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5:00 p.m. — Severe thunderstorm watch discontinued from Interstate 95 westward

While showers and storms are scattered across the region, the severe thunderstorm watch was discontinued for areas along and west of Interstate 95 as they are generally below severe limits.

East of Interstate 95, an isolated severe storm is still possible through 7 p.m. or so.

This will be our final update.

3:10 p.m. — Scattered downpours west of Interstate 95, pushing southeast

Showers, with briefly heavy rain, are scattered from Columbia to Potomac in Maryland and are pushing southeast. They’ll cross the I-95 over the next 30 to 45 minutes but won’t last long.

Not a lot is happening in Northern Virginia although an area of downpours in southern Loudoun and northern Fauquier counties may push southeast along Route 50 into our western suburbs over the next hour.

None of this activity is severe, but if it strengthens and becomes hazardous, we will update.

2:10 p.m. — Initial storminess pushing into Southern Maryland while new storms build to northwest

The severe storm that charged through Alexandria and southern Prince George’s County has pushed into northeast Charles and northern Calvert counties. It has evolved into a cluster of several storms that will push southeast through St. Mary’s and southern Calvert counties over the next hour.

To the northwest, some scattered storms have formed in northern Loudoun and Frederick counties that bear watching as they push east-southeast toward the District’s northern suburbs over the next hour or so. We’ll update as necessary as/when they draw closer. Scroll down below for the general thunderstorm outlook through the afternoon.

1:25 p.m. — Severe storm in Alexandria racing into southern Prince George’s County

Just since 1 p.m., a severe storm erupted near Alexandria unleashing strong winds and hail. It’s currently between the southwest portion of the District and Camp Springs, Md., zipping to the southeast at 25 mph toward Andrews Air Force Base and Clinton. It could produce localized damaging wind gusts up to 60 to 70 mph and small hail in addition to very heavy rain and lightning.

As the storm flared up over Alexandria one eyewitness wrote on Twitter it was like “[t]en minutes of apocalypse.” Another remarked: “literally couldn’t see out my window from all the water but I could hear the hail stones bouncing off the glass.”

Original article

A cold front is coming to kick this hot, smoky air out of town. But first, it may trigger some scattered severe thunderstorms along the way.

A severe thunderstorm watch has been posted for the Washington and Baltimore region through 8 p.m. The watch extends considerably farther north, too, all the way through Philadelphia, New York and Boston.

Around Washington and Baltimore, the storms are most likely to zip through between about 3 and 6 p.m., traveling from northwest to southeast. However, some isolated showers and storms could fire up in the region before that in the early afternoon.

In addition to heavy rain and lightning, the storms could produce hail and bursts of damaging winds. The most widespread concentration of storms may focus east of Washington, with the highest risk of severe weather in New Jersey and southern New York. But a few intense storms around the District and especially our eastern suburbs are possible.

A severe thunderstorm watch means ingredients are in place for possible severe storms, but they are not guaranteed. If a severe thunderstorm warning, however, is issued for your location, it means severe storms are imminent and you should seek shelter in a strong building.

Discussion

Today’s storms will erupt along a slowly advancing cold front, as shown in the diagram below, with initially scattered cells clumping into larger aggregates and line segments as the afternoon continues.

The storms will advance from a bit of an unusual direction, the northwest. They are being fueled by an unstable, humid air mass on westerly-southwesterly winds. In the upper atmosphere, a trough or dip in the jet stream, is approaching, increasing the dynamic uplift over the Mid-Atlantic.

Within the trough, mid-level winds are intensifying, and this creates a wind shear (shown below). The wind shear acts to tilt the updraft away from the downdraft in storm cells, so that the two do not interfere, increasing storm vigor.

The 35-to-40 mph of wind shear will work with the strong buoyancy of the air (instability) to create longer-lived aggregates of storm cells, termed multicell thunderstorms. One thing to note is the paucity of cell coverage portrayed in the high-resolution models (see below). This could be because the low-level airflow is descending mountains to our west, which tends to suppress storm development and dries out the lower atmosphere a bit.

Any severe storms that congeal close to the Interstate 95 corridor will be capable of locally torrential downpours, intense cloud-to-ground lightning, hail up to a quarter size, and peak wind gusts of 60 to 70 mph.