Still, Hilary is expected to bring widespread heavy rain to California and the US Southwest, raising risks of power outages, mudslides and flooding along with disruptions to ground transportation and air travel. The rain could start in the region as early as Saturday, with the worst of it arriving late Sunday into Monday.

A wide area could get 2 to 4 inches, Douty said, which would be similar to the powerful winter storms that sometimes hit California and bring flooding. Desert communities, such as Palm Springs, as well as the many railroad lines that cross the area and pass through the mountains could also be imperiled, he said.

Flood watches cover parts of California, Arizona, Nevada, and Utah, including Los Angeles, San Diego and Las Vegas. Flooding could be worst across Southern California, especially around San Diego, the California Governor’s Office of Emergency Services said in a statement.

Eoin Treacy's view

The cold (bone chilling) water off the coast of California is usually enough to deter the formation of hurricanes in the region. By the time Hilary makes it to Southern California it will have lost intensity. The biggest challenge from storms over the last several years has been rain rather than wind speed. On this occasion, the storm could drop anywhere from 2 to 4 inches of rain on Southern California.