A 'substantial' atmospheric river will drench the west coast this week

Another atmospheric river arrives in the northwestern United States on Tuesday, bringing excessive rain and mountain snow along the entire West Coast as it drifts south through Thursday.

A plume of deep moisture moving into Northern California on Wednesday and Southern California on Thursday has the potential to bring excessive rain that could lead to flash flooding. Forecasters at the Weather Prediction Center are already pointing to potential risk areas for Wednesday and Thursday, including northern parts of the San Francisco Bay Area on Wednesday and Los Angeles on Thursday.

California has not been hit with the same level of atmospheric river conditions this season as last year, when the state experienced the longest stretch of uninterrupted atmospheric river conditions in the 70 years of data collection on these events. With nine consecutive atmospheric rivers from December 2022 through January 2023, farmlands turned into lakes and snow piled up well above homes in the Sierra.

This midweek storm is expected to be of moderate intensity, but another possible storm following it could escalate the situation.

“We could very well receive February's total normal precipitation within the first five days of the month,” forecasters at the National Weather Service's San Diego office wrote early Monday morning.

Here's what you need to know about the timing.

• Monday: A much weaker atmospheric river makes landfall in British Columbia, and some rain falls in the Pacific Northwest.

• Tuesday: The more robust storm, which forecasters in Seattle have called “a more substantial atmospheric river system,” arrives late in the Pacific Northwest. Recent heavy rains may cause flooding of streams and rivers.

• Wednesday: The main stream of heavy precipitation is focused on Northern California. The worst conditions are expected from the northern Bay Area to the Oregon-California border.

• Thursday: Most of the heavy precipitation will drift south into the Los Angeles area, with rain arriving in Southern California later in the day.

The slower the storm moves, the more precipitation will fall. As of Monday morning, forecasters believe it will be a faster-moving storm, reducing some of the rain totals that could accumulate and reducing the potential risk of flash flooding.

Initially, this will be a warmer storm, limiting snow levels to higher elevations, but colder air will return as the storm moves through.

There is a lot of confidence that the Sierra will receive over a foot of snow with this storm. The highest peaks will likely reach even three times that amount.

Next week's storm

Another storm could hit California late this weekend and into next week. But this far away there is great uncertainty about its potential impact.