The NWS Storm Prediction Center’s technical background for today’s severe storm threat says, “Sufficient CAPE [convective available potential energy]...and rather strong low/mid level winds will promote a few organized storms capable of large hail and damaging winds.”
What in the world is CAPE?
It’s a measure of potential thunderstorm energy that uses actual and calculated temperatures and humidity at all altitudes up to more than 40,000 feet. In other words, CAPE estimates the “fuel” available to accelerate air upward.
The faster and farther the air goes up, the stronger thunderstorms can be.
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.
Meteorologists knew as far back as the 1930s that CAPE could be calculated, but the calculations are too complex to be done for day-to-day forecasting until the late 1970s when the National Weather Service began installing computers in its offices around the country that could handle the calculations.
The calculations use the latest weather balloon observations from each office’s forecast area. For the Washington D.C. area, they are based on balloons launched at the NWS office in Sterling on the northwestern edge of Dulles International Airport.
Today’s CAPE values (measured in joules per kilogram) may exceed 1,000 according to the NWS.
Meteorologists use a rule of thumb (an approximate mathematical formula) to estimate the fastest possible thunderstorm updrafts:
Multiply CAPE by two and take the square root of the answer for updraft speed in meters per second. This is only rough guess of possible updraft speeds, but it gives you an idea of how fierce thunderstorms could be on a particular day.
For example, if the CAPE is 1,000, you could expect updrafts of approximately 100 mph.
Another rule of thumb is that a thunderstorm’s downdrafts will be about half as fast as the updrafts.
As thunderstorms move in airplanes taking off and landing in the Washington area will be avoiding them by a few miles for several reasons, turbulence among them.
You, as well as the pilots, don’t want to fly into a thunderstorm on a day when the CAPE is 1,000. This could mean flying into a 50 mph downdraft and a second later into a 100 mph updraft.