS. WIND SPEEDS ASSOCIATED WITH THE LOW-LEVEL JET
WILL BE ABOVE 50 KNOTS JUST A COUPLE THOUSAND FEET FROM THE SURFACE. INSTABILITY WILL BE LIMITED SO THERE IS STILL A QUESTION AS TO HOW MUCH OF THESE WINDS WILL REACH THE SURFACE. THE BEST CHANCE APPEARS TO BE JUST AHEAD OF THE COLD FRONT IN WHAT WILL LIKELY BE A LINE OF SHOWERS AND T-STORMS…QLCS [QUASI-LINEAR CONVECTIVE SYSTEM].

Also on Capital Weather Gang

D.C. area forecast: Mild and muggy ahead of stormy cold front