Bomb cyclone swinging high-impact atmospheric river into California

Almost all of northern and central California is under flash flood watches and high wind warnings, with damaging gusts up to 60 mph possible. Strong to severe thunderstorms are also possible, in addition to 2 to 4 inches of rain in the lowlands and more in the mountains. , the heavy rain will move to a height of 2 to 4 meters of heavy snow. Along the coast, the beaches will be battered by large waves and areas of coastal flooding.

Southern California will see heavy rain, strong winds, dangerous coastal surf and the potential for flooding, especially from Los Angeles north Wednesday night into Thursday.

The disruptive storm followed a barrage of other atmospheric rivers that dropped 11.6 inches of rain on San Francisco in December. The already saturated soils will cause renewed flooding to occur more quickly, making it easier to uproot the trees.

“Damaging winds will bring down trees and power lines,” the weather service warned. “Extensive power outages are expected.”

before the storm, Ordered a mandatory evacuation for the city of WatsonvilleIn the Monterey Bay area of ​​California’s central coast, as it is prone to flooding.

On Wednesday morning, the threatening storm system resembled a strong comma-shaped vortex on water vapor satellites as it lurked ominously off the West Coast. It was both meteorologically striking and embarrassing — even the Bay Area Weather Service tweeted: “As we prepare for incoming weather, let’s take a moment to pause and look at the candid images and marvel at what Mother Nature is sending our way.”

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

Ahead of the storm’s core, warm, somewhat moist air flows northward, leading to moderate rain across central and southern California. Rain rates of 0.1 inches per hour were common, and will continue until about mid-afternoon. This “appetizer” rainfall is associated with the warm front.

Then a break will come, followed by a stronger amount of rain along the actual cold front. The second band is the one that will take the punch, and will include the threat of damaging winds, thunderstorms and coastal flooding.

“Don’t let your guard down if things look quieter than you expected this morning,” the weather service office serving the Bay Area wrote.

Conditions will worsen on Wednesday night

The cold front should arrive along the Northern California coastline around the start of the evening drive and will include torrential rain, rain rates of 0.25 to 0.5 inches per hour, possible thunder and lightning, and the strongest winds. Immediately ahead of the front, southerly gusts of 35 to 55 km/h will be common, followed by a sudden shift to southwesterly winds behind the front. This is when the winds will pick up, with gusts over 60 km/h possible along the coastline, and 45 km/h likely “S or more inland.

The National Weather Service’s National Storm Prediction Center has declared a 1 in 5 risk for severe thunderstorms along the central California coastline, indicating the potential for storms with lightning, hail and 60 mph winds. These storms will accompany the cold front itself.

As the front passes and the winds shift, the onshore flow can cause water to pile up on the shoreline. Large breaking waves of 22 to 27 feet are expected, along with minor coastal flooding. It could affect west San Francisco during Thursday morning through the early afternoon tidal cycle. High surf advisories and coastal flood advisories are in effect.

Around Sacramento and California’s Central Valley, the bulk of the storm is expected Wednesday night into early Thursday, with the weather service expecting flooding in creeks, streams and small rivers. “Some of the recent burn scars will also be at increased risk of mudflow and debris,” wrote the weather service office that serves the area.

Toward Southern California, the worst conditions are expected 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 flooding or debris flows across the region in and out of recent burn scars,” the weather service office serving the region wrote.

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

The march of storms means an improvement in drought, an escalation in the risk of flooding

With a prolonged wet pattern in the forecast, the fear is that a series of stronger than spaced storms could continue to bombard the country next week.

“There will be some flooding, the question is how problematic it becomes, and that will largely depend on the exact sequence of storms next week,” Daniel Swain, a climate scientist at UCLA, said in a video update on Tuesday.

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

“We’re not close to 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 similar to what we saw in the winter of 2016-17, which was an especially wet year with some significant flood-related impacts in Northern California.”

A California “flood” could drop 100 inches of rain, scientists warn

Despite flood fears, the prolific wet pattern was good news for the state’s drought. Most of California is now expected to see improved form during January.

Swain said he expects significant short-term drought relief for northern and central California, though January’s storms will have little impact on the Colorado River crisis.

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

Snowpack is off to one of its best starts in four decades

A very wet December accumulated large amounts of snow in the Sierra Nevada, and the 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. The snow water content was measured to be 177 percent of the site average, which can be compared to the current national 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 last 40 years,” Sean de Guzman, director of the department’s Snow Surveys and Water Supply Forecasting Unit, said during the survey. “However, this does not mean that we are out of the woods yet, and we must continue to be vigilant and continue to save water.”

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

“The significant Sierra snowpack is good news, but unfortunately those storms are bringing flooding to parts of California,” Water Resources Director Carla 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 as a result of our changing climate.”

The storm Wednesday and Thursday is expected to produce up to 2 to 4 additional feet of snow in the Sierra Nevada, prompting winter storm warnings. The heaviest snow is expected above 7,000 feet; Snow levels are expected to be between 4,000 and 5,000 feet as the storm begins, rising to 6,500 to 7,500 feet Wednesday night and then dropping to around 5,000 feet as the storm ends Thursday.

It’s far from the last atmospheric river to hit the Golden State in the coming weeks. While 80 percent of the country faces severe or worse drought according to the U.S. Drought Monitor, too much water in a short period can easily overwhelm soils and cause devastating floods.

At least three more atmospheric rivers are expected to drench the state in the next week or so — one over the weekend, Monday through 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 current one, weather systems bombard the Pacific Northwest. But at least for now, the weather systems in place are gliding in the jet stream directly into California.

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

Jason Semanov contributed to this report.