Around 7 p.m., intense storms bubbled up over Loudoun and Montgomery counties, generating brief torrential downpours, lightning and strong winds.
There were pockets of wind damage, one of which struck the Bethesda area. There, large trees were toppled, limbs came down and thousands were left in the dark. Many utility customers were still without power this morning as cleanup and repair continued. The Woodhaven and Stratton Woods areas of Bethesda were among those hit the hardest.
Sunday’s storms were not widespread. The wind shear that creates longer-lived, intense agglomerations of storms was lacking, but the atmosphere became strongly destabilized on the worst day of our heat wave. Recall that Washington reached 99 degrees, while Dulles and Baltimore hit 100.
That tremendous mass of hot, humid air rose upward, as in a chimney, into the cooler reaches of atmosphere aloft. Thunderstorms rapidly developed, culminating in a brief period of fury, then quickly extinguished themselves.
Without wind shear, storm cells do not sustain beyond 30 to 45 minutes or so. The updraft surges upward, then a downdraft of cool, dense air develops and causes the storm to collapse. The collapse often sends out violent blasts of wind, called downbursts. Meteorologists call these cells “pulse severe” storms.
Not long after the National Weather Service issued a severe thunderstorm warning for the Bethesda area, the storms began dying out.
The wind damage in and around Bethesda was violent though brief, probably caused by a downburst in one of those rapidly cycling cells.
The Doppler image below captures two snapshots, four minutes apart, as the downburst developed and intensified.
The colors show wind direction and intensity. “X” marks the spot where the downburst impacted the ground. The pocket of green is flow toward the radar (located at Dulles), to the west; the pocket of red and pink is flow away from the radar, toward the east.
The spreading apart of opposing flows, on either side of the X, is a signature called divergence. It was most likely created when a bubble of rain-chilled, dense, rapidly descending air struck the ground and then strong winds blasted radially away. Those winds, lasting just a few minutes, reached at least 65 to 70 mph and felled many trees.
Within 15 to 20 minutes of the downburst ending, the storm complex that birthed it literally rained itself out and evaporated, leaving no trace behind in the radar.
What goes up must come down.
Such downbursts are possible in storms that might develop in the Washington region yet again Monday.
Here are more views of the storm and some of the damage it caused from Loudoun, northern Fairfax and Montgomery counties.