Northern California and the San Joaquin Valley are bracing for potential flooding this weekend, as a massive warm storm builds up off the coast. Forecasters say rainfall amounts could reach totals not seen in more than a decade. Already, some regions north of Sacramento are issuing voluntary evacuation orders.
A flood watch was issued Thursday by the National Weather Service from Saturday afternoon through most of Monday for 24 northern counties, including Sacramento.
If the storm follows its current trajectory, forecasters say it could send a particularly large gush of water to the southern Sierra Nevada mountains and into the drought-ravaged San Joaquin Valley. Fresno, Madera, Mariposa, Tulare and Tuolumne counties are under are flash-flood warnings.
The weather service on Thursday predicted that rainfall totals from Sunday and Monday could add up to more than 10 inches in some parts of the Sierra. The foothills near Grass Valley could see as much as 15 inches fall though Monday, the weather service reported.
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In Sacramento, forecasters say up to 5 inches could fall by the time the storm passes.
All told, the weather service’s flood watch advisory says rainfall amounts not seen since December 2005 could be recorded in parts of the State. That storm featured localized flooding, mostly from creeks in the Sacramento area. In Sacramento County, an estimated 60 structures were inundated.
In those storms, some of the worst local flooding occurred in Arden Arcade, where about 40 of 700 owner-occupied town homes in the Woodside complex were damaged, some under more than a foot of water when a nearby slough overflowed.
Heavy rainfall Sunday and Monday, combined with already saturated soil and snow levels above 7,000 feet, is expected to substantially swell rivers and local tributaries. The Cosumnes River south of Sacramento could experience flooding. Rivers with flood-control dams, such as the American and Sacramento, also will rise.
Forecasters say this weekend’s storms are almost certain to flood the Yolo Bypass by late Sunday afternoon. By Monday, Sacramento River flows are expected to reach about 119,000 cubic feet per second.
If that happens, it will be the highest flows since January 2006 when Sacramento River flows topped 200,000 CFS.
U.S. Bureau of Reclamation officials also are increasing flows on the American River in anticipation of large amounts of rainfall upstream of Folsom Lake. Louis Moore, spokesman for the bureau, said 19,000 cubic feet of water was pouring into the reservoir every second on Thursday morning.
Federal water watchers expect to increase the flow out of Folsom Dam and Nimbus Dam to 15,000 CFS by Thursday afternoon in order to make room for the water that’s expected to wash in this weekend.
“The storms that are forecasted are expected to be quite wet,” Moore said. “Operationally that would mean we need to evacuate more water. So our releases would go up a bit more.”
There is plenty of room between the banks of the American River to contain the outflow from the dams below Folsom Lake.
“The channel system is capable of 115,000 CFS,” Moore said. “We are nowhere near that.”
In anticipation of high water in many areas, the Sacramento County Department of Regional Parks announced that it had closed areas of the American and Dry Creek parkways. County park rangers were warning park visitors of the rising water levels.
A handful of residents and businesses along parts of the Yuba River outside Marysville also receive voluntary evacuation notices on Thursday. Properties along Simpson Lane and Dantoni Road could have some flooding, county officials said.
Orchards in the region southeast of Marysville also could see encroaching water. Sandbags and sand are available at the Yuba County Corporation Yard, 1420 Sky Harbor Drive at the Yuba County Airport in Olivehurst.
The weather service warned residents near burn-scarred hills and mountainsides to be wary of debris flows from big downpours. Sacramento County residents were warned to be mindful of falling trees and downed powerlines.
Before the arrival of the weekend rain, skies will clear on Thursday and Friday. Highway 50 and Interstate 80 are open after Wednesday’s closures due to avalanche control, spin-outs and poor visibility.
So far, the rainfall total for the season stands at 10.98 inches, which is 147 percent of normal.
The detailed, seven-day forecast from the weather service for the Sacramento region:
Thursday: A 20 percent chance of showers before 10 a.m. Patchy fog before 10 a.m. Otherwise, partly sunny, with a high near 51. North northwest wind 5 to 8 mph.
Thursday night: Patchy fog after 4 a.m. Areas of frost after 1 a.m. Otherwise, partly cloudy, with a low around 32. North northwest wind 3 to 5 mph.
Friday: Patchy fog before 10 a.m. Areas of frost before 10 a.m. Otherwise, increasing clouds, with a high near 49. North northwest wind 3 to 5 mph.
Friday night: Rain, mainly after 4 a.m. Low around 40. Light south wind. Chance of precipitation is 80 percent. New precipitation amounts between a tenth and quarter of an inch possible.
Saturday: Rain. High near 53. Northeast wind 5 to 10 mph becoming south southeast 11 to 16 mph in the morning. Winds could gust as high as 21 mph. Chance of precipitation is 90 percent.
Saturday night: Rain. The rain could be heavy at times. Low around 50. Chance of precipitation is 100 percent.
Sunday: Rain. The rain could be heavy at times. High near 60. Chance of precipitation is 100 percent.
Sunday night: Rain. Cloudy, with a low around 50.
Monday: Rain likely. Cloudy, with a high near 56.
Monday night: A chance of rain. Mostly cloudy, with a low around 47.
Tuesday: Rain likely. Cloudy, with a high near 55. Windy.
Tuesday night: Rain. Cloudy, with a low around 46. Breezy.
Wednesday: Rain likely. Cloudy, with a high near 55. Breezy.