DENVER (AP) _ The fire of lightning and the crack of thunder are unsettling, but meteorologists say there’s more danger when winds are so calm the storm sits still, deluging the land below.
Over the last 20 years, such stalled thunderstorms have been blamed for nearly 400 deaths in Colorado, Wyoming and South Dakota.
The storm that settled over Cheyenne, Wyo., on Thursday evening left at least 11 people dead, with seven others reported missing. In less than four hours, more than 6 inches of rain fell.
In each case, the winds aloft made the difference between a heavy thunderstorm and a killer, hydrologist Larry Powell of the National Weather Service in Denver said Friday.
″Thunderstorms are normally steered by the winds at about 14,000 to 18,000 feet above the surface,″ he said. ″If the winds are very light or essentially calm, the thunderstorms don’t move. They just stay in one place.″
On June 9, 1972, a thunderstorm stalled over the Black Hills and dumped 15 inches of rain overnight. When the overflow tore through Rapid City, S.D., 238 people died.
On Aug. 1, 1976, 139 people died when a thunderstorm stuck over the northern Colorado canyon of the Big Thompson River dumped 12 to 15 inches of rain in five hours.
In June 1965, 14 people died from flooding that came after several storms in eastern Colorado. Most died near the small town of Granada along the Arkansas River.
Late-afternoon thunderstorms are common in eastern Colorado and southeastern Wyoming during the summer months, when daytime heat combines with moisture from the Pacific or the Gulf of Mexico, Powell said.
The warm air rises, condenses when it gets high enough and thunderstorms result.