Don’t put away those umbrellas
Another atmospheric river is coming, but it won’t be as strong as last week’s
By Paul Rogers and Mark Gomez
Staff writers
One week after an atmospheric river storm pounded Northern California, causing flooding, mudslides and traffic headaches, another one appears to be forming in the Pacific and is set to arrive early next week.
Computer models show the storm hitting Monday or Tuesday, with the North Bay and parts of California farther north taking the brunt, although that could change, experts say.
“It is an atmospheric river,” said Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego. “It looks to be weak or moderate in terms of its intensity, although it might be longlasting and could stall. It’s still three or four days out, so things could change.”
Ralph, one of the nation’s top experts on atmospheric river storms, helped develop a new system to rank such storms on a 1-to-5 scale — similar to the one scientists use for hurricanes — but based on their duration over land and their volume of water.
Last week’s storm, the wettest of the winter so far, was a 4, the kind that can prompt floods and other disasters. The storm expected next week looks more like a 1 or 2, which would be mostly beneficial, expanding California’s water supply without serious flooding he said, although more will be known in the coming days.
The specifics of how much rain is expected and precisely where the storm will be focused remain difficult to pinpoint because computer models have delivered contrasting forecasts this week, said Matt Mehle, a meteorologist with the National Weather Service in Monterey.
According to the weather service office in Sacramento, up to 5 inches of rain are expected in the Sonoma County area between Monday and Wednesday, with 1 inch or so over Bay Area cities.
Ralph noted that the storm
could move to the north or the south in the coming days. That would change rainfall totals.
“The finer details are changing day to day, but the overall trend has not changed,” Mehle said. “Our storm door is remaining open and holding onto an active weather pattern into the first week of March.”
Atmospheric rivers are a particularly moistureheavy, 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 exact a high cost in damage and flooding.
When high-pressure ridges block the rivers, California can enter a drought. When they make it to landfall, the rain can be substantial: 92 percent of the wettest three-day rain events on the West Coast in one study were linked to atmospheric rivers. And 81 percent of Central Valley levee breaks occurred during them.
Earlier this month, a powerful atmospheric river soaked much of Northern California with 3 inches of rain in Oakland and San Francisco and 8 to 10 inches of rain at higher elevations, causing flooding along the Russian River in Guerneville and the Carmel River in Monterey County. Several Bay Area creeks and streams reached slightly above flood stage, and roadway flooding was widespread.
In San Jose, city officials issued an evacuation recommendation on Feb. 14 for residents living near the Guadalupe River, where waters briefly rose just above flood stage. However, no flooding occurred.
Rainfall also caused a levee to break along Highway 37 near Novato, closing parts of the roadway for several days, and it triggered a mudslide in Sausalito that destroyed three houses.
The earlier storm also brought massive amounts of snow to the Sierra Nevada — up to 8 feet in some places. On Thursday, the statewide Sierra Nevada snowpack was at 144 percent of its historical average for this time of year, up from just 69 percent on New Year’s Day.
With the huge snowpack and brimming reservoirs around the state, water agencies around the Bay Area and other parts of California say they do not expect any water shortages this summer.
Even before the potential arrival of the new atmospheric river storm next week, the Bay Area will have a chance of rain starting Saturday afternoon, but mainly north of the Golden Gate, according to the weather service. The latest computer models show the heaviest downpours in the North Bay, with some “precipitation possible as far south as San Jose.”
Mehle said rainfall estimates for the remainder of the month include 5 to 7 inches in the North Bay and 1 to 1.5 inches in San Francisco.
If that holds true, San Francisco could end February with a monthly rainfall total ranking among the city’s top 10 wettest, based on records dating back to 1850. As of Wednesday, San Francisco had received 6.86 inches of rain this month, a figure that falls just outside the top 20 wettest Februarys in the city’s history. But with another 1.2 inches of rain this month, 2019 would enter the top 10.
The wettest February of all time in San Francisco? 14.89 inches in 1998.
In San Jose, 5.29 inches of rain has been recorded so far this month, the 17th wettest February dating back to 1893. Another 0.45 inch would send this February into San Jose’s top 10. In San Jose, 1998 was also the wettest February with 10.23 inches.
“When it comes to getting more precipitation and snowpack for the state, it’s looking pretty good over the next 7 to 10 days,” Mehle said.
Rainfall totals across California for the water year that began Oct. 1 are mostly above average for this time of year, including Redding (25.21 inches, or 111 percent of its historical average), San Francisco (17.94 inches, 108 percent), Oakland (13.96 inches, 100 percent) and San Jose (11.66 inches, 112 percent). Contact Paul Rogers at 408-920-5045 and Mark Gomez at 408-920-5869.
