Tropical Storm Hilary formed Wednesday morning along Mexico’s western coast and is forecast to bring intense rains to Southern California early next week. If it makes landfall, it would become only the fourth storm of at least tropical storm strength to hit the area.

Conditions are favorable for Hilary to increase in intensity over the next two to three days. The National Hurricane Center forecasts the storm’s peak intensity Saturday morning as a Category 3 hurricane with 120 mph winds. The storm is expected to weaken soon after that due to cooler ocean temperatures and potential land interaction with Mexico’s Baja, just south of California. By Sunday, heavy rainfall is expected to reach Southern California and southwest Arizona.
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The National Hurricane Center, a division of the National Weather Service, forecasts that Hilary will transition from a tropical storm to an extratropical storm by Monday morning near the California-Mexico border with 60 mph maximum sustained winds. Long-range computer models show the potential for 40-60 mph wind gusts Monday in Southern California, especially in the mountains.
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The wind damage potential appears to be a minor concern compared to the rainfall threat. There is already a slight risk of flash flooding issued for the region early next week, which is likely to be upgraded to a moderate or high risk if the forecast trend toward Southern California continues.

However, an air mass can sometimes block a storm, driving it north or northeast toward the Baja California peninsula and the west coast of Mexico. Occasionally, a storm can move farther north, as with post-tropical cyclone Kay last year, bringing damaging wind and intense rain to Southern California. Sometimes storms even move across the state; in 1997, Hurricane Nora made landfall in Baja California before moving inland and reaching Arizona as a tropical storm.

Hilary is expected to become a hurricane Thursday, the National Hurricane Center said, as it gains strength from extremely warm waters in the eastern Pacific Ocean. While Hilary is likely to weaken as it tracks northward over colder waters closer to Baja California this weekend, the remnants could bring several inches of rain — and possibly flooding — to areas in the midst of drought.

“Confidence continues to increase on a heavy rainfall, potentially high impact, event unfolding across parts of the Southwest and California Saturday to Monday associated with likely tropical development in the east Pacific,” the National Weather Service said Wednesday.
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Hilary is unlikely to sustain its hurricane strength once it encounters much colder waters by the weekend.

Hilary’s future track becomes more uncertain as the storm moves farther north this weekend, with some models hugging the Baja California coast while others keep it farther west off the coast. That makes it hard to predict the rainfall and wind impacts with confidence.