Simulation of moisture stream into Central California on Monday from GFS model. (PivotalWeather.com)
A simulation of the moisture stream into Central California on Monday from the Global Forecast System model. (PivotalWeather.com)

Historic if not unprecedented amounts of rainfall have fallen over parts of the northern half of California this winter. Between Sunday and Tuesday morning, a new blast of torrential rain raises the specter of a serious flooding event in the region.

The San Francisco, Monterey, and Sacramento regions are all at risk of “excessive rainfall” and “rapid onset flooding,” the National Weather Service said.

Widespread rainfall totals of two to five inches are likely along the coast and at low elevations. In the mountains, four to 10 inches of rain are likely. Rain will change to snow between 6,000 and 8,000 feet, and several feet may pile up in the highest terrain.


Forecast rainfall totals through Tuesday morning. (National Weather Service via WeatherBell.com)

The heaviest rainfall is expected in the San Francisco Bay area Sunday night into early Monday, during the day Monday around Sacramento, and late Monday into Monday night in northeast and east central California along the Sierra Nevada range.

Because of all of the recent precipitation, the ground is saturated and many streams, rivers and reservoirs are at capacity.

“Recent storms have left the region highly vulnerable,” the National Weather Service office serving the Sacramento area warned.

The Weather Service said locations that frequently flood during heavy rain events at risk as well as those that “haven’t flooded in a long time.”

The Weather Service is urging residents to prepare for the possibility of flooding and be ready to evacuate at a moment’s notice. “The time to act is now,” the Weather Service office serving the Bay Area said Saturday.

The onslaught of moisture “will be a stress test for California water infrastructure,” tweeted Daniel Swain, a climate scientist at the University of California at Los Angeles who writes a blog about West Coast weather.

A week ago, dire concerns arose at the Lake Oroville, about 80 miles north of Sacramento, after water topping an emergency spillway started to cause erosion, prompting an emergency evacuation order. But California’s Department of Water Resources (DWR) said the flow of water has since slowed and the situation has stabilized.

“The reduction in flow has allowed assessment teams to view debris buildup and dredge debris piles below the flood control spillway,” the DWR said in a news release Sunday morning. “Lake elevations continue trending downward and had fallen to 852 feet, by 4:00 a.m. this morning, 48 feet below the emergency spillway.”

The DWR suggested it should be able to handle the next surge of water. “As runoff flows into the reservoir, water levels will likely fluctuate but will remain within acceptable and typical depths during times of storm activity,” it said.

But Swain, in a blog post, cautioned that this storm or one in the future could still test Oroville’s dam system. He noted “dam operations cannot release water at as high a rate as they would under normal circumstances, meaning that the Oroville Dam may once again fill to the brim if storm or snowmelt-related inflows become high enough.”

Swain added that other reservoirs in the region may also be tested by this storm: “… serious issues have started to arise in the vast levee system that protects much of the Central Valley and Delta region from flooding. So far, there have not been any major levee failures in California this winter. But if the current forecast holds, the levee system will be under more strain … than it has been in many years.”

This latest storm will draw a powerful stream of tropical moisture sourced from the central Pacific Ocean, known as an atmospheric river, straight into the Central California coast. In the mountains, the rain (and snow) will intensify as the terrain forces the fire hose of moisture upward, where it can focus and unload.

The Weather Service office in Sacramento said the incoming atmospheric river is carrying an amount of moisture that only comes inland about once every five to 10 years.


A GFS model forecast of integrated water vapor transported by the atmospheric river Monday morning on the California coast. (Scripps)

In addition to the heavy rain, strong winds are forecast along the coast with gusts over 45 mph possible.