Radar courtesy MyRadar | © OpenStreetMap contributors

* Flash flood watch through 11 p.m. for Prince William and Charles counties *

4:55 p.m. — Heavy storms so far focused south of immediate D.C. area, but more could flare up this evening closer to town

Radar shows storms mostly south and southwest of the Beltway with the heaviest activity in Virginia’s Northern Neck and the west central part of the state from Culpeper through Charlottesville to near Lynchburg.

Storms may develop or expand somewhat north this evening and could produce heavy rain, before exiting to the east by 10 or 11 p.m. But they may end up being widely scattered rather than widespread.

This is the last update in this article. We’ll post our P.M. forecast update at 5:30 at washingtonpost.com/capitalweathergang and, if any severe weather develops, post updates there.

Original article from early afternoon

Temperatures have soared above 90 degrees for a record 26th time this month, and, adding in the humidity, the soupy air is primed to fuel showers and thunderstorms this afternoon and evening. These storms may be intensified by a slow-moving cool front entering the region.

Some of the storms could be heavy and produce damaging winds. Because of the high moisture content in the air and the possibility of multiple rounds of storms in some areas, the National Weather Service has issued a flash flood watch for the region between 3 and 11 p.m. Up to 2 to 3 inches of rain could fall in a few locations in a short amount of time.

“These rainfall amounts may result in rapid rises of water on small streams and creeks, as well as in urban and poor drainage areas,” the Weather Service wrote. Also, consider the ground is soggy in some areas from heavy storms last week and unable to absorb much more water. Remember never to attempt driving across a flooded road.

The most likely timing for storms is between 4 and 8 p.m. in the immediate Washington region, although a few storm cells could flare up between 2 and 4 p.m. in our western areas. Particularly in our eastern areas, storms could linger after sunset.

Although heavy rain and lightning will be the primary hazards with these storms, damaging wind gusts that bring down trees are possible in a few storms.

Some areas may miss out on the storms. Forecast models are inconsistent in their predictions for how widespread the storms will be.

Discussion

The weather later today features a typical summertime setup for the possibility of a few storms, which necessitate warnings because of their severity, for both strong wind gusts and flash flooding. For the past couple of days, daily storminess has been suppressed, because of a high-pressure ridge in the jet stream — which causes air to sink and dry.

Today, we are back in the soupy “return flow,” in which humid air is being drawn into the region, ahead of a weak cold front approaching from the northwest. At higher altitudes, a weak dip in the jet stream flow is encouraging air to rise across the region.

The timing of the front’s arrival is perfect, to act as agent for “scooping up” the unstable air, encouraging storms to break out along a broad arc from the Mason-Dixon Line and south.

Additionally, we are monitoring for a small region of low pressure that may develop near the bay later this afternoon. That feature may encourage some additional convergence of low-level, humid air, further stoking the air’s ascent.

We expect the air mass to become modestly unstable, given strong solar heating and abundant low-level moisture. One caveat: The morning weather balloon launch from Dulles shows a stable layer in the middle atmosphere, left over from the previous days’ environment of sinking air. It will still take some time to erode this feature, and it may initially put the brakes on widespread storm coverage.

In the upper levels, winds from the west have strengthened as a jet stream trough swings through. This has increased wind shear across the Mid Atlantic and Northeast. Wind shear promotes larger, longer-lived complexes of thunderstorms, allowing storm cells to intensify.

An analysis of the shear is shown below.

Note the pocket of 35 to 40 knot shear values across northwest Pennsylvania. To the south, on the edge of the pocket, values are smaller but still in the 30-knot range for the D.C. region. That will be sufficient to help organize small storm cells into larger clusters and small lines. The tendency for increased organization may be greatest north of D.C. proper, across central Maryland.

The combination of modest shear and a modestly unstable atmosphere is, in fact, a favorable balance of conditions for promoting stronger storms. The Storm Prediction Center recognizes this and has placed our region in its marginal-risk zone for severe weather.

Aside from some of the cells producing sporadic strong to locally severe gusts (in the form of microbursts), our greater threat today may be from flash flooding.

Low-level moisture is expected to increase through the day, as convergent winds ahead of the front and the area of low pressure accumulate low-level moisture. When thunderstorms erupt, winds aloft from the west will be aligned parallel to the front. This effect may steer or “train” cells repeatedly across the same regions, causing rain totals to quickly add up.

Add in some dynamic uplift from the jet stream trough, and the fact the front will be moving through very slowly — this combination of factors could spell a few trouble spots for heavy rains through the evening, particularly across central Maryland.