A series of Pacific storms will shift southward this weekend, bringing heavy rain and flooding risks to much of California through the week of Christmas. Snow will continue to accumulate in the Sierra Nevada, increasing runoff concerns.
Pineapple Express moves into Northern California
“The upcoming atmospheric river that will focus on Northern California from late Saturday night to Monday qualifies as a pineapple express,” said AccuWeather Senior Meteorologist Brett Anderson. A pineapple express is a moisture-laden atmospheric river that stretches from near Hawaii to the U.S. West Coast.
This storm follows one that will bring heavy rain and flooding to western Oregon later this week, with some rain affecting Northern California into Friday night. Forecasts call for 4–8 inches of rain in the northwestern corner of California, around 0.5 inch in parts of the Sacramento Valley, and up to 4 inches in the Siskiyous and northern Sierra Nevada.
“The storm from Saturday night to Monday will bring more substantial rain to Northern California,” said AccuWeather Lead Storm Warning Meteorologist Michael Stahlman. Rainfall could reach 6–12 inches in a few hours on west-facing slopes of the coast ranges, Siskiyous, and northern Sierra Nevada, raising the risk of rapid small stream and short-run river flooding.
San Francisco is expected to see 2–4 inches of rain. Slick roads, ponding in low-lying areas, and urban flooding where storm drains are blocked are likely.
New storm forms mid-Christmas week
The atmospheric river will ease Monday night as a new storm develops off the West Coast, spreading rain across much of California during the week of Christmas. “Additional rain from Tuesday to Thursday in parts of Northern California, particularly the west- and southwest-facing slopes of the mountains, will lead to a medium to high risk of flooding,” said AccuWeather Storm Warning Meteorologist Julia Williams.
Coastal Southern California may see rain as early as Tuesday. Drenching downpours and urban flooding are expected in Los Angeles and San Diego from Wednesday to Friday, potentially disrupting outdoor holiday activities. Showers could reach inland areas, including the California deserts, Las Vegas, and Phoenix.
“As the rain expands into the southern part of California, there is a medium to high risk of flooding, especially in the south- and west-facing slopes of the Transverse Mountains from Tuesday to Thursday,” Williams said.
Sierra Nevada snow forecast
Snow levels will start high this weekend but will fluctuate throughout the week. Several feet of snow are expected on ridges and peaks, benefiting ski resorts and contributing to spring runoff, as the season has seen little snow so far. Snow may occasionally dip to Donner Pass along Interstate 80, which could create travel difficulties. Motorists are advised to monitor local forecasts for updates on heavy snow timing.
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