Unique Impacts From Hilary In The United States

From a tropical cyclone rainfall impact perspective, Hilary was an unusual event for much of southern California and a unique event for Death Valley, California as well as portions of the Great Basin and the Intermountain West. The upper level trough that caused Hilary’s recurvature, and Hilary itself, were extremely anomalous events for mid to late August as borne out by NCEP reanalysis standardized anomalies (1979-2009). Sea surface temperatures along the storm’s path were above average, which helped maintain it as a stronger system despite vertical wind shear and land interaction as it approached the American Southwest. As Hilary moved near the coastal ranges of Southern California and moved up the Sierra Nevada, a broad area of heavy rainfall fell across the Desert Southwest as moisture and instability streamed in from the Gulf of California at a scale not seen since at least 1976.

State rainfall records from a tropical cyclone, post-tropical cyclone, or remnant from an eastern Pacific tropical system were set across several western United States from Nevada northward through the Pacific Northwest into Montana. Death Valley, California set a new all-time daily rainfall record when it received more rain on August 20 – 55.9 mm – than it averages over an entire year. Los Angeles, San Diego, and Palm Springs (in California) saw their wettest summer day (June-August) on record. Lakes established by the rains in Badwater Basin persisted into November. It is hypothesized that increasing sea surface temperatures should bring stronger systems out of the tropics and more associated moisture and instability into the American Southwest in the future – and by proxy heavier rainfall. However, the unusual nature of Hilary’s track appears to have been more important to the rainfall across desert and upslope areas of California on the eastern side of the terrain (Peninsular Ranges, eastern end of the Transverse Ranges, and Sierra Nevada) where anomalous moisture transport was 10-20 standard deviations above the 30-year (1979-2009) NCEP reanalysis climatology for mid to late August, with Integrated Vapor Transport values qualifying it as being associated with an extreme to exceptional atmospheric river.

Several low pressure records for the month of August were set near the coast of Southern California close to Hilary’s track, including a couple at sites with longer databases that extend back into the nineteenth century (Los Angeles and San Diego). The upper trough that escorted Hilary inland also helped set August low pressure records much farther up the coast of California into Oregon, including one site (Eureka) with a database back into the late nineteenth century. The downstream impacts of these two systems was the strengthening of a warm core ridge which helped promote an intense heat wave across the Central Plains.