One of the key metrics for evaluating prospects for explosive thunderstorm development is called Convective Available Potential Energy, or CAPE. It’s an estimate of “fuel” available to developing thunderstorms.
The CAPE levels Friday night as the derecho approached Washington, D.C. were - in short - astronomical.
CAPE is measured in Joules per kilogram - which is simply a unit of energy. Amounts between 1,500 and 2,500 are considered large, and anything over 2,500 extreme. The CAPE calculated based on weather balloon observations near Dulles airport Friday night was over 5,000!
One of the indicators of the tremendous energy in the atmosphere Friday night was the almost continuous display of lightning. The video above shows the storm over Montgomery Village, Maryland at 10:15 p.m, uploaded by ravens32681.
This incredible amount of CAPE demonstrated the remarkably hot, moist and extremely unstable air mass in place that evening.
Ryan Hanrahan, a broadcast meteorologist in Connecticut, wrote a fascinating (somewhat technical) blog discussion about the derecho and said: “...the environment preceding Friday’s derecho was one of the most impressive, and unstable, you’ll ever see in the northeast.”
In his blog post about CAPE in late March, CWG’s Jack Williams described a back-of-the-envelope way to calculate how strong a storm’s updraft (the air the storm is ingesting) would be based on CAPE:
Multiply CAPE by two and take the square root for the updraft speed in meters per second.
Doing that calculation using Friday night’s CAPE values, we get an updraft speed of 100 m/s, which converts to 225 mph. Jack noted a fair approximation of a thunderstorm’s potential downdraft - the wind gusts we feel on the ground - is half the updraft speed. So Friday night, the CAPE suggested the possibility of downdrafts of over 100 mph. They didn’t quite reach that potential, but close!
Fortunately, typical CAPE values for summer thunderstorms in Washington, D.C. are in the 1,000-2,000 range - which can still produce damaging thunderstorm gusts. But when this metric climbs higher than that, it really demonstrates explosive potential.
Of course, potential is just that. As Jack noted: “CAPE doesn’t say whether that day’s thunderstorms will even form, much less tap all of the potential energy, but it’s a good measure of the potential danger if thunderstorms do form as they are very likely to do today.”
In case you were curious, this morning’s CAPE near Dulles airport was calculated to be around 1,500, and is forecast to rise to above 2,000 this evening. There is a slight risk of thunderstorms, and given this CAPE, they have the potential to produce very strong wind gusts should they develop.