Flash Floods Slow moving or Stalled thunder Storm, especially in canyon areas 144 deaths in July 31, 1976 flood 12 inches of rain in 4 hours (normal ~16 inches /year)
Instead, the storm remained virtually stationary for more than three hours and dumped a foot of rain into the canyon. Eight inches of rain fell in one hour-long stretch, and turned the normally placid two-foot-deep trickle into a raging torrent of water 19 feet high. Sweeping 10-foot boulders in front of it, the wall of water sped down the canyon slope. Cars, campers, and buildings in its path had no chance of survival. NOAA News OnlineNOAA News Online
In two hours, the Big Thompson Canyon flood killed 145 people (including six who were never found) destroyed 418 houses and damaged another 138 destroyed 152 businesses and caused more than $40 million in damages.