Mandatory evacuations were underway and some officers were set to go door-to-door to warn residents in Riverside County of a pending storm approaching Southern California Wednesday evening.
Authorities had already asked residents near the Holy Fire burn scar area to voluntarily evacuate Tuesday, before the threat of debris flow worsened.
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The brunt of the storm is expected to pack a wallop Thursday morning.
"It's really going to pick up as we go to bed tonight and then into tomorrow," said NBC4 forecaster Shanna Mendiola. "You'll have a (Thursday) morning commute that's going to be soggy with a break in between, then another round of rain in the afternoon."
Rain totals will range from .75 inch to 2 inches. Three inches are possible in some mountain communities.
The following areas were under mandatory evacuation Wednesday at 3 p.m.:
- Glen Ivy A
- Glen Eden
- Horsethief A
- Laguna A
- McVicker A
- Withrow A
"People in these zones MUST GO NOW," a news release read.
Authorities also advised residents check the city's maps to see which zones were being evacuated.
"Now is the time to grab your documents. Now is the time to get your spare medication, extra water, and make arrangements for wherever you're going to stay whether it's a local hotel or with a family that's outside of the area," said Riverside County Fire Capt. Fernando Herrera.
A reception center was set up at Temescal Canyon High School, located at 28755 El Toro Rd. in Lake Elsinore.
Debris flows were not expected in the areas near the Cranston Fire burn scar.
Residents near the Woolsey, La Tuna Canyon, Creek and Skirball fire areas were advised to be on alert for potential evacuations, though none were in place Wednesday afternoon. The Los Angeles County Fire Department and The Los Angeles County Sheriff’s Department have staffed additional resources in the area as well as a precaution.
CalTrans was also warning drivers to be on alert in Los Angeles and Ventura counties as rocks, debris and mud may spill onto roadways.
The city of Beverly Hills was providing sandbags to residents at 342 N Foothill during business hours.
NBC4's Fritz Coleman said the heaviest rain will fall between 4 a.m. and 7 a.m. Thursday in Orange, Riverside, and San Bernardino counties.
Everyone can expect a soggy morning commute Thursday morning.
Even moderate rainfall can produce flooding in burn areas because they lack the vegetation that would normally absorb water. Fire-scarred hillsides are left with a repellent layer that blocks water absorption. If it's not absorbed by the soil, rainwater simply washes down the hillside, sometimes with enough force to move boulders, tear out trees and damage buildings and bridges.
The potential for destruction was illustrated in January when an early morning downpour in the Thomas Fire burn area triggered a mudflow that killed 21 people in the Santa Barbara County community of Montecito.
NBC4's Jonathan Lloyd contributed to this report.