The first tropical threat to the US this season isn’t on the East Coast or the Gulf of Mexico, it’s in Southern California.
Tropical Storm Hilary formed on Mexico’s west coast Wednesday morning and is expected to bring heavy rain to southern California early next week. Should it make landfall, it would be only the fourth storm of at least tropical strength to hit the region.
Conditions are favorable for Hilary to ramp up her intensity over the next two days. The National Hurricane Center is forecasting the storm’s highest intensity as a Category 3 hurricane with winds of 120 miles per hour for Saturday morning. The storm is expected to weaken soon due to cooler ocean temperatures and possible interaction with Baja Mexico in southern California. Heavy rains are expected to reach southern California and southwestern Arizona through Sunday.
The recorded storm never crossed California as a hurricane, and only three storms reached California as tropical storms: Nora in 1997, Kathleen in 1976, and the Long Beach storm in 1939.Still, there are dozens of tropical rain events drifting into Southern California and the Southwest from the remnants of tropical storms and hurricanes. Most recently, Hurricane Kay killed one person in 2022 when their home in San Bernardino County was engulfed in debris.
Two factors tend to keep Southern California safe from such storms: Colder sea surface temperatures, which take their fuel away, and upper-level steering winds in the eastern Pacific. The El Nino phenomenon this year has resulted in significantly greater ocean temperatures than usual; today, the sea in Malibu registered 70 degrees. The warmest coastal ocean temperatures Los Angeles typically experiences in the summer are 67 or 68 degrees.
Even still, four days in advance is a challenging time to anticipate a hurricane’s path. According to the most recent forecast from the National Weather Service, “There remains a very large spread in the ultimate track so this remains a very low confidence forecast in terms of the track as well as the impacts.” Having said that, almost all of the members of the GEFS ensemble predict moderate to heavy rainfall, especially in the area south of Pt. Conception.
That rainfall is now associated with a large summer accumulation in Southern California, with “multiple resolutions at or above 2.5” and up to 2.9, according to the NWS. So the likelihood of very unusual rainfall is definitely increasing. The official forecast for total rainfall is currently around 1 to 2 inches Sunday through Tuesday, but there is potential for much higher (or lower) amounts depending on the actual runway.
Lightning can also occur which, if accompanied by wind but not rain, can pose a fire hazard.
As for the waves, the NWS says Hilary “will bring potentially hazardous sea conditions in all coastal waters beginning this Sunday.” While the wind/weather impact of these systems is highly uncertain in the region, sustained steep southerly winds and severe storm surges are expected through possible for the coming weekend and beyond.
Surfline’s surf forecaster is currently forecasting waves of 4 to 5 feet in northern Los Angeles County and 6 to 8 feet in northern Orange County.However, these estimates could decrease significantly over the 24-hour period as the storm approaches the Baja California coast, or increase significantly as it advances further into Southern California’s swell window. In fact, as recently as Wednesday, Surfline predicted waves of 15 to 20 feet in north Los Angeles, which is rare in that region in the summer.