Potential Category 3 atmospheric river to hit Northern California

Southern California is set to experience a drastic drop in temperatures on Monday while a powerful winter storm is expected to hit Northern California starting late Monday and bring several inches of rain and heavy winds.  

An atmospheric river, or a weather system that moves high concentrations of water vapor outside of the tropics, could on Tuesday bring up 1 to 3 inches of rain to the coastal areas, with higher elevations receiving upward of 3 to 5 inches, the National Weather Service told CBS San Francisco. The atmospheric river could be as strong as a Category 3, with a scale that tops out a 5.

“Locally up to 7 inches are possible over favored peaks and higher terrain of the Sonoma Coastal Range where prolonged moderate to heavy precipitation and higher rain rates are currently forecast,” the NWS told CBS San Francisco. “Last but not least, if that was not enough, there is a slight chance of thunder which has expanded southward to just around San Francisco. Not expecting much more than a rumble of thunder here and there.”

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

Meanwhile in Southern California, the National Weather Service forecasted drastic changes in weather with temperatures dropping up to 20 degrees, as a storm system sweeps through the area into Wednesday.

“Say goodbye to the warmth,” NWS Los Angeles tweeted. “Big drop in temperatures on track between today and tomorrow (Tuesday). Expect 15-20 degrees of cooling thanks to the approaching storm system”

NWS forecasts temperatures in downtown Los Angeles are expected to drop from a high of 79 degrees on Monday to 61 degrees on Thursday, CBS Los Angeles reported. 

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

The plume will move slowly through San Luis Obispo and Santa Barbara Counties all day Tuesday resulting in 1.5 to 3 inches of rainfall, with some foothills like the Santa Lucias garnering up to 5 inches, the NWS said. 

Ventura and Los Angeles Counties could see anywhere between half an inch to an inch of rainfall between four to six hours, the NWS said. 

Meanwhile, Northwest Oregon and Southwest Washington face a storm system that is expected to bring heavy rain and strong winds starting Monday evening that could lead to minor flooding along rivers and creeks, according to NWS Portland. 

“A 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 into Tuesday. Windy conditions inland late Tuesday morning and afternoon.”

The shift in weather comes as millions of Americans deal with a frigid winter storm that has gripped much of the United States.

CBS News has confirmed at least 38 weather-related deaths nationwide from that storm.

The scope of the storm has been nearly unprecedented, stretching from the Great Lakes near Canada to the Rio Grande along the border with Mexico. About 60% of the U.S. population faced some sort of winter weather advisory or warning, and temperatures plummeted drastically below normal from east of the Rocky Mountains to the Appalachians, the National Weather Service said.

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