Improved Prediction of Atmospheric Rivers That Drive Flood Damages in the Western United States

Abstract

Atmospheric rivers (ARs) are extratropical storms that produce extreme precipitation on the west coasts of the world's major landmasses and have been shown to be an important source of variations in precipitation and streamflow in the western U.S. and globally. ARs have been identified as the primary source of hydrologic flooding in the western U.S., yet their costs remain largely unquantified. A systematic analysis of 40 years of data from the U.S. National Flood Insurance Program (NFIP) establishes that ARs are the primary drivers of flood damages in the western United States. The NFIP claims and payments are combined with a catalog of ARs classified according to a recently developed AR scale, which varies from category 1 to 5. The data reveal that flood damages increase exponentially with AR intensity and duration. AR1 and AR2 storms are mostly beneficial, replenishing the water supply while causing median flood damages of less than a million dollars; AR4 and AR5 storms cause median damages in the tens and hundreds of millions of dollars, respectively. A research effort to improve forecast skill of AR landfall and duration has produced an informative set of AR forecast tools. Discussion of these tools along with examples of their use by water resources and emergency management officials is presented.