Why Tropical Storm Hilary lashed California

Aug 21, 2023, 6:06am EDT
A man looks at the sea on the beach as Tropical Storm Hilary hits Baja California state, in Tijuana, Mexico August 20, 2023.
REUTERS/Aimee Melo
North America
Jenna Moon/

Tropical Storm Hilary hit California and Mexico, bringing wide-spread flooding and record rainfalls. The storm weakened from a Category 4 hurricane as it made landfall Sunday. In Mexico, one person was killed after flash-flooding destroyed roadways.

California is typically protected from the hurricanes and tropical storms that lash the east coast. A rare set of conditions all had to transpire for Hilary to hit the state: July, the hottest month ever recorded, meant warmer-than-usual ocean temperatures in the Pacific. Typically, cold waters that traverse down from Alaska act as a deterrent against tropical storms. "To see a storm of this magnitude in this part of the world — and at this time of year — is highly unusual," Kristy Dahl, principal scientist at the Union of Concerned Scientists, told the Los Angeles Times.1

Scientists have been linking extreme weather events to a changing climate. Extreme event attribution, a new branch of climate science, links abnormal weather patterns to the anthropogenic climate change. As the earth heats past 1.1 degrees Celsius of warming, hurricanes are getting more intense. In Hurricane Hilary's case, the storm intensified from a tropical storm to a Category 4 storm in just 24 hours. The rapid intensification of storms has become more common with climate change.2

A year's worth of rain is expected to fall over the course of the storm. Climate change is leading to heavier precipitation, because the atmosphere and oceans are warmer, and storms carry more moisture as a result.3