A cold front sliding into the region promises to set off scattered thunderstorms this evening. The most likely timing for the storms is between 5 and 10 p.m. from west to east. Short-range computer models focus most activity in the immediate metro area (within one county of the District) between about 7 and 9 p.m. – though this is subject to change.

More isolated (hit or miss) storms could pop-up as soon as this afternoon.

Heavy rain and lightning are likely to be the primary hazards in these storms, although isolated damaging wind gusts are possible.  The majority of storms should remain below limits considered severe (winds of at least 58 mph and/or hail at least 1 inch in diameter), but we would not be surprised to see a few severe thunderstorm warnings issued.

Technical discussion

Today, we have a setup similar to other days this past summer: A strong Bermuda High pumping in hot, humid air and a slowly approaching cold front. The air mass contrast across this front (an indication of its strength) is not as great as last week. Additionally, an upper level disturbance (shortwave trough) is approaching from the Great Lakes region.

Weather map forecast at 8 p.m. tonight (NWS)

Instability today is expected to become significant, predicted to rise to 2000-2500 J/kg of CAPE (convective available potential energy). This is a moderately large value for summer. This will enable thunderstorms to become strong, and approach severe levels in spots. In spite of large CAPE, one factor that may inhibit storms is a stable layer in the middle atmosphere, called a temperature inversion, which robs the buoyancy from cloud updrafts. Whether this inversion will persist through the day or break down remains to be seen.

CAPE simulation at 6 p.m. from high resolution NAM model (WeatherBell.com)

Wind shear – the change in wind speed with altitude – is very, very weak, less than 10 knots. Without wind shear, thunderstorms do not become well-organized nor long-lived. However, with the approaching disturbance aloft, the shear will increase slowly through the day, and particularly during the overnight, when values could reach 30 knots, which is moderately strong for summer.

As we progress through the afternoon, and the cold front approaches, we expect showers and thunderstorms will erupt area-wide. However, the high resolution model guidance suggests that coverage will initially be spotty and unorganized. There is no suggestion that a solid squall line of storms will develop ahead of the front. Storms may trigger in the usual spots – over the elevated terrain, along a surface pressure trough over the Piedmont, and also in the vicinity of the front.

With the significant instability, initial storms during the afternoon may be isolated, pulse-type strong to severe – producing very heavy rain, small hail and localized damaging wind gusts. The Storm Prediction Center (SPC) barely includes the D.C. region in a 5% risk of severe wind gusts. The larger threat is further north, over New York, where wind shear is stronger. As we move into evening, and wind shear increases, pulse type storms may transition into more organized and longer-lived storm clusters and short segments (multi cells), but coverage may still be scattered. Here are two high resolution model radar simulations around 8 p.m.:

HRRR model radar simulation for 8 p.m. this evening (WeatherBell.com)

High resolution NAM model simulation at 8 p.m. tonight (WeatherBell.com)

The front is expected to take its time crossing our region, and may still not have pushed completely through the D.C. region until pre-dawn Thursday. Accordingly, the shower and thunderstorm risk will linger through the overnight (though the best chance is in the 5-10 p.m. window). Locally heavy rain this afternoon and tonight is possible in any of these storms, given the very humid air mass in place (surface dew points in the mid-70s, quite oppressive!).

Tomorrow, the region of showers and thunderstorms shifts to central and southeast Va. as the front pushes into that region.