An atmospheric river is starting to slam Northern California, bringing up to 15 inches of rain over the Bay area, focusing on the North Bay and making travel in the Sierra nearly impossible a week before the Thanksgiving travel season.

The atmospheric river is poised to drench the Sacramento Valley by Tuesday night, according to the National Weather Service. The new storm will surge in strength Wednesday, bringing rain, snow, and high winds that make travel dangerous.

National Weather Service meteorologists predict a ‘moderate to strong’ atmospheric river will arrive on Tuesday and persist into the weekend. Extended periods of rain and mountain snow are anticipated, with the most intense precipitation occuring north of Interstate 80.

Atmospheric rivers are long, flowing regions of the atmosphere that carry water vapor through the sky, about 250 to 375 miles wide, with some growing to more than 1,000 miles long, according to the NOAA. While rivers on land generally flow downhill, atmospheric rivers flow in the direction of moving air created by weather systems.

It’s one of two separate storm systems traveling from the Pacific. As bursts of cold and hot air meet, they develop into different weather phenomena. As the bomb cyclone dissipated, the pressures carried the atmospheric river right over the Pacific Northwest.

“This one is particularly different because it will kind of stall out along Northern California and bring many days of moderate to steady rainfall,” said Courtney Carpenter, a meteorologist at NWS Sacramento. “So, were not expecting widespread flooding at this time but again, a lot of rain is going to cause some issues across the northern portion of the state.”

Northern California’s coastline could receive between 8-12 inches of rain Wednesday through Friday, with “peak figures potentially reaching 20 inches,” according to AccuWeather Local StormMax.

Soaking rain from the storm will reach the San Francisco Bay area, but not until later in the week, predicts Accuweather. The region could face life-threatening flash flooding, swift debris flows, and landslides, according to the service.

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Numerous local schools have planned closures ahead of the expected 15 inches of rain, as local governments gave out warnings. Meteorologists in the area called for a safety alert beyond Interstate 80 and Northern Sacramento Valley, where ‘leaf-littered storm drains’ may heighten the chances of flooding.

California’s Governor Gavin Newsom addressed the looming concerns: As California braces for an onslaught of heavy rain in Northern California and fire weather in Southern California, the California Governor’s Office of Emergency Services is taking proactive measures to protect communities from dual threats: debris flows in recently burned areas and heightened fire risks due to high winds,” he communicated on X.

This area is no stranger to atmospheric river storms, which affected California and Washington last winter as well, causing multiple deaths and millions of dollars in damage.

“The heaviest rain will pummel regions north of the North Bay towards the Northern California coastline. San Francisco and the South Bay skirt the fringe of the intense precipitation zone where “Rainfall will diminish quickly from north to south,” says the forecast.

While Western Washington may dodge the most severe downpours, the state still faces considerable rains, estimated to be between 2 and 4 inches area-wide. Washington state was hit with a cyclone bomb on Tuesday night, leaving nearly 600,000 people without power and killing one person as the entire region was slammed with two heavy storm systems. Thanks to the atmospheric river, they’ll also get a not-insignificant amount of rain as locals work to clean up downed lines and trees.

Los Angeles and its neighbors will get to avoid this atmospheric fallout but are expecting a second, less intense storm trailing from the Pacific anytime later this weekend to early next week. Signs suggest this system could veer further south and escalate into another bomb cyclone.