Ventura County Fire Department officials announced around 11:15 p.m. that crews responded to the 700 block of Trueno Avenue after a 30-by-30 feet sinkhole opened up between homes on nearby Grada Avenue.
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Those two homes were red-tagged as a precaution, VCFD firefighter Andy VanSciver said.
"Flooding is impacting the hillside behind two homes. Sand bags are in place and the storm is diminishing," VCFD tweeted just after 3:30 p.m. "Four homes under evacuation orders."
No injuries were immediately reported.
A county geologist also responded to the scene to assess the sinkhole.
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The storm marked California's 10th atmospheric river of the winter, storms that have brought enormous amounts of rain and snow to the state and helped lessen the drought conditions that had dragged on for years.
The storm is expected to weaken as it moves down the coast and shouldn't bring as much rain as previous storms.
The Associated Press contributed to this report.