Flash Flood in Ada County, Idaho

A training thunderstorm persisted near Arrowrock Dam for about 45 minutes with rain rates between 3 and 6 inches per hour. Rocky cliff faces and steep valleys allowed water to accumulate, though soil/brush retention absorbed most of the rain. While Arrowrock Road was still drivable afterwards, there was evidence of significant water erosion, with multiple spots estimated to have at least 6 inches of running water. The road would have been undrivable during the event.

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.