The biggest storm of the season so far is expected to flood the San Francisco Bay area from Tuesday to Thursday, with rainfall in urban areas ranging from 5 to 10 centimeters.
Meteorologists are calling the system an atmospheric river, a wet storm that pulls a long column of water vapor into the atmosphere – on average 250 to 375 miles wide – and often called a river in the sky. On the West Coast, atmospheric rivers originate more frequently in the South Pacific, and as they travel from the tropics across the ocean, they collect incredible amounts of moisture.
“It is the first atmospheric river of the season and we expect stronger rains than we saw the rest of the season,” he said. Brayden Murdock, meteorologist at the National Weather Service office in Monterey, California.
Radar images from Tropical Tidbits show the atmospheric river over California on Tuesdays and Wednesdays.
Tropical Tidbits / tropicaltidbits.comThe storm’s door opened in northern California in recent days, with the system sweeping the area on Thursday and Friday, marking the beginning of an unstable period of time.
After a dry and sunny day on Saturday, a second typical winter system will blow up the region from Sunday morning until Monday morning with a breath of cold air, more rain and potentially low snow.
Mount Hamilton in the South Bay, Mount Diablo in the East Bay and Mount St. Helena in the North Bay could be fully pulverized, as could the surrounding hills and mountains.
Snow can impact some roads, such as Skyline Boulevard that crosses the mountains in the counties of Santa Cruz, Santa Clara and San Mateo.
Monday night is expected to be dry before the big soaker arrives on Tuesday night. The strongest rain will occur from Tuesday night until Wednesday, and meteorologist Drew Peterson said that areas like San Jose, protected by mountains, can see up to 1.5 inches, while more exposed areas, like San Francisco and Oakland, up to 3 inches and local mountains six to eight inches.
Meteorologists are watching this event closely, as it can bring potential for mud and rockslides and floods in lower, poorly drained areas. Higher rainfall rates can trigger debris flows in scars from forest fire burns, especially in the Santa Cruz Mountains, where the CZU Lightning Complex charred 86,509 acres in August.
A cyclist crosses a street during a rain storm in the Richmond district of San Francisco on November 17, 2020. The storm is the first strong storm of the season to hit the San Francisco Bay area.
Douglas Zimmerman / SFGATEThe cold and unstable climate continues until the end of next week and the following weekend.
“We are seeing a shift to cooler weather with the reminder of the month and possibly later, as La Niña weakens,” said Peterson.
La Niña, the cousin of the best-known El Niño, is the cooling of equatorial waters in the east and center of the Pacific Ocean that can impact atmospheric conditions worldwide. Under La Niña conditions, the bay area can see a drier climate with storms being pushed into the Pacific Northwest and this is what happened at the beginning of this rainy season.
“We started to see La Niña weaken and it has an immediate effect on our climate,” said Peterson. “At least, I think this is part of what we are seeing here. It is part of the story, but not the whole story.”