Extreme Rainfall Rates and Flash Flooding in San Diego, California on 22 January 2024

Thunderstorms with climatologically extreme rainfall rates produced flash flooding in the city of San Diego and adjacent suburban areas to the east during the mid-late morning of 22 January 2024. Local rainfall rates from ALERT and ASOS precipitation gauges between 50 and 75 mm (2 to 3 inches) in one hour resulted in rainfall return intervals of 500 to 1000 years, based on NOAA Atlas 14. This resulted in reported damage to around 1000 homes and businesses, numerous road closures and nearly 200 swift-water rescues. An atmospheric river along and just south of the US-Mexico border, associated with a jet stream which circumnavigated the entire North Pacific had estimated synoptic-scale integrated water vapor transport (IVT) reaching the coast of 400 kg m^-1s^-1. However, a southerly low-level jet over San Diego with 850 hPa winds of nearly 20 ms^-1 and associated with precipitable water of 28-30 mm brought locally higher IVT of over 600 kg m-1s-1. The low-level jet also combined with west to southwest winds of similar speeds at 700 hPa to bring strong shear in the unstable environment. Thunderstorms trained along an approximate west to east line from Coronado through San Diego approximately along California Highway 94 to Spring Valley, maximizing rainfall amounts locally to 100 mm during a 3-hour period.

While details in the location and amounts of an extreme precipitation event like this are difficult to predict as a slight misplacement of the synoptic and mesoscale features by the numerical models that drive the strongest convective thunderstorms, a meteorologist can detect and communicate the possibility of this occurring. For this event, high-resolution models, including the HRRR and a WRF-ARW model run locally by the National Weather Service in San Diego, showed local hourly rainfall amounts of 50 mm or more in one hour up to 36 hours in advance of the rainfall, though not in the exact location. Ensemble models, including ENS, also showed the potential for very high rainfall amounts, including 95th percentile values as high as 125 mm for a 12-hour period over San Diego on 22 Jan 2024 from the ensemble run issued 24 hours in advance. As a result, meteorologists at the National Weather Service Forecast Office in San Diego were able to issue a flood watch the day before the event and produce messaging for emergency managers and other National Weather Service partners mentioning a risk of heavy rain five days before the event. Operational meteorologists creating forecasts and impact-based decision support services need to identify the weather elements creating major events like this which result in around 1000 homes damaged or destroyed and an estimated $90 million of damage.