Severe flooding in northern California has turned the small river town of Guerneville into an “island,” county officials said Wednesday (Feb. 27), rendering the community inaccessible by anything other than watercraft, according to The Washington Post

In an update to an evacuation notice Wednesday morning, the county reminded area residents that even though rains have eased, the worst of the flooding is yet to come. The Russian River surpassed its flood stage in the town Tuesday evening, resulting in dozens of road closures, and is expected to crest Wednesday night.

In addition to Guerneville, 24 nearby towns and communities were ordered to evacuate. Flood levels are expected to decrease Thursday and into the weekend, according to the National Weather Service.

“Good morning. Guerneville is officially an island,” the Sonoma sheriff’s office posted to Facebook on Wednesday. “Due to flooding all roads leading to the community are impassable. You will not be able to get into or out of town without a boat today.”

The rain, winds and flooding were spurred by a potent atmospheric river, the National Weather Service said.

Atmospheric rivers develop when strong storms pull humid air from the tropical Pacific Ocean to the West Coast. The result is a fire hose of extreme rainfall that can trigger dangerous flooding and deadly landslides. They’re most common during winter, and the effects are exacerbated when heavy rain falls on burn scars from recent wildfires.

Parts of the Sonoma region were hit by the deadly wildfires that ravaged northern California in 2017, resulting in mass evacuations and the deaths of dozens in surrounding counties. At least six people died in Sonoma County.

Those who have opted to wait out the storm in their homes could be stuck for days, the Los Angeles Times reports. The newspaper reports that this is expected to be the most severe flood in the area since 1995, when the Russian River crested in Guerneville at about 48 feet, 16 feet above its flood stage.

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