Parts of Southwest California on Sunday braced for heavy rains — potentially as much as an inch an hour — that could lead to life-threatening floods, officials said, prompting evacuation orders in some places and at least one school district to cancel classes for Monday.

The Weather Prediction Center issued a rare “high risk” prediction of excessive rain in an area that includes Santa Barbara, Ventura and Oxnard, saying eight or more inches of rain could fall in a 24-hour period.

The heavy rains pose flooding risks that could be “especially problematic” in areas previously scarred by wildfires, the center said.

On Saturday night, the National Weather Service said “rain totals still look to be very impressive” — of at least three to six inches across coastal areas and six to 12 inches for the foothills and mountains.

The storm is also expected to bring heavy mountain snowfall and widespread gusty winds, Eric Schoening, an emergency response specialist with the National Weather Service, said during a briefing on Saturday.

“The most significant rainfall totals for this event are expected across the central and southern coastline, including the Los Angeles and San Diego metro areas,” Mr. Schoening said.