Flooding rain and damaging winds possible in storms in DC area

The watch spans from central Virginia through central Pennsylvania, where the storms are developing and sweeping southward. It does not include counties along the Chesapeake Bay where storms are projected to be somewhat less numerous and intense.

Remember that a severe thunderstorm watch means conditions are favorable for intense storms, but not a guarantee. Stay weather aware. If a severe thunderstorm warning is issued for your location, it means a severe storm is imminent and that you should seek shelter.

The initial round of showers and storms that has already passed will make way for another wave developing in southern Pennsylvania arriving during the late afternoon. It may be more intense.

We’ll post a new update in another hour — around 3:10 pm

Original article from midday

The blissfully dry weather of the past several days has departed; a very warm, humid pattern is taking its place. But cooler air lurks to our northeast and northwest. We’re stuck in the transition zone where these contrasting air masses meet, a ripe setting for intense thunderstorms.

Storms are most probable between about 3 and 11 pm, and some may be severe — containing damaging winds and hail in addition to heavy downpours and dangerous lightning. Some areas could be hit by heavy storms repeatedly — increasing the risk of flooding.

The National Weather Service has issued a flood watch for much of the region, except for Southern Maryland and counties next to the Chesapeake Bay, where showers and storms will probably be less numerous.

The heaviest rain and greatest flood threat will likely focus between Interstates 95 and 81. “Rainfall amounts of 1 to 3 inches are possible within the span of a couple of hours, with locally higher amounts possible,” the Weather Service cautions.

Rain totals will be highly variable throughout the area, depending on where the heaviest storm cells track — which can’t be predicted before they start to form. Some areas could see less than a tenth of an inch while some models show maximum totals over 5 inches, which is a serious amount of rain. This amount of rain would require heavy storms forming and reforming while tracking over the same area repeatedly — a phenomenon known as training. The greatest threat of training storm cells is west of Route 15, running from Frederick to Warrenton.

The Weather Service has placed the western half of our region in a Level 2 out of 4 risk zone for excessive rain; our eastern areas are under a Level 1 risk.

The flooding threat may be mitigated somewhat by the fact June has been dry so far — but if 2 or more inches falls in short amount of time, that could quickly cause streams to overflow and for poor drainage areas to be overwhelmed.

“Excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations,” the Weather Service writes.

Remember to never attempt to drive across a flooded road as the water level is difficult to judge. Turn around, don’t down.

In addition to the heavy rain threat, the Weather Service’s Storm Prediction Center has placed the area in a Level 2 out 5 risk for severe storms that could produce “damaging winds and isolated large hail.” An isolated tornado also cannot be ruled out.

The most probable timing for severe storms would be in the late afternoon and early evening before the threat wanes toward dark. However, the risk of heavy rainfall could continue until 10 or 11 pm in parts of the area.

An unusual pattern is coming together for the next 12 to 18 hours.

As shown in the forecast surface chart (valid 8 pm) below, we have an approaching cold front from the west. Over the Bay and I-95 corridor, another slow-moving frontal boundary is approaching from the east: an odd direction, in fact, a process that’s called retrograding. Along this boundary, a weak area of ​​low pressure is expected to develop.

So the region will be positioned in a zone in which the humid, unstable air mass in between the fronts is getting squeezed from both directions. This is called convergence of air, and the result will be a large mass of air forced to ascend.

Adding to the potency is a very high humidity content of the air. The morning weather balloon at Dulles revealed aggressive moistening of the deep atmosphere is underway, to the point where the “precipitable water” (total liquid equivalent depth of water vapor) will be between 2 to 2.5 inches — a value that is quite excessive for our region in late June—near record levels. These abnormally high values ​​at 8 this evening are shown by the ribbon of red colors in the map below.

So we have very high moisture content, getting squeezed upward over the region between two fronts, in an atmosphere unstable enough to generate thunderstorms. These factors will intensify late this afternoon and likely be sustained until about midnight.

The deep airflow aloft is also anomalous for this time of year, from due north — so storm cells will develop in Pennsylvania and drift south into the Baltimore-Washington region.

We think the retrograding front draped along I-95 will act as a conduit along which storm cells will repeatedly fire and track from north to south. It’s difficult to say a priori the exact counties/locales impacted, but this “training” effect could lead to impressive rain totals for some, upward of 2 to 3 inches. One of the high-resolution forecast model simulations of radar coverage for through tonight is shown below — you can pick out the enhanced corridor of storm cells along and west of I-95.

Another area of ​​focused, heavy rain may be near or just west of the I-81 corridor, where enhanced lifting of humid air by the mountains and the approaching cold front may wring out extra atmospheric moisture. Note, however, the above radar simulation is only a rough guide as to how storms may evolve; the actual timing and placement of storms could end up being quite different.

Locally severe storm cells may also generate damaging wind gusts, intense lightning and, perhaps, even a weak tornado. We don’t expect the severe weather coverage to be as widespread as the flood threat. But the wind shear (or increase in wind speed and change in direction with altitude) is sufficient, along with local “spin” generated along the retrograding front, for the threat of an isolated tornado.

Damaging straight-line winds are more likely, in the form of downbursts, in which the heavy mass of descending water in cell downdrafts drags the air down to the surface in a high-velocity impact.