Potential Category 4 atmospheric river to hit Northern California

Southern California experienced a drastic drop in temperatures on Monday, with wet weather expected through the week, while a powerful winter storm hit Northern California, starting late Monday and bringing several inches of rain and high winds.

An atmospheric river, or weather system that moves high concentrations of water vapor out of the tropics, could bring 1 to 3 inches of rain to coastal areas on Tuesday, with higher elevations receiving 3 to 5 inches, the National Weather Service said CBS San Francisco. An atmospheric river can be as strong as a category 4, with a scale that exceeds 5.

Accompanied by a mix of weather signals, a rising jet stream pushed a powerful winter storm system toward California’s Bay Area on Monday. CBS San Francisco reported. The National Weather Service has issued a series of updated warnings targeting area communities, including a flood watch for cities such as San Francisco, Watsonville, Pacifica, Santa Cruz, Scotts Valley and Boulder Creek. These warnings went into effect late Monday evening and are currently set to remain in effect until Tuesday afternoon.

By Tuesday morning, experts predicted significant runoff from the surrounding mountain ranges could lead to flooding in rivers, creeks, streams and low-lying areas, the weather service said in consultative, adding that flooding can occur in urban areas as well as in areas with poor drainage. Storm drains and ditches may become clogged with debris, the weather service warned.

“Locally, up to 7 inches is possible over favored summits and higher terrain of the Sonoma Coast Range, where continued moderate to heavy rainfall and higher rain levels are currently forecast,” the NWS told CBS San Francisco. “Last but not least, if that’s not enough, there is a small chance of thunderstorms that have extended south to around San Francisco. I don’t expect much more than a rumble of thunder here and there.”

The National Weather Service said a flood watch is in effect for the North Bay, San Francisco and the coast. A wind advisory has also been issued for coastal areas from Sonoma County to Santa Cruz County.

The agency’s San Francisco bureau shared updated guidance early Tuesday morning and noted at the time that “moderate to heavy rain” was expected throughout the day, with 5 inches expected locally and between 2 and 4 inches expected along the coast. Between 2 and 3 inches of rain are expected farther inland, with slightly less precipitation forecast for valley communities, the weather service said. The advisory is expected to remain in effect until 6:00 PM PST Tuesday evening.

“Localized flooding is likely to occur, particularly in urban areas where ponding on roads or poor drainage is a common problem,” forecasters said. “Excessive runoff can lead to flooding of rivers, creeks, streams and other low-lying areas prone to flooding. Travelers should plan for wet travel on Tuesday and allow extra time to get to their destination.”

Monday night’s wet weather is already causing problems for BART trains in the Bay Area, the agency said, saying at the time that transit passengers should brace for delays of up to 20 minutes across the system. The weather service noted that rain began falling in areas around the San Francisco airport around 5:50 p.m. Monday, according to CBS San Francisco.

The weather service also issued a wind advisory that went into effect late Monday night and was scheduled to remain in effect through Tuesday morning for coastal communities stretching from Sonoma County to Santa Cruz County.

“South wind 20 to 30 mph with gusts to 50 mph expected,” the weather service said. “Local gusts up to 60 mph over ridges and tops.”

Authorities in California warned of the potential for falling trees and tree limbs and possible power outages as a result of the wind forecast, CBS San Francisco reported, noting that beach hazard statements and high surf warnings were also issued for coastal beaches due to “dangerous swimming and surfing conditions”.

Meanwhile in Southern California, the National Weather Service predicted drastic weather changes with temperatures dropping into the low 20s as a storm system moves through the area on Wednesday.

“Say Goodbye to the Heat,” NWS Los Angeles tweeted. “A big drop in temperatures is expected between today and tomorrow (Tuesday). Expect 15-20 degrees of cooling thanks to the approaching storm system”

The NWS forecast that temperatures in downtown Los Angeles are expected to drop from a high of 79 degrees on Monday to a low of 61 degrees on Thursday. CBS Los Angeles reported.

A low pressure system currently forming is set to move across Washington late Tuesday and pull a plume of very moist air over California early Tuesday into early Wednesday, according to the NWS.

The column will move slowly through San Luis Obispo and Santa Barbara counties throughout the day Tuesday, bringing 1.5 to 3 inches of rain, with some foothills like the Santa Lucias collecting up to 5 inches, the NWS said.

Ventura and Los Angeles counties could see between half an inch and an inch of rain between four and six hours, the NWS said.

Meanwhile, Northwest Oregon and Southwest Washington are facing a storm system expected to bring heavy rain and high winds starting Monday night, which could lead to minor flooding along rivers and streams, according to NWS Portland.

“Strong frontal system brings heavy rain and strong winds to NW OR and SW WA through Tuesday,” NWS Portland tweeted. “Strongest winds along the coast increasing tonight Tuesday. Windy conditions inland late Tuesday morning and afternoon.’

The weather change comes as millions of Americans deal with frigid temperatures winter storm which covered much of the United States.

CBS News has confirmed at least 62 weather-related deaths across the country from this storm.

The storm’s reach is nearly unprecedented, stretching from the Great Lakes near Canada to the Rio Grande along the Mexican border. About 60 percent of the U.S. population faces some kind of winter weather warning or advisory, and temperatures fell sharply below normal from east of the Rockies to the Appalachians, the National Weather Service said.

Thousands of American flights were canceled Saturday and nearly 3,000 as of Sunday night, according to the tracking site FlightAware.