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Bomb cyclone threatens California with risk of high winds, floods, mudslides

By Alex Sosnowki, Accuweather.com
Watery, sticky mud from floods the 101 Freeway in Montecito, California on January 14, 2018. Authorities have no idea when the six-lane U.S. Highway 101 will reopen but acknowledge it could be weeks if the damage is as bad as feared. Photo by Jim Ruymen/UPI
Watery, sticky mud from floods the 101 Freeway in Montecito, California on January 14, 2018. Authorities have no idea when the six-lane U.S. Highway 101 will reopen but acknowledge it could be weeks if the damage is as bad as feared. Photo by Jim Ruymen/UPI | License Photo

A major storm that will rapidly intensify and become a bomb cyclone off the coast of California will slam into the Golden State, unleashing the risk for life-threatening flooding, damaging winds and power outages from Wednesday into Thursday, AccuWeather meteorologists warn.

The storm will arrive just days after the deadly flooding and heavy snow that developed in the state over the holiday weekend, threatening travelers and causing hundreds of thousands of power outages.

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An atmospheric river, or massive plume of moisture originating from the tropical Pacific Ocean some 2,500 miles away, will fuel the heavy precipitation.

Such a weather setup can lead to excessive rainfall and flooding, as well as a thump of snow that can shut down travel where the atmosphere is cold enough.

In this particular case, the event can be classified as a Pineapple Express, since the plume of moisture will extend from near Hawaii.

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"There is a significant risk for flash flooding, and people should watch for and avoid rapidly rising water which can quickly become life-threatening," AccuWeather chief meteorologist Jon Porter said, adding that flooding may result in street and highway closures.

"This will be a dangerous and high-impact storm for California, capable of producing life-threatening conditions and significant disruption which may last several days."

Mudslides are among the dangers that can persist for several days even after the rain ends, Porter explained.

"Not only will this storm be intense tapping into a substantial atmospheric river, but it is also arriving just days after the previous storm brought heavy rainfall and created significant flooding, increasing the impacts and risks that can occur," he said.

The rapid intensification of the storm, in a process known as bombogenesis, will add to the dangers it poses, including strong winds on top of the extreme precipitation. When the barometric pressure falls at least 0.71 inch (24 millibars) in 24 hours, a storm has undergone bombogenesis, and meteorologists will often refer to this type of strengthening system as a bomb cyclone.

"The atmospheric river will initially be directed into Northern California on Wednesday, Central California Wednesday evening and then Southern California later Wednesday night into early Thursday," AccuWeather senior meteorologist Brett Anderson said.

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"Rainfall rates may exceed 2 inches per hour in some areas, especially over higher terrain."

Multiple areas face excessive rainfall on the order of 4-8 inches of rain. Rainfall of this magnitude on top of the several inches of rain that fell on New Year's Eve could result in a dangerous flash flood and mudslide situation in a number of communities of the Golden State.

Areas at greatest risk for upwards of 4 inches of rain from the new storm include coastal areas of California, just north of San Francisco, as well as areas just south of San Francisco to the mountains just north of Los Angeles, AccuWeather meteorologists say.

Another pocket where 4 or more inches of rain is likely to pour down is the west-facing lower slopes of the Sierra Nevada just east of Sacramento to just east of Redding.

San Francisco, Sacramento and Los Angeles will still receive heavy enough rain from the storm to lead to flash flooding, forecasters say. San Francisco is likely to pick up 2 to 4 inches of rain from the storm with 1.50 to 3 inches on the way for Sacramento and Los Angeles.

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Much of that rain may fall within a 12-hour window or less with quick runoff collecting on area streets and highways.

"Motorists traveling through the region may encounter washed-out secondary roadways and may have to take detours," Anderson said.

Another significant problem from this storm will be caused by strong winds, which can approach hurricane force of 74 mph or greater in some locations. The high winds will be created as the barometric pressure plummets in the storm and causes air to rush in toward the system's center.

The force of the wind can cause poorly rooted trees in saturated soil to topple over. Where utility lines are in the way, power outages will result. Hundreds of thousands of utility customers could experience power outages from the storm.

"Residents should make preparations for this storm by filling up their cars with gas, having enough gas to power generators, and stocking up on non-perishable food items in case there is an extended loss of power," Anderson said.

As tree limbs break or trees come crashing down, people in stopped vehicles and pedestrians beneath can be seriously injured.

"Travel during the height of this storm is strongly discouraged," Anderson said.

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"Snow levels in the Sierra Nevada are expected to be mostly above pass level on Wednesday but then begin to lower Wednesday night and Thursday with several feet of snow, especially above 7,000 feet," Anderson said.

"Travel through the higher passes of the Sierra Nevada Wednesday night into Thursday may become very difficult due to high snowfall rates, near-zero visibility and strong winds."

There is the potential for road closures along I-80 due to heavy snowfall. The highway in the Sierra Nevada was closed for many hours over the New Year's weekend as snow piled up fast and stalled vehicles.

Snow levels are likely to remain above Southern California passes, but snow could fall for a time at Siskiyou Summit along I-5 near the Oregon/Northern California border.

The atmospheric river that targeted Northern and Central California Saturday produced the second-highest daily rainfall in the history of downtown San Francisco. The 5.46 inches of rain that fell on the city was second only to the 5.54 inches of rain that fell on Nov. 5, 1994. Records date back to 1849.

The same setup brought a staggering hourly snowfall rate of 7.5 inches per hour Saturday afternoon at the Central Sierra Snow Lab and deposited several feet of snow on the high country of the Sierra Nevada in 12-24 hours.

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Without question, the barrage of storms will help tremendously with the long-term drought in California and many other parts of the West. Some reservoir levels may surge to full capacity before the storm train eases down later in the month.

Reservoir levels as of Jan. 3, 2023.

Many additional storms will roll ashore from the Pacific through the middle of January. The storms may be as little as 12 hours apart at times.

"Residents of the West Coast should continue to frequently check AccuWeather as additional storms are expected to parade into the West Coast through January bringing additional episodes of damaging winds, heavy rainfall and heavy snow in the mountains," Porter said.

Each of these storms will have the potential to produce rounds of flooding rain at low elevations and heavy snow in the high country, AccuWeather meteorologists say.

However, since the overall storm track is likely to shift farther to the north along the West Coast, many of the storms may avoid bringing much rain to Southern California. Storm-related problems are likely to increase in Oregon and Washington.

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