San Francisco, Los Angeles to receive more heavy rain as 2023 gets underway

More moisture-packed storms are expected to pay a visit to California during the first week of 2023 following an atmospheric river that dropped a historic amount of rain on the San Francisco Bay area as well as heavy snow in higher elevations on New Year's Eve.

Because of the saturated state of the ground, another atmospheric river during the middle and latter part of the week is likely to lead to more flooding and mudslides in the Golden State, AccuWeather meteorologists warn.

Meteorologists describe an atmospheric river as a plume of rich moisture that extends from the tropical part of the ocean to land and can produce copious amounts of rain and mountain snow should the firehose effect linger.

Not only did Downtown San Francisco experience its wettest calendar day since Nov. 5, 1994, due to the atmospheric river on Saturday, but it was also the city's second wettest day in 174 years of record-keeping.

The downtown area picked up a whopping 5.46 inches of rain and was just 0.08 of an inch shy of tying the current record. The single-day rainfall accounted for nearly 50 of the entirety of the rain that fell in December.

There were numerous incidents of road closures, flooding and mudslides in Northern and Central California. A 72-year-old man was killed after he was struck by a falling tree at Lighthouse Field State Beach in Santa Cruz, a city located along the Central California coast.

Drenching rain also swept through Southern California with close to 1 inch falling in Downtown Los Angeles and San Diego. Just under 0.50 of an inch of rain fell in the desert city of Palm Springs, Calif.

More rain is coming to much of California this week, forecasters say.

A weaker storm, when compared with the one that moved through on Saturday, will roll in across Northern California on Monday afternoon and Monday night before it reaches into Southern California on Tuesday. Rainfall with this storm will be relatively light, ranging from a few tenths of an inch to about 1 inch.

The system from Monday to Tuesday will precede a larger and more disruptive storm later in the week.

A much more potent storm will be waiting in the wings over the Pacific and that system is poised to overspread Northern California late Tuesday night and Wednesday. While this storm may have a less-intense firehose effect from the ocean, compared with Saturday's storm, it will still unload heavy rain and result in flooding and mudslides.

"The storm is likely to gain strength and slow down as it approaches California at midweek," AccuWeather Meteorologist Joe Bauer said. A narrow ribbon of tropical moisture will extend from near Hawaii to parts of California for a time.

As the storm stalls, it will pump large amounts of moisture over much of California for an extended period from Wednesday night to Thursday night.

"The slow-moving nature of the storm and its moisture will lead to multiple hours of moderate to heavy rain at low and intermediate elevations of California," AccuWeather Senior Meteorologist Joe Lundberg said.

A general 2-4 inches of rain will fall from the storm in San Francisco, as well as coastal areas and the west-facing mountains farther to the north and farther south through Big Sur and perhaps to Santa Barbara, Cali., according to AccuWeather's team of meteorologists. Locally higher amounts of 6 to perhaps 8 inches may fall in the rainiest locations.

The Los Angeles area is likely to receive about 1-2 inches of rainfall from the storm with lesser amounts forecast toward San Diego. Should the atmospheric river set up farther south than the Big Sur area, then more of coastal Southern California could receive heavier rain, which would lead to a greater risk of flash flooding and mudslides.

The mid- to late-week storm will result in a high risk of dangerous and perhaps life-threatening flash flooding and mudslides, especially in the northern and central parts of California. Motorists may need to seek alternative routes as water flows across and collects on low-lying portions of streets and highways. Winding, hilly secondary roads may be especially dangerous in this situation where debris is deposited or the pavement is washed away.

The saturated state of the ground combined with winds from the storms can lead to trees toppling over.

Unlike the storm during the New Year's weekend, which began as rain before transitioning to heavy snow, the storms this week will have cooler air to work with.

The quick change from rain to intense snow on Saturday caused Interstate 80 to become clogged with stalled vehicles as inches of snow evolved into feet of snow.

Snow levels this week will be at or just below Donner Pass and Truckee, California, for the duration with periodic wintry conditions. However, since the main focus of the storm's moisture will be directed toward coastal areas of California, rather than from the coast to the Sierra Nevada, snowfall is likely to average 6-12 inches with locally higher amounts over the mountains, instead of the several feet of snow that quickly piled up on Saturday.

As more snow piles up in the high country of the Sierra Nevada, the risk of avalanches will increase.

The Pacific storm train is likely to continue through next week, but conditions will continue to evolve during the middle portion of January.

Changes in the jet stream and surface weather patterns will put some resistance on the amount of moisture available to the storms in California, moving forward during the middle of the month, Bauer said.

"There will be storms with some episodes of rain and mountain snow in California during the second and third weeks of January, but the strongest storms and heaviest precipitation are likely to be focused more on the northwest U.S., rather than farther to the south," Bauer said.

The Pacific storm train is likely to continue through next week, but conditions will continue to evolve during the middle portion of January.

Changes in the jet stream and surface weather patterns will put some resistance on the amount of moisture available to the storms in California, moving forward during the middle of the month, Bauer said.

"There will be storms with some episodes of rain and mountain snow in California during the second and third weeks of January, but the strongest storms and heaviest precipitation are likely to be focused more on the northwest U.S., rather than farther to the south," Bauer said.

In terms of drought relief, the storm train has already been beneficial. Many reservoir levels in California were on the rise during late December. Runoff from prior and upcoming storms will continue to raise water levels in the many rivers and reservoirs in the coming days.

As runoff continues over the next several weeks, it is possible for some reservoirs to approach full capacity in the northern and central parts of the state.