At 4 p.m. on a sultry Tuesday afternoon in Washington, the weather was calm but for some billowing cumulus clouds perched over the city. Just 15 minutes later, the mass of clouds quadrupled in size, shooting 40,000 feet into the atmosphere, morphing into a raging thunderstorm with 60 mph winds and hail.
It was the first of two storms that rocked the District over a 90-minute span. The storm duo downed numerous trees and unleashed hail as big as ping-pong balls.
How did these storms develop with such haste and vigor?
The intense thunderstorm cells that erupted over the District (the first, over northern D.C. about 4 p.m.; the second, over southern D.C., closer to 5:30 p.m.) were a particular type called “pulse severe” storms. They develop in a strongly unstable air mass (although they lack a significant increase in winds with altitude or wind shear that helps intensify and prolong the storm’s updraft).
Pulse storms typify heat waves; the extreme heating of the atmosphere into the mid-90s — coupled with high humidity levels — supercharges the lower layers of the atmosphere with tremendous buoyant energy. Updraft air takes off like a rocket, and for a brief period, the updraft can become intense, without the assistance typically provided by wind shear.
In a pulse-type storm (a diagram is shown below), the updraft explodes upward, drawing up copious amounts of water vapor that then rapidly condenses into supercooled water drops and small ice particles called graupel.
The image below illustrates radar snapshots showing the rapid growth of the 4 p.m. pulse storm erupting over northern D.C. These images were created using special volume-rendering software that captured 15 minutes in the evolution of this cell, each frame shown at five-minute intervals. That 15-minute time period shows the complete developmental phase of this storm.
Between the first two frames (top row), the budding storm cloud grew 10,000 feet in five minutes. Also note how the core of heaviest rain and hail (yellow and red colors) initially developed aloft and remained aloft for as much as 10 minutes (thus the updraft must have been very strong, to suspend that huge mass of water), then crashed to the surface by the last frame.
Here’s an animated look at this storm:
The mixture of ice and water in very cold regions of the middle atmosphere causes strong electrical fields to develop (and hence frequent lightning), and also triggers the growth of hailstones. Large hail, like the half-dollar-sized stones falling over D.C., can develop if the updraft is strong enough to levitate large, heavy stones high up in the cloud.
Once the storm’s downdraft develops (from evaporation of raindrops in the subcloud air, and also the “drag” or push of heavy rain downward on the air), it rapidly takes over, spreads throughout the cloud and smothers the updraft — much like throwing a wet towel on a campfire.
A pulse-type thunderstorm is “up-boom-down-fizzle” in its sequence. The period of severe weather may last only 10 to 15 minutes. But in those 10 to 15 minutes, very frequent lightning can trigger house fires and knock out utility transformers. Portions of the downdraft can accelerate to very high speeds, unleashing a microburst of intense gusts (60 to 80 mph) that down trees. Large hailstones can pelt cars, siding and windows.
Days that feature weak wind shear often have weak winds in general throughout the deeper atmosphere. This means the storm cells are not pushed rapidly along. They loiter, or even become stationary, such that the torrent of rain falls in one spot for the duration of the storm. This can lead to localized flash flooding.
Several locations affected by these storms Tuesday afternoon picked up a quick two to three inches of rain.
When a pulse storm starts to fade, new cells can erupt downwind from the cool air it exhausts, which is what gave rise to the follow-on storm that unloaded large hail on the Mall, Capitol Hill and near Nationals Park about 5:30 p.m. That new storm became particularly intense when it combined or merged with the decaying first storm that helped feed it.
Below find some images and video of the storms along with their hail and wind damage.
Storms in action
Matthew Cappucci and Ian Livingston contributed to this report.