Thursday, December 01, 2022

Atmospheric River Storm: How It Affects California Drought

Atmospheric River Storm: How It Affects California Drought

The conga line of Pacific storms hitting northern California over the past week was a record.

With 4.05 inches of rain in 24 hours, Sunday was the wettest October day in San Francisco history and the fourth rainy day in history since the 1849 gold rush. It’s an astounding feat for October, which isn’t usually a very wet month. Likewise, San Jose received 50% more rainfall over the weekend than the entire previous year. In East Bay, a foot fell during a week at Tilden Park in Berkeley and on the summit of Mount Diablo. And Marin County, the focus of the powerful atmospheric river this weekend, culminated in a spectacular 26 inches above Mount Tamalpais as all was said and done.

Does the flood mean California’s severe drought is over? No, experts said on Monday when the sunny skies returned.

But they noted that it was a great start to the winter rainy season this year, sending tens of billions of gallons into anemically low bodies of water, nearly ending the fire season across much of the state and embarking on a long task of rebuilding rivers and groundwater.

“It was a choker frog, but not a drought cure,” said Geoffrey Mount, emeritus professor of earth and planetary sciences at the University of California, Davis. “We are still in a drought. But this storm has smoothed out some of its rough edges. “

He estimates California needs five to seven more severe atmospheric river storms to fill reservoirs and end drought.

The forecast for Northern California suggests generally dry weather this week. Water managers and meteorologists are hoping for a repeat of the storm in November to maintain momentum, given that the last two years have been the driest in Northern California since 1976-77.

“You just don’t make up for it in one storm – even if it’s an all-time record,” said Mount, a senior fellow at the Center for Water Policy at the California Institute for Public Policy.

As a result of this storm, San Francisco’s monthly rainfall for October was 7.04 inches, making it the second wettest October since 1849 when its modern weather records began, second only to October 1889 when it fell 7.28 inches.

“This is more than a drop in the bucket,” said Ian Null, a meteorologist at the Golden Gate weather service in Half Moon Bay. “But that doesn’t fill the bucket.”

Why? Here’s the math: In the past two years ending June 30, San Francisco has received only 20.67 inches of rainfall, Null noted.

San Francisco is commonly used as an indicator of weather trends in the Bay Area because it has the oldest set of accurate weather data.

The historic biennial average is 45.78 inches. Thus, the city began this winter season with a 25.11-inch slump – in other words, there was no rain for an entire year. To make up for this deficit, the city will need a staggering 48 inches of rain by June next year.

This happened only once in recorded history, in the winter of 1861-1862, when newly elected Governor Leland Stanford rode a rowboat through the streets of Sacramento to reach his inauguration and fell 49 inches. The Bay Area came close in 1997-98 when El Niño storms reached 47 inches in San Francisco and caused massive flooding in Northern California.

Atmospheric River Storm: How It Affects California Drought
MILLBRAY, CALIFORNIA – OCTOBER 25: Flooding on Millbray Avenue off Highway 101 in Millbray, California disrupts traffic on Monday, October 25, 2021 in the Bay Area Sunday. (Mercury News)

The impact of last week’s storms continued on Monday.

The National Weather Service on Tuesday issued a high tide warning from Point Reyes National Coast to Big Sur. Forecasters have warned of 20 to 30 feet of waves crashing onto the beach and life-threatening rip currents.

Atmospheric River Storm: How It Affects California DroughtWater managers thoroughly checked the reservoirs on Monday and generally liked what they saw.

In Lake Oroville, California’s second-largest reservoir in Butte County, the lake has risen 23 feet and added 135,000 acres of water from last Monday to Monday afternoon – enough to meet the annual needs of 675,000 people. But Oroville was only 26% full, up from 22% a week ago.

California’s largest reservoir, Lake Shasta, near Redding, rose 2 feet, but that only took it from 21% to 22%. Despite billions of gallons of water leaking from the wet watershed, experts say, the lake is 35 miles long and will take time to recover.

In the Bay area, 10 reservoirs belonging to the water area of ​​the Santa Clara Valley increased from 10% to 11% over the week.

“We have a long way to go to make up for the last few dry years,” said Tony Estremera, chairman of the Santa Clara Valley Waters District. “We need everyone to continue to contribute to saving water.”

Likewise, the seven reservoirs operated by the East Bay Municipal Community added 5,000 acre feet, up 56% from 55% a week earlier. Partially dried out reservoirs in Marine County performed better, with fill rates increasing from 32% to 43%.

In the Santa Cruz Mountains, the Loch Lomond reservoir near Ben Lomond, which received 9 inches of rain in a week, rose 1 foot, raising it from 53% to 54%.

Atmospheric River Storm: How It Affects California DroughtWater managers said most of the water has seeped into dry land, but they expect reservoir levels to rise slowly this week as stormy streams flow down.

In the Sierra Nevada, more than three feet of snow fell in some areas between Sunday and Monday morning.

“It’s pretty crazy,” said Chris Beam, a chef at Donner Ski Ranch, 10 miles northwest of Lake Tahoe. “I have not seen snow like this since March. As we speak, the CEO is plowing the parking lot. ”

County spokesman Jason Hoppin said that in the Santa Cruz Mountains, where a decision was made on Sunday to evacuate 3,300 homes amid concerns about landslides in areas burned by last year’s wildfires, several trees have been cleared and some flooding has occurred, but there was no major damage.

Null noted that of the previous nine wettest October in San Francisco, six of those years ended with above average rainfall.

“This is a good start,” he said. “A really good start. But this is not a guarantee that we will have a good result. “

Bay Area News Group reporters Summer Lean and Will Houston contributed to the story.

Atmospheric River Storm: How It Affects California Drought
Donner Ski Ranch, 10 miles northwest of Lake Tahoe, fell over 2 feet of new snow on Sunday, October 24, 2021 during an atmospheric river storm earlier in the season. (Photo by Claudia Salas, Donner Ski Ranch)

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