Tropical Cyclone Hilary - NWS San Diego provides weather decision support for Tropical Cyclone impacts in Southern California

The first ever tropical storm landfall in San Diego County occurred August 20, 2023. WFO San Diego recently became an official full tropical program office, including tropical hazards, and as a result, this was the office’s first official tropical product issued (TCV). Prior tropical cyclones to make landfall nearby were in 1939 in Long Beach and Hurricane Nora in 1997, which made landfall in the Imperial Valley. Given the rarity of a tropical system reaching Southern California and the typically dry climatology in August (other than monsoon thunderstorms), the storm received a lot of attention.

At one point TC Hilary was a category 4 strength cyclone south of the Baja Mexico Peninsula, and was consistently forecast by the National Hurricane Center to make landfall in San Diego County. Partner Impact-based Decision Support Services went into overdrive, including five live webinars, five YouTube recordings, daily email updates, as well as specific rainfall rate forecasts for burn scars and urban runoff. On August 20, mandatory evacuation orders and warnings (volunteer) were issued for several communities, including burn scar areas. The San Diego County Emergency Operations Center was fully staffed, including WCM Alex Tardy, and featured a media press conference and dozens of interviews. Other regional and county EOC activations were provided virtual briefings up to twice daily, including CalOES Southern Region. The partner briefings and dozens of local, national, and international media emphasized the potential for nearly a year's worth of rainfall in the lower deserts and 5 to 10 inches in the mountains, with potential for a foot, plus a range of 1 to 3 inches for urban areas of San Diego, Irvine and Riverside. Along with the rainfall, strong winds in a Santa Ana-like fashion were predicted in the mountains and passes, including the deserts and parts of the immediate coast and valleys.

Tropical Cyclone Hilary left significant impacts on Southern California. All areas received 1 to 3 inches of rain and wind gusts of at least 30 to 40 mph, while many desert and surrounding mountains received 3 to 8 inches of rainfall in less than 18 hours. The largest rainfall total was 13.54 inches on the desert side of the San Bernardino mountains at 8000 feet, resulting in massive flooding and debris flow that crossed and shut down I-10 in the Coachella Valley. The strongest wind gust was 84 mph in the foothills of the San Diego mountains. Widespread flooding of rivers, low lying areas, and deserts resulted in search and rescue efforts, and floodwaters led to major road and railroad closures and bridge washouts. Debris flows in flash floods moved boulders and mud, trapping residents and cutting off access for multiple bridges and washing out state and county roads with as much as 6 feet of rock and debris. Flash flooding and rainfall rates were as high as 500 year return intervals in Mission and Whitewater drainages into the Palm Springs region. Mud, debris and water flows trapped residents throughout the Coachella Valley including inundation of I-10 resulting in a closure and leading to rescues and stranded dozens of vehicles around Palm Springs. State highway 38 was severely damaged when the Santa Ana River carved a new path during flash flooding following 9 inches of rain in 18 hours. This presentation will cover the impacts in detail as well as the partner feedback about the NWS services in a rare event.

NWS core partners users were surveyed following the tropical cyclone impacts and these results will be shared in the presentation including damage assessments and evaluation of services.