The latest ongoing atmospheric river was unleashing heavy rain and thunderstorms, mountain snow and gusty winds over much of the West Coast and parts of the Intermountain West into Wednesday.

The big picture: "Southern California is once again 'under the gun' for numerous instances of flash flooding as the main band of atmospheric river rainfall focuses through the overnight hours there," per a Wednesday morning National Weather Service forecast discussion.

• Flood watches stretched across much of California's coast and the state could expect showers and thunderstorms streaming inland associated with the atmospheric river that began Sunday and saw Santa Barbara Airport shut Monday due to flooding.
• Flood concerns were especially high for the Los Angeles metro area, as heavy rain moved in from the Pacific Ocean throughout the day. Thunderstorms that could cause flash flooding were possible, the NWS stated. There's a moderate risk of excessive rainfall across this area.

Of note: Soils in southern and central California "remain well saturated" from Saturday's lighter rains and previous days' precipitation, per the NWS' Weather Prediction Center.

Context: Parts of California are still recovering from earlier storms that brought mudslides and record rainfall to Los Angeles this month and record rains to San Diego in January.

• There was a moderate risk of excessive rainfall in parts of Los Angeles, San Bernardino, Orange, Riverside and San Diego counties on Tuesday through Wednesday morning. This risk category was a level three out of four when it comes to the NWS' risk ranking system.
• The moderate risk area in effect includes about 15 million people, according to the NWS.
• Portions of Santa Barbara County had seen nearly 10 inches of rain in the two days leading up to Monday evening, according to preliminary NWS data. Parts of Ventura County saw up to nearly 7" and L.A. up to almost 4".

State of play: California Gov. Gavin Newsom activated the State Operations Center on Sunday to help coordinate state, local and federal response to the atmospheric river.

• Santa Barbara County officials issued evacuation warnings over the weekend ahead of the storm and some of these remained in place on Tuesday.
• There were multiple water rescues, including in San Luis Obispo County, which borders northern Santa Barbara County, where crews said they conducted three rescues Monday in Paso Robles city.

#TRAFFICALERT: One-way traffic control is in place on State Route 84 just east of Palomares in Alameda County due to a slipout. For live traffic conditions go to Caltrans Quickmap 🔗https://t.co/cipKtcceGE pic.twitter.com/NCveKYMNXO

— Caltrans District 4 (@CaltransD4) February 19, 2024

In San Francisco, a rare flash flood warning was issued for downtown as thunderstorms rolled through on Tuesday.

• Powerful wind gusts of up to 50 mph Sunday and 60 mph Monday were reported across the region.

Zoom out: President Biden approved California's request for a disaster declaration for San Diego County and offered federal assistance in response to January's severe storm that hit the city budget.

• San Diego and Orange counties each saw over an inch of rain in the 24 hours to Sunday afternoon, per the NWS.

How it works: Atmospheric rivers are highways of concentrated water vapor in the middle atmosphere that can stretch for thousands of miles, channeling water from the subtropics to temperate regions.

• Climate change is adding more moisture to atmospheric rivers, leading to higher rain and snow totals, studies show.

What's next: The storm is likely to wind down across much of the state on Wednesday.

Go deeper: The biggest factors behind California's historic flooding

Editor's note: This article has been updated with new details throughout.