Hurricane Hilary was a powerful Eastern Pacific tropical cyclone that gained significance for its extremely rare track into the southwestern United States in August 2023. This storm was the first system to prompt a Tropical Storm Warning for Southern California. Its remnants delivered record-shattering rainfall across parts of the desert Southwest. Hilary’s existence was brief but intense, spanning just five days from its birth as a depression to its dissipation over the Western U.S.
Genesis and Peak Intensity
Hilary’s origins trace back to a tropical wave, developing into a tropical depression by Wednesday, August 16, 2023, south of Acapulco, Mexico. Just six hours later, the system strengthened into Tropical Storm Hilary. Fueled by warm ocean waters, the storm underwent rapid intensification. Within forty-eight hours, Hilary achieved its maximum strength, reaching Category 4 status on the Saffir-Simpson Hurricane Wind Scale. This peak occurred on Friday, August 18, when the storm’s maximum sustained winds reached 140 miles per hour well south of the Baja California Peninsula.
The Northward Track and Transition to a Tropical Storm
After reaching its peak intensity, the hurricane began to accelerate north-northwestward, paralleling the western coast of Mexico. As it moved over cooler waters and encountered increasing wind shear, the storm began a rapid weakening trend on Saturday, August 19. This fast movement was a factor in its subsequent track toward the U.S. border. By Sunday morning, August 20, Hilary had weakened significantly and was reclassified as a tropical storm. The storm’s center made landfall along the western coast of the Baja California Peninsula in Mexico, near San Quintín, later that day. As the storm crossed into northern Baja California, it lost the necessary tropical characteristics and was officially downgraded to a post-tropical cyclone on the evening of August 20.
Timing of the Historic Southwest US Rainfall
Despite the loss of its tropical status, the system’s immense moisture continued its northward surge into the southwestern United States on Sunday, August 20. The storm center, still producing tropical-storm-force winds, moved into Southern California later that evening. Torrential rain began falling across Southern California, breaking numerous historical records. Downtown Los Angeles recorded 2.48 inches of rain, setting a new record for the wettest August day. San Diego similarly broke its daily rainfall record, logging 1.82 inches. Death Valley National Park received 2.20 inches of rain, surpassing the park’s average annual rainfall in a single day. The remnants of Hilary continued pushing inland into Nevada by Monday, August 21, and the system was finally absorbed by a non-tropical low-pressure area later that day.