Moisture Sources for Flash Floods in the United States

Flash floods are among the deadliest and costliest natural disasters that affect the United States and are especially difficult from a forecasting perspective as they involve both predicting where, when, and how much rainfall will occur as well as the hydrological response to that rainfall.

In this study, backward trajectories from 19,253 flash flood reports from 2007-2013 published by the National Weather Service were used to assess the nonlocal contribution of the land surface to the moisture budget for flash flood events in the conterminous United States. Parcels were released from flooded locations and traced backward in time for 120 hours using North American Regional Reanalysis data. For moisture increases that occurred within the boundary layer, the land surface properties were recorded. These data were then used to assess the state of the land surface that contributes to parcel moisture for six regions previously identified to be especially “flashy” (Saharia et al. 2017) based on their streamflow records (the West Coast, Arizona, the Front Range, Flash Flood Alley, the Missouri Valley, and the Appalachians).

This methodology, uniquely applied here to the flash flood problem, elucidates that many well-known climatological mechanisms are key components for heavy rainfall in these regions. The land surface was found to have a positive contribution to the moisture budget for flash flood events for the United States, though the extent of this effect varies by region and season.