SAN FRANCISCO (CBS SF) — A Category 3 Atmospheric River, fueled by moist air from the remnants of a tropic storm that ravaged Hawaii, took aim at Northern California Tuesday, packing torrential downpours that could dump as much 8 inches of rain over the next 48 hours.
The storm was the first ranked on a 1 to 5 system similar to hurricanes by researchers with the National Weather Service and UC San Diego’s Center for Western Weather and Water Extremes.
The weather service has issued several watch and warnings in anticipation of the storm fronts fury. A flash flood watch and a high wind warning have been issued for the region from 10 p.m. Tuesday to 10 a.m. Thursday.
“An atmospheric river with origins near Hawaii will take aim at the San Francisco Bay Area and Central Coast Wednesday through Thursday morning,” weather service forecasters warned. “Rainfall rates of at least half an inch per hour are possible during the heaviest showers, with the highest rates in the higher terrain.”
A plume of the storm front was being carried by the jet steam from the Hawaiian Islands to the West Coast. Over the weekend, the Hawaiian storm brought heavy downpours, high winds, pounding surf and a historic dusting of snow above 6,000 feet on Maui.
By the time the system clears the Bay Area on Thursday, 2-3 inches of rain could fall on the North Bay, 1-2 inches in San Francisco and Oakland and 4-6 inches in the coastal mountains.
Forecasters warned some areas may see as much as 8 inches.
“Periods of heavy rain and strong winds will continue Wednesday leading to flooding concerns across the region,” the weather service warned.
The rain began steadily on Tuesday evening, but the brunt of the storm was predicted to hit the San Francisco Bay Area during the Wednesday morning commute.
“The Wednesday morning commute will be adversely impacted with flooding roadways and ponding of water expected,” the weather service warned. “Periods of heavy rain are also possible during the Wednesday afternoon and Thursday morning commutes.”
Mudslides could also occur in areas where the hillsides have already become saturated by storms in early February.
“Recent rainfall has saturated soils across the area, and the additional heavy rainfall will create the potential for widespread shallow landslides, rockslides, and debris flows,” the weather service said. “Periods of heavy rainfall will likely inundate storm drains with rapid rises on small creeks and streams.”
Some heavy duty excavators were brought in to clear dirt and rock that came down on Hicks Road in San Jose Tuesday.
The mess was between Pheasant and Alamitos roads right near the Gaudalupe Reservoir. Officials said they make keep that stretch of road closed because of the approaching storm.
Santa Clara county is expected to see threats of flooding, reservoir levels increasing and likely more mudslides with this week’s next round of storms.
“Mudslides happen when the soil gets very saturated from the rain, and when the rain starts to run off it often takes the loose soils with it,” said San Jose State Department Chair and Professor of Civil Environmental Engineering Laura Sullivan-Green. “That’s the type of slide we’ll see now.”
She said while it’s difficult to predict large mudslides like the one that shutdown Highway 17 in 2017 during a particularly heavy rainy winter, we will likely see smaller scale slides this week because the ground is already saturated from recent storms.
The threat of slides and mud flows, she said, lasts briefly after the rain stops. A mudslide Tuesday in Los Gatos on Pheasant and Hicks happened between storms.
“We’ll see a lot of small landslides or mudslides and we may see possibly some larger events, but I would definitely expect a lot of small movements,” Sullivan-Green said.
Last month, Maryam Hashemi of Los Gatos almost lost her home after a nearby creek flooded it. Now, she’s warning others to clean drains, cut tree limbs and check nearby creeks for debris.
“I think everybody should clean their creeks before the rainy season and trim their trees,” she said. With yet another round of rain on the way, Hashemi is prepared for the worst.
In the mountains, the storm system will create chaotic conditions. The initial pulse on Wednesday will be fueled by warm, moist air that will raise the snow levels to 6,000-7,000 feet with rain falling at lower levels.
But the second pulse of the storm rolling in over the weekend will send the snow levels to 2,000 feet.
“This has been one of the most chaotic forecasts of my career with many adjustments and changes made to the existing forecast,” lamented one of the weather service forecasters.
The high water content of snow will create a condition called “Sierra Cement” and not they powdery conditions most skiers enjoy.
“Total snow accumulations of 6 to 10 inches for Truckee and South Lake… 2 to 5 feet possible above 8000 feet including Mammoth Lakes,” the weather service said. “Winds could gust as high as 55 mph with ridge gusts possibly exceeding 150 mph.”