Severe flooding in Northern California has turned the small river town of Guerneville into an “island,” county officials said Wednesday, rendering the community inaccessible by anything other than watercraft.

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 floods were caused by record-breaking rainfall in the region, according to the Weather Service. The Sonoma sheriff’s office has urged residents to heed evacuation warnings, writing Wednesday afternoon that roads “keep flooding, and flooding FAST.”

The rain, winds and flooding were spurred by a “potent atmospheric river,” the 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.

The river is expected to crest at about 46 feet Wednesday night, according to the National Weather Service.

In Monte Rio, a town about five miles south of Guerneville, firefighters worked overnight to help people trapped in their vehicles and homes as water levels rose, the AP reports. Fire Chief Steve Baxman told the Press Democrat that rescuers took “17 people out of cars and houses during the night.”

“Too many people are driving into the water,” he told the publication.

Sonoma County Supervisor James Gore said in a video Tuesday evening that the rainfall could be “100-year storm” material and that he was preparing to move his family to another location overnight, stating: “Family most important, property second.”

“We’re all about preparedness. We’re not trying to have people stand on their roofs and get saved in the middle of the night,” Gore said Tuesday. “If you get an evacuation order, pay attention.”