‘A truly brutal system’: atmospheric river to slam California, West Coast

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

The National Weather Service office that serves the Bay Area has adopted an unusually stern tone in warnings about the forthcoming storm, calling it a “truly … brutal system … that needs to be taken seriously.”

“This will likely be one of the most impactful systems on a widespread scale that this meteorologist has seen in a long while,” wrote one of the agency’s forecasters. “The impacts will include widespread flooding, roads washing out, hillside collapsing, trees down (potentially full groves), widespread power outages, immediate disruption to commerce and the worst of all, likely loss of human life.”

The atmospheric river is connected to a rapidly intensifying low pressure system over the northeast Pacific, which will strengthen at a sufficient rate to be classified as a “bomb cyclone.” Meteorological bombs are named due to their explosive development, which entails a rapid drop in air pressure, which generates a vacuum-like effect that allows the storm to ingest air and brew strong winds.

The Center for Western Weather and Water Extremes, a research agency that rates atmospheric rivers on a 1 through 5 scale based on their intensity and duration, is calling the impending episode a category 3 event. That rating does not take into account the saturated ground ahead of the storm from earlier rains, however, which will enable flooding to occur more easily and heighten the risk of downed trees.

A broad 2 to 4 inches of rain will come down north of the Bay Area between Wednesday and Thursday with amounts of 1 to 3 inches more probable to the south. Rain and howling winds may strike in two waves: the first Wednesday morning and a second late Wednesday afternoon into early Thursday.

At higher elevations, precipitation totals of 3 to 6 inches are likely through Thursday, with isolated amounts up to 8 inches.

The anticipated rain comes atop a whopping 11.60 inches that fell in downtown San Francisco in December, including 5.46 inches on Dec. 31 — the city’s second-wettest calendar day since record-keeping began in 1849. The average monthly precipitation for December is only 4.76 inches.

The weekend’s atmospheric river event unloaded torrential rain in Northern California and Central California, closing major highways and breaching levees. Three to four dozen people were rescued because of high water along Route 99 south of Sacramento, and one person was found dead inside a submerged vehicle, according to the Associated Press.

Flood watches blanket large parts of Central California and Northern California, from the coastline to the Sierra Nevada. San Francisco, Oakland, Sacramento, Fresno, San Jose and Napa are all included.

“Forecast rainfall over already saturated soils will result in widespread flooding impacts regionwide,” the Weather Service office serving the Bay Area wrote in a forecast discussion. “Of greatest concern will be flooding of rivers, creeks, streams, and other low-lying and flood-prone locations. This will also result in extensive street flooding in portions of the Bay Area and Central Coast. Additionally, these conditions will also lead to an increased threat for widespread shallow landslides.”

Winter storm warnings, meanwhile, cover the Sierra Nevada, with snow levels primarily above 5,000 feet. For elevations more than 7,000 feet, 2 to 4 feet of accumulation is expected.

The atmospheric river, which will take the form of a moisture conveyor belt with ties to near Hawaii, is being steered into California by a low pressure system that, by Wednesday morning, will be about 500 miles west-southwest of Medford, Ore. That counterclockwise-spinning low is drawing in tropical moisture, focusing it into a band and then channeling it directly at the coastline.

In California, conditions will deteriorate Tuesday night into early Wednesday as a first surge of moisture arrives ahead of the atmospheric river. The heaviest precipitation is predicted as the system’s cold front approaches from the north Wednesday afternoon and evening before passing to the south Thursday morning.

Near the cold front, rainfall rates may reach a quarter to a half-inch per hour, with snowfall rates in the Sierra Nevada exceeding 2 inches per hour. That slug of heaviest precipitation should only last six to eight hours in most locales, and abruptly end from northwest to southeast on Thursday.

That thick band of frontal precipitation could also include some strong to locally damaging wind gusts. At the coastline, gusts of 40 to 60 mph are possible in Central California, with isolated 70 mph gusts possible. Farther inland, gusts of 45 mph are likely in the lower elevations, except gusts of 55 to 75 mph in the highest mountain summits of the Sierra Nevada.

High wind watches are in effect from the Bay Area to Sacramento. “These winds combined with already saturated soils will likely result in downed trees/limbs causing isolated to scattered power outages as well as potential property damage,” the Weather Service office serving the Bay Area cautioned.

By late Thursday, most of Central California and Northern California will be in the parent low pressure system’s “dry slot,” or a wedge of dry air wrapping into the circulation behind a cold front. That will quickly shut off precipitation, though a few additional downpours are likely due to “dry slot convection” — or the development of isolated pockets of rain beneath the core of high-altitude cold air at the center of a low pressure system.

The expected rainfall would ordinarily be a welcome blessing in a state that’s 80 percent encapsulated by a severe or worse drought, but water is simply arriving at too swift a pace. That’s why the National Weather Service Weather Prediction Center is warning of a level 3 out of 4 red-zone moderate risk of flash flooding and excessive rainfall in its outlooks for the California coastline, writing “numerous flash floods and runoff concerns are likely across this” area.

“Have a ‘go bag’ ready in case you need to evacuate quickly (go to ready.gov/plan or ready.gov/kit if you’re not sure what to pack),” the Weather Service office serving the Bay Area wrote.

The barrage of atmospheric rivers will continue well into next week, at least. Another is slated to hit this weekend, followed by a second Monday into Tuesday next week and a third later in the week. All appear robust and could bring several additional inches of precipitation each. While the rain and snow will help the state’s water supply, flooding could become even more severe as waterway levels rise.

Meteorologists blame an unusually zonal, or west to east, weather pattern for the onslaught of storminess. Often, during La Niña winters like the present, weather systems bombard the Pacific Northwest, including Washington and Oregon. But, at least for the moment, weather systems are instead surfing the jet stream directly into California.

Jason Samenow contributed to this report.