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Bomb cyclone slinging high-impact atmospheric river into California

The Washington Post 1/4/2023 Matthew Cappucci, Diana Leonard
Pedestrians walk around a tree that fell during a storm in downtown Sacramento, Calif., on Jan 4. © Fred Greaves/Reuters Pedestrians walk around a tree that fell during a storm in downtown Sacramento, Calif., on Jan 4.

A dangerous storm system is slamming California, with meteorologists at the National Weather Service warning of imminent “widespread flooding, impassible roads, mudslides/landslides [and] rapid rises in rivers/creeks.” The system, which prompted the Weather Service to take the unusual step of urging residents to have “go bags” at the ready and prepare insurance documentation in advance, is set to unleash its harshest conditions Wednesday night into early Thursday.

Virtually the entirety of Northern and Central California is under flood watches and high-wind warnings, with damaging gusts to 60 mph possible. Strong to severe thunderstorms could be in the offing as well, in addition to 2 to 4 inches of rain in the lowlands and more in the mountains. In the highest terrain, the heavy rain will transition to up to 2 to 4 feet of heavy snow. Along the coast, beaches will be battered by large waves and areas of coastal flooding.

Southern California will see heavy rain, strong winds, hazardous surf along the coast and the potential for flooding, too, especially from Los Angeles northward Wednesday night into Thursday.

The disruptive storm comes on the heels of a barrage of other atmospheric rivers, which dropped 11.6 inches of rain on San Francisco in December. The already saturated soils will make renewed flooding occur more quickly, and make it easier for trees to be uprooted.

“Damaging winds will blow down trees and power lines,” the Weather Service warned. “Widespread power outages are expected.”

Crews work to remove fallen trees blocking U.S. Highway 101 in Humboldt County near Trinidad, Calif., on Jan. 4. © AP/AP Crews work to remove fallen trees blocking U.S. Highway 101 in Humboldt County near Trinidad, Calif., on Jan. 4. Floodwaters at Discovery Park, located at the convergence of the Sacramento River and the American River, on Jan. 4. © Fred Greaves/Reuters Floodwaters at Discovery Park, located at the convergence of the Sacramento River and the American River, on Jan. 4.

Ahead of the storm, a mandatory evacuation was ordered for the city of Watsonville, in the Monterey Bay area of California’s Central Coast, because it is prone to flooding.

Happening now

On Wednesday afternoon, the threatening storm system resembled a potent comma-shaped swirl on satellite imagery as it lurked ominously off the West Coast. It was both meteorologically striking and foreboding — even the Weather Service office in the Bay Area tweeted: “As we prepare for the incoming weather, let’s take a moment to pause and look at the visible imagery and marvel at what Mother Nature is sending our way.”

That parent low-pressure system is a “bomb cyclone,” a term describing the storm’s rapid intensification since early in the week. Its minimum air pressure plummeted by about 3 percent in 24 hours, signifying a vacuum-like ingestion of air that is resulting in strong inward winds.

Ahead of the storm’s core, warm, somewhat humid air is streaming north, leading to moderate rainfall across Central and Southern California. Rainfall rates of 0.1 inches per hour were common Wednesday morning. This “appetizer” rainfall was associated with the warm front.

Then a break will come, followed by a more potent batch of rainfall along the actual cold front. That second band is the one that will pack the punch, and include the threat of damaging winds, thunderstorms and coastal flooding.

City flood barriers line the street at Folsom and 17th streets on Jan. 4 in San Francisco. © Marlena Sloss for The Washington Post City flood barriers line the street at Folsom and 17th streets on Jan. 4 in San Francisco.

On Wednesday afternoon, the front was just starting to come ashore along the coast of far Northern California. Weather radar showed heavy rain starting to affect Crescent City, Eureka and Fort Bragg. Winds were gusting to 40 to 60 mph along the coast and over 80 mph on the ridgetops of coastal mountains. More than 20,000 customers were already without power.

Even around the Bay Area, winds were gusting over 40 mph ahead of the front Wednesday afternoon.

The National Weather Service's forecast for rainfall. (WeatherBell) The National Weather Service's forecast for rainfall. (WeatherBell)

Conditions to worsen Wednesday night

The cold front should arrive in the Bay Area by the start of the evening commute and will feature torrential downpours, rainfall rates of 0.25 to 0.5 inches an hour, possible thunder and lightning, and the strongest winds. Immediately ahead of the front, southerly wind gusts of 35 to 55 mph will be common, followed by an abrupt switch to southwesterly winds behind the front. That’s when winds will increase, with gusts of over 60 mph possible at the shoreline, and 45 mph or greater likely inland.

The National Weather Service Storm Prediction Center declared a Level 1 out of 5 risk for severe thunderstorms along the Central California coastline, suggesting the potential for storms with lightning, hail and winds gusting to 60 mph. These storms would accompany the cold front itself.

As the front passes and the winds switch, the onshore flow could result in water being piled against the coastline. Large breaking waves of 22 to 27 feet are expected, along with minor coastal flooding. That could affect western San Francisco during the Thursday morning into early afternoon high-tide cycle. High-surf advisories and coastal flood advisories are in effect.

Around Sacramento and California’s Central Valley, the brunt of the storm is predicted Wednesday night into early Thursday, when the Weather Service expects flooding in creeks, streams and small rivers. “Some of the recent burn scars will also be at elevated risk of mud and debris flow,” wrote the Weather Service office serving the region.

Toward Southern California, the worst conditions are anticipated late Wednesday night into Thursday. Up to 4 to 8 inches of heavy rain on south-facing mountains in Santa Barbara and Ventura counties “could cause significant flash flooding or debris flows across the region in and outside of recent burn scars,” wrote the Weather Service office serving the region.

Two to 4 inches of rain is predicted in Los Angeles, which is under a flood watch.

Parade of storms means drought improvement, escalating flood risk

With a prolonged wet pattern in the forecast, the concern is that a series of closely spaced, stronger storms could continue to bombard the state next week.

“There will be some flooding, the question is just how problematic it becomes, and that’s going to depend mainly on the exact storm sequencing next week,” Daniel Swain, a climate scientist at UCLA, said in a video update Tuesday.

The onslaught of atmospheric rivers has drawn comparisons to a California “megastorm” scenario that could impact the state in the coming decades, in which relentless storms drop 60 to 100 inches of precipitation on the Sierra Nevada, causing widespread catastrophic flooding.

“We’re nowhere near that yet and we’re probably not headed there, but this is definitely one of the higher-impact wet periods we’ve seen in recent years,” Swain said. “Right now, it looks like it could be comparable to what we saw in the 2016-17 winter, which was an exceptionally wet year with some significant flood-related impacts in Northern California.”

A ‘megaflood’ in California could drop 100 inches of rain, scientists warn

Despite the flood concerns, the prolific wet pattern has been good news for the state’s drought. Most of California is now expected to see drought improvement during January.

Swain said that he expects significant, short-term drought relief for Northern and Central California, although the January storms will have little effect on the Colorado River crisis.

“The drought situation is going to look a lot better when we see the next major drought update in either two or four weeks,” he said.

Snowpack off to one of best starts in four decades

The National Weather Service's forecast for snowfall in the Sierra Nevada. (WeatherBell) The National Weather Service's forecast for snowfall in the Sierra Nevada. (WeatherBell)

A very wet December racked up big snow totals in the Sierra Nevada, and statewide snowpack remains well above average for this time of year. On Tuesday, the California Department of Water Resources conducted its first snow survey of the season at Phillips Station, about 15 miles south of Lake Tahoe. Snow water content measured 177 percent of average at the site, which is comparable to the current statewide average of 174 percent. Cold storms this week will only add to those numbers.

“Our snowpack is actually off to one of its best starts in the past 40 years,” Sean de Guzman, manager of the department’s snow surveys and water supply forecasting unit, said during the survey. “However, that doesn’t mean that we’re out of the woods quite yet, and we must continue to remain vigilant and continue to conserve water.”

If February and March turn dry, the picture could look drastically different by April 1, a key date for measuring the water supply expected from mountain snowpack.

“The significant Sierra snowpack is good news but unfortunately these same storms are bringing flooding to parts of California,” Department of Water Resources Director Karla Nemeth said in a statement. “This is a prime example of the threat of extreme flooding during a prolonged drought as California experiences more swings between wet and dry periods brought on by our changing climate.”

The storm Wednesday and Thursday is forecast to produce up to 2 to 4 additional feet of snow in the Sierra Nevada, prompting winter storm warnings. The heaviest snow is forecast above 7,000 feet; snow levels are predicted to be between 4,000 and 5,000 feet as the storm begins, rise to 6,500 to 7,500 feet Wednesday night and then lower to around 5,000 feet toward the storm’s conclusion Thursday.

Active pattern ahead

This is far from the last atmospheric river that will plague the Golden State in the weeks ahead. While 80 percent of the state is facing a severe or worse drought per the U.S. Drought Monitor, too much water in a short period can easily overwhelm soils and cause destructive flooding.

At least three additional atmospheric rivers are expected to drench the state in the next week or so — one over the weekend, one Monday into Tuesday, and another late next week. A “zonal,” or west-to-east jet stream pattern, is largely to blame. Often, during La Niña winters like the present one, weather systems bombard the Pacific Northwest. But at least for the moment, weather systems are instead surfing the jet stream directly into California.

“This is not a ‘one and done’ storm,” the Weather Service office serving the Bay Area wrote Wednesday. “Of course, timing and details of subsequent systems will be subject to change. Be sure to stay tuned to the latest information in the coming days.”

Jason Samenow contributed to this report.

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