It looks like these storms will reach the Beltway area between about 3:30 and 4 pm
Our next update will post between 3 and 3:15 pm
1:55 pm — Severe thunderstorm watch until 10 pm
As showers and storms reach the Interstate 81 corridor, the National Weather Service has issued a severe thunderstorm watch for the Washington-Baltimore area until 10 pm; the watch also includes Roanoke, Richmond and Philadelphia — a total population of more than 20 million people.
“Damaging winds are the primary hazard,” the watch states, although torrential rain and lightning are also a threat.
Remember that a severe thunderstorm watch means conditions are favorable for severe storms, but not a guarantee. On the other hand, if a severe thunderstorm warning is issued for your location it means a dangerous storm is imminent and you should seek shelter immediately.
Original article from 1:30 pm
Monday will probably mark the eighth day in a row with highs in the 90s in Washington, but a cold front barging into the region will put an end to that streak. This front will set off widespread showers and storms as we transition to this less hot air mass.
The heaviest storms will sweep through swiftly, arriving in our western areas by 2-3 pm, right around the Beltway close to 3-4 pm and pushing over the Chesapeake Bay by 5 or 6 pm — although this timing could shift slightly. There could be a weaker, trailing line of showers and storms closer to sunset that exits by 10 or 11 pm
The afternoon round of storms could produce heavy downpours, lightning and a few bursts of damaging winds. The National Weather Service, which placed the area under a level 2 out of 5 risk for severe weather, issued a bulletin indicating it is “likely” (80 percent chance) to issue a severe thunderstorm watch.
A severe thunderstorm watch has already been issued for much of the Northeast from the New York City area through northern Maine until 8 pm
The storm potential Monday afternoon and evening is predicated on the arrival of a cold front from the northwest, intersecting an unstable and humid air mass along and ahead of it. The surface forecast chart below shows the position of the front at 8 pm:
Meanwhile, in the middle atmosphere, a belt of strong winds will be oriented from Pennsylvania to Maine, and, parallel to the front, will shift toward the northeast.
Storms have begun to develop along the crest of the Appalachians to our west; these storms will then drift toward Interstate 95 during the mid to late afternoon.
Given some sunshine and plentiful low-level moisture, we expect the atmosphere to destabilize to modest levels by midafternoon. This will energize and enlarge cloud updrafts. However, thick clouds drifting over the area from storms further northwest may prevent the atmosphere from reaching full destabilization Monday afternoon.
With moderate instability, the degree of storm intensity and organization is determined by wind shear, which is the increase in wind speed with altitude. Wind shear levels as measured by weather balloon Monday morning at Dulles Airport did not reveal exceptional levels, only around 20 mph (18 knots). With the approaching ribbon of faster wind aloft, however, we expect these values to climb to closer to 40 mph (30-35 knots) through the afternoon.
The image below shows the forecast wind shear values at 5 pm The DC region will lie along the southern edge of higher wind shear values to the north; the core of strongest shear will be well to our north. Such strong wind shear values in an unstable atmosphere may trigger isolated supercell thunderstorms, with the best chance for damaging wind, tornadoes and large hail.
In the Washington region, we expect torrential rain, lightning and a few pockets of strong to damaging wind or downbursts. The activity will be progressive, so we don’t expect storm cells passing repeatedly over the same area or training. But due to the intense updrafts and abundant low level moisture, some spots may pick up a quick one to two inches of rain — enough to trigger isolated flash flooding in areas that have received large amounts of rain this month.