Timeline: When will rains hit California from new atmospheric river storm?

California will be hit by another atmospheric river storm this week. But this one will be warm instead of cold, raising concerns about potential flooding and snowmelt.

The storm

The inbound storm is forming as a northern pressure ridge links with subtropical moisture moving up from Hawaii. Such systems are sometimes referred to as a Pineapple Express and are known to drop heavy precipitation.

Northern California

Thursday: The storm will move in in the evening, bringing rain and snow to high elevations.

Friday: Intense rain will fall, with the possibility of flooding in some river, mountain and foothill areas.

Southern California

Thursday: Rain will begin in the evening.

Friday: The most intense part of the storm is expected.

Saturday: The rains will taper off.

Areas of concern

• According to the National Weather Service, rivers that could see flooding include the Russian River at Hopland; the Salinas River at Bradley and Spreckels; the Merced River at Stevinson; the Tuolumne River at Modesto; the Cosumnes River at Michigan Bar; the Mokelumne River at Benson’s Ferry; and Bear Creek at McKee Road.
• Nearly two dozen additional river locations across the state may surge above their “monitor stage” — meaning they could potentially overflow their banks and cause minor flooding in low-lying areas. That includes multiple locations along the Sacramento and the San Joaquin rivers.
• Officials in Fresno and Madera counties have issued evacuation warnings for residents in their areas, warning of potentially serious flooding from the incoming storm. Fresno County Sheriff’s Office spokesperson Tony Botti said officials are particularly concerned about flooding around Pine Flat, as well as Millerton Lake, which feeds into the San Joaquin River and flows west toward Mendota, Tranquility and other communities.

• At Oroville Dam — which suffered a near catastrophic failure amid a series of atmospheric rivers in 2017 — state operators said they could begin releasing water down the dam’s rebuilt main spillway as early as Friday.
• In the San Bernardino County Mountains, where some residents still remain trapped by snow after historic blizzard conditions dropped more than 100 inches of fresh powder in the area, officials are warning that rain could increase the weight of snow atop structures, potentially leading to roof collapses or other issues. Residents and authorities already have reported several roof collapses from mounting snow, including a grocery store providing critical supplies in Crestline last week.
• Up to 8 inches of rain are possible in the coastal foothills from Morro Bay north, while the rest of San Luis Obispo and Santa Barbara counties could see up to 4 inches. In Ventura County, rainfall of up to 2 inches is possible, while the Los Angeles metropolitan area could get up to 1.5 inches. Temperatures across the Ventura and L.A. County coasts and valleys will be mostly in the mid- to upper 60s, with a chance of 70 degrees in the warmest valley location.