A flash flood watch is in effect as a once-a-decade rainstorm barrels toward Los Angeles

A powerful series of rainstorms is headed for California Thursday and through the weekend — arriving after a period of already severe weather in the state.

A flash flood watch is in effect throughout Los Angeles, Ventura, and southern Santa Barbara counties.

Up to a foot of rain is expected in some places, as the storms drench a region from Northern California down to San Diego, raising safety concerns. Those concerns are perhaps most serious at the Oroville Dam, where unusually high waters damaged an emergency spillway February 9, forcing 188,000 residents to flee the area.

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Lake Oroville dam -- emergency spillway and evacuation
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Lake Oroville dam -- emergency spillway and evacuation
TOPSHOT - The Oroville Dam spillway releases 100,000 cubic feet of water per second down the main spillway in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
OROVILLE, CA - JULY 20: In this handout photo provided by the California Department of Water Resources, Full water levels are visible behind the Oroville Dam at Lake Oroville on July 20, 2011 in Oroville, California. (Photo by Paul Hames/California Department of Water Resources via Getty Images)
California Department of Water Resources personnel monitor water flowing through a damaged spillway on the Oroville Dam in Oroville, California, U.S., on February 10, 2017.
FOLSOM, CA - JULY 20: In this handout photo provided by the California Department of Water Resources, Full water levels are visible behind the Folsom Dam at Folsom Lake on July 20, 2011 in El Folsom, California. (Photo by Paul Hames/California Department of Water Resources via Getty Images)
OROVILLE, CA - AUGUST 19: The Oroville Dam spillway stands dry at Lake Oroville on August 19, 2014 in Oroville, California. As the severe drought in California continues for a third straight year, water levels in the State's lakes and reservoirs is reaching historic lows. Lake Oroville is currently at 32 percent of its total 3,537,577 acre feet. (Photo by Justin Sullivan/Getty Images)
65,000 cfs of water flow through a damaged spillway on the Oroville Dam in Oroville, California, U.S., February 10, 2017. REUTERS/Max Whittaker
California Department of Water Resources crews inspect and evaluate the erosion just below the Lake Oroville Emergency Spillway site after lake levels receded, in Oroville, California, U.S., February 13, 2017. Kelly M. Grow/ California Department of Water Resources/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.
California Department of Water Resources personnel monitor water flowing through a damaged spillway on the Oroville Dam in Oroville, California, U.S., on February 10, 2017.
A damaged spillway with eroded hillside is seen in an aerial photo taken over the Oroville Dam in Oroville, California, U.S. February 11, 2017. California Department of Water Resources/William Croyle/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. THIS PICTURE WAS PROCESSED BY REUTERS TO ENHANCE QUALITY. AN UNPROCESSED VERSION HAS BEEN PROVIDED SEPARATELY.
A damaged spillway with eroded hillside is seen in an aerial photo taken over the Oroville Dam in Oroville, California, U.S. February 11, 2017. California Department of Water Resources/William Croyle/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. TPX IMAGES OF THE DAY
Riverbend Park is seen under flood water in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
TOPSHOT - Crews work on a damaged section of the Oroville Dam in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
TOPSHOT - A home is seen marooned as the surrounding property is submerged in flood water in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Riverbend Park is seen under flood water in Oroville, California on February 13, 2017. Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States' tallest dam. Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 75 miles (120 kilometers) north of Sacramento, have eased. But people were still being told to stay out of the area. / AFP / Josh Edelson (Photo credit should read JOSH EDELSON/AFP/Getty Images)
Water is released from the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 16, 2017. REUTERS/Jim Urquhart TPX IMAGES OF THE DAY
A road is closed by the flooded Feather River outside town below the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 16, 2017. REUTERS/Jim Urquhart
Rocks are hauled to the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 15, 2017. REUTERS/Jim Urquhart
A helicopter flies rocks to the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 15, 2017. REUTERS/Jim Urquhart TPX IMAGES OF THE DAY
Trucks carry rocks as water is released from the Lake Oroville Dam after an evacuation order was lifted for communities downstream from the dam in Oroville, California, U.S. February 14, 2017. REUTERS/Jim Urquhart
Staff with the California Department of Water Resources watch as water is released from the Lake Oroville Dam after an evacuation was ordered for communities downstream from the dam in Oroville, California, U.S., February 14, 2017. REUTERS/Jim Urquhart
OROVILLE, CA - FEBRUARY 15: Reconstruction continues in a race to shore up the emergency spillway at Oroville Dam in Oroville, Calif., on Feb. 15, 2017. (Brian van der Brug/Los Angeles Times via Getty Images)
OROVILLE, CA - FEBRUARY 16: Clouds drift in the sky after morning showers at the Oroville Dam overlook where helicopters are ferrying sand and rocks to the dam's emergency spillway reconstruction project in Oroville, Calif., on Feb. 16, 2017. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
OROVILLE, CA - FEBRUARY 16: California Water Service district manager Toni Ruggle (CQ) surveys the Feather River at Riverbed Park downstream from Oroville Dam in Oroville, Calif., on Feb. 16, 2017. For decades, city officials have used the steps at the park to gauge the height of the river. Water up to the fourth step is 100,000 cubic feet per second. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
OROVILLE, CA - FEBRUARY 16: A mallard floats past a picnic table along the swollen Feather River at Riverbed Park downstream from Oroville Dam in Oroville, Calif., on Feb. 16, 2017. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
Cots are washed and set to dry at the Silver Dollar Fairgrounds evacuation center in Chico, California, U.S., on Tuesday, Feb. 14, 2017. State officials rushed to repair an emergency spillway for the Oroville dam -- just 150 miles (241 kilometers) north of San Francisco -- that is threatening to submerge an entire region of northern California after a recent deluge of rain. Photographer: Michael Short/Bloomberg via Getty Images
OROVILLE, CA - FEBRUARY 14: A dog sits in the back of a pickup truck as a helicopter picks up a bag of rocks at a staging area near the Oroville Dam on February 14, 2017 in Oroville, California. More than 188,000 people were ordered to evacuate after a hole in the emergency spillway in the Oroville Dam threatened to flood the surrounding area. (Photo by Elijah Nouvelage/Getty Images)
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Efforts have been underway through the week to buttress the spillway, but a new rush of rainwater poses risks.

Los Angeles in particular will experience unusual weather, which is expected to begin Thursday night. Two to six inches are expected in the area, with up to an inch of rain falling per hour.

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