Genesis locations of the costliest atmospheric rivers impacting the western United States

Abstract:

Extreme weather events associated with atmospheric rivers (ARs) have the potential to produce substantial economic damages on the West Coast of the USA. Recorded flood damage costs, sourced from the National Flood Insurance Program, are paired with an AR tracking algorithm to examine the life cycles of the most damaging ARs in the western United States over the last 40 years. The genesis location of landfalling ARs on the West Coast of the USA shifts westward with increasing damages, particularly for ARs that make landfall across northern regions of the West Coast of the USA. Landfalling ARs in Washington and Oregon that originate from the northwest region of the Pacific, westward of 170°W, are more than twice as likely to cause extreme damage (90^th percentile of damage costs) than ARs originating from the northeast region of the Pacific. In Oregon, only 5% of ARs originating eastward of 170°W are found to cause extreme damage (exceeding $25 million) which increases to 12% for ARs originating westward of 170°W. Furthermore, in Washington, the majority (61%) of ARs that bring extreme damages originate from the northwest Pacific, while only 40% of all Washington ARs originate in this location. For ARs that make landfall in California, the genesis location does not change the probability of extreme damage. ARs generating further west (in the northwest Pacific Ocean) are typically associated with higher moisture fluxes and persist for longer, both of which contribute to a higher potential for severe flood damage. A better understanding of the conditions favorable for AR genesis in these key regions may help improve forecasts of these most damaging events.