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Intense Pacific storm to hammer Southern, Central California

AccuWeather 3/21/2023 Alex Sosnowski

The latest major storm in a series from the Pacific Ocean began affecting California Monday night. The storm will seriously impact Southern and Central California with torrential rain, high winds and heavy mountain snow into Wednesday. The likelihood exists of road closures, widespread power outages and risks to lives and property in the region, AccuWeather meteorologists warn. The storm will also hurl significant rain and snow over the interior Southwest, including the Colorado River basin.

"The heaviest rain will fall along the Central California coastal ranges, in the lower and intermediate slopes of the Southern California mountains, as well as the Sierra Nevada foothills," AccuWeather Senior Meteorologist Heather Zehr said. In these areas, a general 2-4 inches of rain will fall with pockets where 4-8 inches will pour down with an AccuWeather Local StormMax™ of 10 inches.

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Rain will be heavy enough to aggravate flooding over the San Joaquin River Valley and some of its tributaries. Prior storms have already led to flooding in the region, and fears remain this spring as many reservoirs have become filled to capacity. Water management officials are now releasing water in anticipation of melting snow from the mountains later this spring.

Rain overspread much of coastal Southern California during Monday night and will continue at varying intensity through Wednesday. The heaviest rain in the Los Angeles and San Diego metropolitan areas will occur into Tuesday night with a high risk of street flooding, debris flows and road washouts in the surrounding hilly and mountainous terrain.

Those who must travel amid the storm will likely face challenging travel conditions in coastal areas of Southern California, according to AccuWeather meteorologists. Flooded roads could cause some vehicles to stall. Fast-moving water could sweep some vehicles away, while road washouts and mudslides pose a significant risk to motorists.


Downtown Los Angeles has received twice its historical average rainfall since Dec. 1, with 22.30 inches of rain, compared to the average of 11.06 inches through March 20. The storm into Wednesday could push the total past 2 feet of rain since the start of December.

The San Francisco and Sacramento metro areas will be on the northern edge of this storm. San Franciso has picked up 26.30 inches of rain since Dec. 1, which is 1.9 times that of the historical average through March 20. Sacramento has received 1.6 times the historical average rainfall during the same period.

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"In the Bay Area, the bulk of the rain will occur into Tuesday afternoon," Zehr said. "Through Tuesday afternoon and into Tuesday night, the rain will be focused from the Santa Cruz mountains down through Monterey Bay and toward San Luis Obispo County, as well as across much of Southern California."

The storm will pack a high amount of wind energy, similar to a hurricane in some cases, through Tuesday night. In Southern California, AccuWeather Local StormMax™ wind gusts to 75 mph (hurricane force) are possible along the coast and to 100 mph over the ridges, peaks and passes.

"Gusts near 80 mph can occur over the southern deserts including in the Antelope and Lucerne valleys," Zerh said. Winds over the interior deserts and in parts of the Southwest are likely to remain strong through Wednesday.

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"There is the risk of a large number of trees to be toppled with the associated risk of a large number of power outages during the storm, especially along the coasts of Central and Southern California from just south of the San Francisco Bay Area to San Luis Obispo and Santa Barbara, with another high-risk zone from Long Beach to San Diego," AccuWeather Chief Meteorologist Jonathan Porter said. The risk could be increased if an intense area of low pressure makes landfall in Central California rather than a storm that meanders slowly offshore.

The storm may end up being one of the most intense March storms for the Bay Area, based on central atmospheric pressure, on record. At San Francisco International Airport, the lowest pressure observed for any storm in March is 29.24 inches of mercury (990.2 millibars).

Along with trees being top-heavy from the amount of water they have absorbed over the winter while bobbing in saturated ground, the wind direction of the strongest gusts could prove detrimental.

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The strongest winds during the storm will tend to be from the south and southwest. Typically the strongest winds come from other directions in the region, such as the west, northwest or some easterly component, Porter explained. Because of this, the trees' roots grow in such a way to resist the winds from these directions, but not so much from gusts from the south and southwest. As a result, it may take less wind than usual to knock over trees when the wind is blowing from the south and southwest.

Not only will falling trees bring a heightened risk of power outages but also another hazard for motorists, pedestrians and property owners.

While the heaviest snow will fall on the central and southern Sierra Nevada, enough snow will fall along Interstate 80 and Donner Pass, California, to lead to travel delays and possible road closures into Wednesday.

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Snow levels will dip as the storm progresses in Southern California.

"Following slightly warmer air that will cause snow levels to rise into Tuesday evening, an invasion of colder air into the storm will cause snow levels to dip to around 3,000 feet Tuesday night into Wednesday morning," Zehr said. This means that up to several inches of snow may fall on the high ground of interstates 5 and 15 in Southern California, known as Tejon Pass and Cajon Pass. Motorists will experience significant delays and possible road closures due to high winds, heavy snow and slippery conditions.

Up to several feet of snow will fall on the high-elevation communities north and east of Los Angles, such as Big Bear Lake and Wrightwood. These locations were pummeled with yards of snow from storms in recent weeks.

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As the storm pushes inland Wednesday, enough dry air will likely mix in to cause rainfall to become more showery. However, any breaks of sunshine in between can cause the showers to grow into gusty thunderstorms with hail in Southern California.

The same storm will also pump a significant amount of moisture across the interior Southwest through midweek. Areas of rain, mountain snow and thunderstorms are all in the mix for the storm.

Downpours may be intense enough to lead to flash flooding in Las Vegas and Phoenix. Meanwhile, Flagstaff, Arizona, will pick up 6-12 inches of snow from the storm, with the worst travel conditions from Tuesday night through Wednesday.

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The storm will bring another layer of snow on the mountain in the region, which will melt later this spring and boost water levels on the Colorado River and eventually Lake Mead.

Denver is likely to stay on the storm's warm side, with only a couple of showers in the offing from Wednesday to Thursday.

Following the storm, a break is in store for California from late this week through this weekend.

However, AccuWeather meteorologists are already tracking two storms that may join up early next week with possible adverse impacts in the state.

One storm will move from Alaska to British Columbia and the other will spin over the central Pacific over the next few days before a possible merge just off the West Coast of the United States this weekend.

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The current storm and additional storms early this spring, combined with melting snow later on, will continue to hack away at the remaining drought in California and much of the Southwest moving forward.

A significant amount of drought has already been wiped away, and the severity of the long-term drought has been rolled back in other areas from the storm onslaught this season.

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