Yesterday’s wind damage up in Walker County at Sumiton was very significant, and caused by a “wet microburst”, a short lived wind event that is most often found in summer storms. Wet microbursts are narrow regions of very strong, straight-line winds descending from mid levels of the thunderstorm accompanied by brief intense rain. By definition, the damage is in an area 2.5 miles in diameter or less, and the high winds last five minutes or less. Needless to say, with something that small scale that doesn’t last more than a minute or two, a good warning is almost impossible. There was no severe thunderstorm warning in effect for Walker County when the damage happened, but I sure can’t blame the National Weather Service. There were probably two dozen storms on radar yesterday that looked just like that one, and all of the other ones did not produce that kind of damage.
Many people in Sumiton told us it was a tornado, but the atmospheric conditions yesterday were not adequate for a tornado to form; if you were to go up 15,000 feet, the winds were less than 10 knots. And, even at jet stream level, around 25,000 feet, winds were under 20 knots. Jet stream altitude winds during tornado outbreaks are usually over 100 knots. Those strong upper winds are usually up around the Canadian border this time of the year.
Good ingredients for wet microbursts include very high surface-based instability values and a nice layer of dry air in the mid levels of the atmosphere. As a parcel descends through the dry layer, evaporative cooling makes the air sink faster. Winds at the surface can exceed 75 mph briefly.
We will go back and look at the radar images during the microburst at Sumiton yesterday, we might be able to identify one velocity signature that would have given us a clue, but in real time it is almost impossible to identify a microburst event that lasts for only a few minutes. Just another reminder we all need to keep an eye on the sky when thunderstorms are prowling around the Alabama countryside. Even on a hot, summer afternoon....
Many people in Sumiton told us it was a tornado, but the atmospheric conditions yesterday were not adequate for a tornado to form; if you were to go up 15,000 feet, the winds were less than 10 knots. And, even at jet stream level, around 25,000 feet, winds were under 20 knots. Jet stream altitude winds during tornado outbreaks are usually over 100 knots. Those strong upper winds are usually up around the Canadian border this time of the year.
Good ingredients for wet microbursts include very high surface-based instability values and a nice layer of dry air in the mid levels of the atmosphere. As a parcel descends through the dry layer, evaporative cooling makes the air sink faster. Winds at the surface can exceed 75 mph briefly.
We will go back and look at the radar images during the microburst at Sumiton yesterday, we might be able to identify one velocity signature that would have given us a clue, but in real time it is almost impossible to identify a microburst event that lasts for only a few minutes. Just another reminder we all need to keep an eye on the sky when thunderstorms are prowling around the Alabama countryside. Even on a hot, summer afternoon....