Hurricane Ian was flooding some areas of Florida's west coast with storm surges that could reach up to 18 feet above ground level as it moved across the peninsula after making landfall Wednesday afternoon, the National Hurricane Center said.

Why it matters: Surge numbers that high — 12 to 18 ft — would be unprecedented for the region and some of the highest on record in the U.S.

• "Ian battering the Florida peninsula with catastrophic storm surge, winds, and flooding," the NHC said in a 5pm ET update.
• Although the hurricane weakened after making landfall near Pirate Harbor in the afternoon, becoming a Category 1 storm by 11pm, storm surge and flooding threats remained on Thursday — as power outages hit more than 2 million customers in the state.

What they're saying: Florida Gov. Ron DeSantis (R) warned at a briefing Wednesday evening there would be damage throughout the entire state.

• "Overwhelmingly it's been that surge that’s been the biggest issue and the flooding … as a result," he said. "In some areas, we think it’s hit 12 feet."

The big picture: Storm surge was battering Florida's west coast throughout Wednesday, and 8 to 12 ft was expected between Bonita Beach and the small island of Chokoloskee in southern Florida, and 6 to 10 ft from Englewood to Longboat Key.

• The storm surge was occurring along with high winds, heavy rainfall and considerable flooding.

Zoom in: Ian caused over 9 ft of surge plus the tide level in Naples, Collier County, by 1pm ET Wednesday — a new record for the city, according to a National Oceanic and Atmospheric Administration monitoring station. At least 6.2 feet of that was the surge itself, which set a record there.

• The Collier County Sheriff's Office said officers were "getting a significant number of calls of people trapped by water in their homes," and the Naples fire station was inundated with about three feet of floodwater.

In Fort Myers, the city saw a peak 7.26 ft of surge — a record high, per NOAA.

• Homes in Fort Myers Beach were damaged or destroyed from the surge, while boats were seen floating through streets in Fort Myers.

Meanwhile, the Caloosahatchee River, which flows into the San Carlos Bay and the Gulf of Mexico, was experiencing 7.2 ft of "moderate flooding" as of around 6:10pm ET, according to a National Weather Service gauge in Fort Myers.

• Surge-related flooding was reported on the island of Sanibel and seen on public cameras broadcasting footage of the storm.
• A National Ocean Service station near Ft. Myers reported a water level greater than 7 ft, per a 7pm EST NHC update.

Threat level: The NHC considers storm surge, or an abnormal rise of water generated by a storm, to be the most deadly and destructive aspect of hurricanes.

• The surge is the result of water being thrust toward the shoreline by the winds moving cyclonically around the storm and can cause "extreme" flooding in coastal areas, especially when it coincides with high tides.

Go deeper: Universal Studios, Disney, stores, schools and more closing for Hurricane Ian

Axios' Andrew Freedman contributed to this story.

Editor's note: This story has been updated with additional details.