Flash Flood in Boise County, Idaho

Rain amounts of 0.2-0.3 inches in an hour caused a debris flow that trapped cars near MP 103 of Hwy 21. This surprisingly small amount of water produced a mudslide because of two important reasons. First, the Wapiti burn scar last year burned very intensely in the flood location, leaving lots of ash and debris on the ground. Second, this portion of Hwy 21 is notorious for avalanches because of it's steep, fairly sparse forest. The very steep open slopes that were primed with ash allowed the small amounts of water to run off almost immediately, carrying debris and mud with it.

Monsoonal moisture surged into the region over a three day period, the most intense of which was the 26th. High atmospheric moisture content combined with weak flow in a monsoonal pattern to produce very moist storms moving very slowly. Additionally, as terrain served as the main trigger for convection, storms would train over the same spots for much longer than normal. Precipitable water was at near record levels around 1.2 inches.