‘Atmospheric river’ ready to kick up a storm

A powerful “atmospheric river” storm is expected to pummel Northern California starting tonight and deliver heavy rain, gusty winds, downed trees, power outages and rough driving conditions Wednesday and Thursday.

“It is a classic Pineapple Express,” said Bill Patzert, a research scientist and oceanographer who retired last year from NASA’s Jet Propulsion Laboratory in Pasadena. “It’s coming from deep in the tropics south of Hawaii aiming right at the West Coast.”

The storm should bring up to 5 feet of new snow in the Sierra Nevada, forecasters said. The National Weather Service announced flash-flood and high-wind warnings for the

Bay Area, along with Santa Cruz and Monterey counties, from 10 p.m. today to 10 p.m. Thursday. And fears of mudslides prompted Caltrans on Monday to announce it will close a section of Highway 1 south of Big Sur in both directions starting at 5 p.m. today.

In the Bay Area, the first big push of rain is forecast to arrive over the North Bay late today and spread inland Wednesday.

“Rain will spread over the area early Wednesday morning and impact the morning commute quite a bit,” said Spencer Tangen, a meteorologist with the weather service in Monterey. “Rain may be heavy at times as the atmospheric river moves onto land.”

Late last week, meteorologists and scientists studying computer models included the potential for this atmospheric river storm, which at the time was projected to be centered over Central or Southern California.

But over the weekend the forecast shifted, and the storm is now expected to have a greater impact in the Bay Area and Northern California, they say.

This potent storm is the result of a cold front moving down from the Pacific Northwest, combining with a plume of subtropical moisture moving in from Hawaii, according to the weather service.

In the Bay Area, storm totals from tonight through Thursday morning are forecast to range from 2 to 3 inches in the North Bay, 1 to 2 inches along the Peninsula, East Bay and South Bay, and 4 to 6 inches along the coastal mountains, with some mountain areas receiving 8 inches.

“There is still some uncertainty with how much rain” will fall and how long it will last, Tangen said.

Sustained winds of 30 to 40 mph are expected Wednesday with gusts of 60 to 70 mph in the hills.

“Given the saturated soils and expected rainfall, trees will likely come down,” the weather service wrote in its Monday forecast.

Unlike last week’s storms,

this week’s won’t bring snow to Bay Area peaks. Forecast lows overnight are expected to remain in the 40s with daytime highs in the 50s.

Atmospheric rivers are a particularly moisture-heavy, intense type of storm. They can be 250 miles wide, 1,000 miles long and can carry 20 times as much water per second as the Mississippi River where it empties into the Gulf of Mexico.

Such storms are vital to the water supply of California and other Western states, with about a dozen providing up to 50 percent of the annual water supply in most years. They also can cause damage and flooding.

Patzert said as of Monday afternoon it looked as if the storm will cause some localized flooding and snow blizzards in the Sierra, but that overall it will benefit California’s water supply, particularly by boosting groundwater tables still low in some areas after the 201216 drought.

“It could be the most intense storm so far of this winter,” he said. “There’s always minor damage, but the bottom line here is water, and this is good for the

water supply.”

Wednesday’s storm is expected to rank as a 3 on a new scale of 1 to 5 that scientists at Scripps Institution of Oceanography and other research centers developed recently, Jay Cordeira, a meteorology professor at Plymouth State University in New Hampshire, said on Twitter.

Meteorologists still are watching the trajectory of the storm to forecast where the heaviest rains are expected.

“There will be intense downpours in the coastal mountain ranges,” said Andrea O’Neill, a meteorologist and oceanographer with the U.S. Geological Survey in Santa Cruz. “We already have saturated soils, so that becomes a hazard for streams and flooding and landslides.”

“The potential is there for local impacts to be extreme,” she added. “But it depends on where it plays out and where it makes landfall.”

Caltrans announced Monday it will close Highway 1 south of Big Sur beginning at 5 p.m. today in landslide-prone areas. One

closure is at Mud Creek, near Ragged Point, the scene of a massive landslide in May 2017 in which millions of tons of earth buried the road 9 miles north of the Monterey-San Luis Obispo county line. The second closure is at Paul’s Slide, about 13 miles north of Mud Creek near Lucia.

Although small-stream flooding is expected in some places, major rivers are not expected to flood, according to the weather service. But local agencies are watching closely.

“That kind of intense rainfall can cause localized flooding, but we’re not expecting Anderson to fill or anything like that,” said Marty Grimes, a spokesman for the Santa Clara Valley Water District, referring to Anderson Reservoir near Morgan Hill, the largest of 10 district reservoirs. “Anderson has plenty of room in it.”

On Monday, Anderson Reservoir was 37 percent full. The reservoir filled in 2017 during heavy storms and its releases contributed to $100 million in flood damage to downtown San Jose. The water district has been letting water out

since last week to keep Anderson’s level lower as more storms roll in.

One thing is clear now: 2019 will not be a drought year. With recent storms, seasonal rainfall totals in most Northern California cities are close to or above their historical averages for this time of year, including San Francisco (13.72 inches, 92 percent of average), Oakland (10.84 inches or 87 percent), and San Jose (9.27 inches, 100 percent.)

On Monday, the Sierra Nevada snowpack measured 129 percent of its historical average for this time of year.

More snow is on the way. Jim Wallmann, a meteorologist with the weather service in Reno, said he expects the storm to dump at least 3 feet of new snow in the Sierra at elevations higher than 3,000 feet. Some forecasts called for 5 feet at the highest mountain passes.

“Unlike the fluff this weekend, it’s going to be a wet, heavy Sierra cement type snow,” Wallmann said.