Severe storm and flash flood watches in effect through this evening

Remember that a watch means conditions are conducive to the formation of intense storms, but that they are not a guarantee. Stay alert. But if a severe storm warning is issued for your location, it means a severe storm is imminent and you should seek shelter immediately.

We will post updates as storms develop and move through the area at the top of this article.

Original article from 1:30 pm

After a delightful day with below-average temperatures and humidity Monday, steamy air has surged back into the area. As a cold front clashes with the hot, muggy air in place, strong to severe thunderstorms could erupt across the Washington region, especially this evening.

Computer models suggest the most likely window for storms is between about 6 pm and midnight.

“Damaging downdraft winds will become a concern as thunderstorms gradually increase in coverage and intensity. Some severe hail may also occur,” the National Weather Service wrote in a special bulletin about the storm threat. The bulletin said that a severe thunderstorm watch will probably be issued.

The Weather Service has placed the region in a Level 3 out of 5 “enhanced” risk zone for severe storms.

A flood watch has already been issued for the region.

“Strong to severe thunderstorms will move across the region late this afternoon through the evening hours,” the National Weather Service wrote in the flood watch statement. “Heavy rain will accompany a number of these storms which may drop 1 to 2 inches of rainfall in an hour. Additionally, some regions could see repeat thunderstorm activity leading to an enhanced threat for flooding.”

Areas most prone to flooding include those near creeks and streams as well as low-lying, poor-drainage areas. At additional risk are those areas that were deluged last week with over 4 inches of rain, including parts of Fairfax, Montgomery and Prince George’s counties as well as the District. In these areas, the ground is already somewhat saturated, and stream levels are high.

Timing: Storms are most likely in the 6 pm to midnight window, but a few isolated cells could pop up late this afternoon. Here’s when the majority of storms might first arrive:

• Interstate 81 (Hagerstown to Front Royal): 4 to 6:30 pm
• Route 15 (Frederick to Warrenton): 5:30 to 7:30 p.m.
• Interstate 95 (Baltimore to DC to Fredericksburg): 6:30 to 8:30 pm
• Route 301 (Bowie to La Plata): 7 to 9:30 pm

Some areas could see more than one round of storms.

• Likely: Very heavy rain, gusty winds (30 to 50 mph)
• Possible: Flooding, damaging winds (50 to 70+ mph), small hail
• Small chance: Large hail, isolated tornado

The severe-weather story today focuses on several potential hazards — severe storm activity and the potential for flash flooding late Tuesday afternoon and into the evening.

Let’s start with the forecast weather map for early evening (shown below). A cold front associated with a dip or through in the jet stream will arrive on our doorstep by early evening. Ahead of the front, “return flow” around the western flank of the Bermuda High — from the south-southwest — will combine with abundant sun to raise surface temperatures and dew points, a measure of humidity, into the low 90s and low 70s, respectively. Any time the dew point is over 70, it’s oppressively humid.

Meanwhile, a belt of strong winds in the middle atmosphere will approach from the west, as the trough moves out of the Great Lakes. Both the trough and region of enhanced wind are shown in the late-afternoon forecast chart at 18,000 feet (below).

It’s a solid setup for scattered to widespread intense thunderstorms. The hot, humid air mass at low levels and cooling of the atmosphere in upper levels (due to the approaching trough) will cause significant destabilization of the atmosphere. The belt of strong westerly flow aloft will increase the shear, or changing of wind direction and speed with altitude. Wind shear increases the intensity of storm cells and promotes better organized, longer-lived storm complexes.

One of the factors to note are a few stable layers in the atmosphere. Certain layers are unstable and will become more unstable with continued heating and moistening of the surface. But the morning weather balloon launch at Washington Dulles International Airport indicated a few shallow but significant regions of atmosphere that could rob storm updrafts of buoyancy, and mitigate the severity of storms a bit. It’s also possible that these intervening layers will “mix out” by evening and the entire, deep atmosphere will be fully primed to maintain strong updrafts. This factor will be hard to assess unless the Weather Service launches a special midafternoon sounding balloon.

The morning model runs are unanimous about generating widespread storms along the front, starting in the late afternoon. The cells will quickly come together into better organized multicellular complexes. Some of these complexes may take on a fast-moving, bowing configuration; a few may develop transient supercell-like characteristics, meaning that they are rotating.

Scattered instances of damaging winds are the greatest single storm threat, with likes over 60 mph possible. Some instances of hail are possible, and a brief tornado could initiate from a weak supercell or two.

There is also an indication that waves of storms could move repeatedly over the same regions as we get into evening. This “training” potential is the reason the Weather Service has issued a flood watch across the region.

CWG meteorologists will be monitoring and updating the forecast as we move through the day.