Pair of atmospheric rivers set to bring heavy rain to California. Here’s what to know

Up to eight inches of rain expeced in some areas of northern California, forecasters warned

A massive atmospheric river known as a “Pineapple Express” was barrelling towards the US West Coast on Wednesday, with torrential rain and powerful winds raising the threat of heavy flooding and landslides.

The “Pineapple Express” - so called because it originates near Hawai’i in the tropical Pacific - is the first of two systems to hit the region in the coming days.

Excessive rainfall is expected from the southern Oregon coast down through central California on Wednesday, the National Weather Service (NWS) reported. Up to three inches of rain is expected in the Sacramento Valley.

Heavy snow is also expected at higher elevations, and local authorities warned that scattered flash flooding and landslides, due to oversaturated soils, were possible.

During a webcast on Tuesday, UCLA climate scientist Daniel Swain told reporters that the the approaching storms would be a “classic” atmospheric river and would be “moderate-to-strong”. The upcoming storms will “affect all of California’s major population centers,” according to Mr Swain.

A jaw-dropping view of the storm system approaching the US West Coast on 31 January, 2024 (CIRA/NOAA/NWS)

“Storm number one will be moderate to strong. Nothing earth-shattering. It won’t be any catastrophe, but it could cause fairly widespread urban and small stream flooding in Northern California,” he said.

“The heavier rains and the stronger winds with storm number one are probably going to be from the Monterey Bay area northward. So this will be a storm that affects the [San Francisco] Bay Area during the evening commute.”

Mr Swain noted that colder air will accompany the second dose of stormy weather that is expected to arrive in the state on Sunday.

“With the Sunday-Monday storm in the Sierra Nevada, there actually could be quite a bit more snowfall,” he said.

The NWS also predicted heavy snow in the the Shasta and Sierra Nevada mountains from late Wednesday morning into Thursday morning. On Thursday, the moisture plume will aim at Southern California, producing heavy rain, the forecaster said.

“A front moving onshore over the West Coast will move inland to the Rockies by Friday morning. A plume of moisture will move along the West Coast and then transition into Southern California overnight Wednesday,” the NWS said.

“On Wednesday, the robust plume of moisture will move over California, creating heavy rain over California... The associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, small streams, and burn scars the most vulnerable.”

The Pacific Northwest has seen severe impacts in recent days. On Sunday, a mudslide in Mason County, Washington blocked a major road while a separate slip in Astoria, Oregon, blocked a street and damaged seven homes on Saturday, according to KGW8.

Bay Area forecasters are predicting up to eight inches of rain in some areas of northern California in the coming days.

“Expect issues if traveling on Wednesday, especially in the North Bay and the coastal ranges,” officials with the NWS for the Bay Area wrote on X.

By Thursday, Los Angeles is expected to receive up to four inches of rain.

The Pineapple Express will hit California after southern parts of the state saw widespread flooding from torrential rain last week. More than 4 inches fell within 24 hours, leading to dangerous flooding throughout San Diego.

Hundreds of residents needed rescuing from floodwaters, while the San Diego Fire Department pulled 24 people from the San Diego and Tijuana rivers, the Associated Press reported.

A mudslide blocked a major roadway in Mason County, Washington on Sunday (Washington State Department of Transportation)

Earlier this month, severe winter storms killed at least 89 people across the US as many northern states suffered brutally cold temperatures, heavy snow and ice.

In particular, the Pacific Northwest was hit by a devastating ice storm. In Portland, Oregon, where a quarter-inch of ice coated roads, sidewalks and trees, three people died when a powerline fell on top of their vehicle.

A 21-year-old pregnant woman, her fiteen-year-old brother and her boyfriend were killed as they attempted to leave the car and slipped on a live wire. The nine-month-old baby who was with them survived.