SAN FRANCISCO (CBS SF) — The Bay Area’s saturated hillsides will face a new challenge this week– a moisture-laden atmospheric river that will crash into the region late Tuesday and dump several inches of rain.

The National Weather Service issued a flash flood watch for the entire Bay Area from Tuesday evening through 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.”

“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.”

Forecasters also warned Bay Area drivers to prepare for sloppy commutes on Wednesday and Thursday.

“The Wednesday morning commute will be adversely impacted with flooding roadways and ponding of water expected,” the weather service said. “Periods of heavy rain are also possible during the Wednesday afternoon and Thursday morning commutes.”

The weather service predicted that by the time the system clears the area on Thursday, 2-3 inches of rain would fall on the North Bay, 1-2 inches in San Francisco and Oakland and 4-6 inches in the coastal mountains.

The Santa Lucia range in Monterey County was expected to bear the brunt of the storm’s fury with up to 8 inches predicted.

“The first big push of moisture is forecast to arrive over the North Bay Tuesday night/early Wednesday before a secondary push across the Big Sur Coast Wednesday — a pretty juicy airmass,” the weather service warned.

The rainy onslaught in February has pushed local rainfall totals to near normal. Since the water season began on July 1st, San Francisco has received 13.72 inches of rain or 90 percent of normal.

The downpours have swollen local rivers. The River Forecast Center prediction for the on-coming story shows “a rapid rise on mainstem rivers of 10-12 feet on
Wednesday.”

“Heavy rain and snow runoff would likely lead to flooding concerns along the Central Coast,” the weather service said. “Other locations in the Bay Area will also be of concern with flooding potential given the copious amounts of rain in 12 hours.”

Wind will also be an issue. The National Weather Service said a wind warning was likely for Wednesday.

“Winds really crank up early Wednesday with local (weather) models showing sustained 30-40 mph with gusts 60-70 mph in the hills,” the weather service said. “Given the saturated soils and expected rainfall trees will likely come down. A wind advisory or combination wind advisory/high wind warning will likely be
needed.”

“Simply put, the midweek storm system could be a big impact event with heavy rain, flooding and destructive wind,” the weather service concluded.

In the mountains, the storm front’s warm temperatures will carry with them the threat of sudden snow melt and flooding. A winter storm watch goes into effect for areas above 4,000 feet from late Tuesday through Thursday.

“Heavy snow possible — Travel will be very difficult,” the weather service warned. “Total snow accumulations of 1 to 3 feet, with localized amounts up to 4
feet, are possible.”

Caltrans said chains will likely be required on the mountain passes on I-80 and Highway 50.

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