From a violent storm to a ghost of a storm in less than 30 minutes. This is the interesting story of the birth and sudden death of thunderstorm early Monday evening.
When the storm threatened the metro area, it was the only game in town, a rogue that erupted near Winchester and then charged toward the populated Dulles corridor, unleashing damaging winds near Middleburg.
The National Weather Service was so concerned about the storm that it issued a severe thunderstorm warning for the immediate metro area, including the District at 5:44 p.m., while the storm was still in Loudoun County. It predicted winds up to 70 mph. “Damaging winds will cause some trees and large branches to fall,” the Weather Service cautioned. “This could injure those outdoors … as well as damage homes and vehicles.”
But, in the blink of an eye, the storm disintegrated before even reaching Fairfax County. The Weather Service abruptly terminated the warning at 6:02 p.m. Just a few rain drops fell inside the Beltway.
Here’s how it all happened …
The storm cell initiated over the mountain slopes, along the Blue Ridge, as is often the case when large-scale triggers, such as an approaching front, are weak or absent. The updraft rapidly became quite intense, drawing buoyancy from a large reservoir of hot, unstable air. A large mass of condensed water and frozen ice developed in the updraft’s middle and upper reaches. If you somehow weighted it all, it would tip the scales at well over 1 billion tons.
After 30 minutes or so, gravity was the victor. The heavy mass of water could be suspended no longer. Downward it cascaded, as a torrential downpour, dragging the air down with it. Evaporation cooled the air, increasing its density, accelerating its downfall. As it fanned out, the downrush developed a violent core, with winds in the 75-85 mph range.
Over more rural territory, east of Middleburg, Doppler radar picked up an ominous wind signature: Evidence of 84 mph gusts just a few hundred feet above the surface. Spotty tree damage in this region suggests the core of violent winds, known as a microburst, may have reached the ground.
The storm then moved over the radar site at the National Weather Service forecast office in Sterling, Va., entering the “cone of silence” (the radar cannot scan upward, only sideways). And as it moved away from the radar site, toward the east-southeast, something quite unexpected happened.
Up until then — for an hour — the storm was solid and persistent, with an intense core presented on radar (sometimes approaching 65 dBZ) and an ongoing microburst. Then, “poof.” Gone, in less than 30 minutes. Completely.
We meteorologists found this quite astounding. Jokingly, it’s as if the almighty Doppler radar ate the storm, sucking it inside its radome with invisible microwave rays. But its disappearance while transiting the radar was only coincidence. What really happened was a process called “core collapse.”
A “core collapse” is just that: The storm updraft goes up, develops a huge mass of suspended water and ice, then expends itself in an intense downdraft.
It’s all about the wind shear. Faster winds aloft tilt the updraft sideways. The rain falls out from beneath the tilted updraft, unloading it, unburdening it. But on weak shear days, like Monday, storms sow the seeds of their own destruction. The updraft remains vertical, rather than tilting or leaning over. The downdraft is directed straight down, back through the updraft, choking it, smothering it. While a collapsing core of precipitation sometimes generates a microburst, it also quickly ends the storm. Storms on weak shear days “pulse” more than persist.
Monday’s storm seemed as though it would persist, having been intense for at least an hour, a bit longish for a purely pulse-type cell. Early on, there may have been just enough shear to give it a more organized character, facilitating what we call a multicell storm complex. Call it a hybrid storm.
In the end, though, the abrupt collapse of its large core was too much, destroying the delicate balance between updraft and downdraft. A bit more shear and the storm may have survived, unleashing continued severe weather in D.C.’s western suburbs and even inside the Beltway.
Instead, the ghost of the storm — its cool outflow of wind — fanned out and dissipated — the faint blue arc noted in the right side panel in the figure above. We’ll all take a bit of nature’s air conditioning over a violent microburst any day.
Jason Samenow contributed to this post.