U.S. insurers are bracing for what could be $20 billion in losses as Hurricane Florence barrels toward the Carolinas and Virginia, bringing punishing rains and potentially deadly flooding.

Hurricane Florence, on track to become the first Category 4 storm to make a direct hit on North Carolina in six decades, howled closer to shore on Wednesday, as schools and factories were being shuttered and authorities ordered more than 1 million people in the region to evacuate.

Losses from Florence could be on par with those from two previous storms: Hurricane Hazel, which ravaged North Carolina in 1954, causing $15 billion in losses, and Hurricane Hugo in 1989, which caused $20 billion, according to a report from risk modeling firm RMS on Tuesday.

"As we look back at previous events, although hurricanes making landfall in the Carolinas are rare, the losses through this triple-threat of storm surge, wind and inland flood, could also be significant," RMS said in the report.

The firm will have a better estimate for total insured losses after Florence makes landfall.

The slow-moving storm could dump between 20 and 30 inches of rain in the Carolinas after making landfall, according to a report on Wednesday by risk modeler AIR Worldwide. More than 758,000 homes in the Carolinas and Virginia are at risk, according to risk assessment service CoreLogic.

Liberty Mutual Insurance and Nationwide Mutual Insurance Company write the most commercial property insurance coverage in North and South Carolina, according to a report on Wednesday by Moody's, citing data from SNL Financial. Other top commercial property insurers in the region include The Travelers Companies, Chubb Ltd. and a U.S. subsidiary of Zurich Insurance Group , Moody's said.

The S&P Composite 1500 Property & Casualty Insurance Index fell 0.5 percent on Wednesday, with shares of insurers including Travelers Companies Inc, Allstate Corp, American International Group Inc and Chubb Ltd trading down less than one percent.

"Stocks tend to fall in front of a storm because of uncertainty," said Sandler O'Neill analyst Paul Newsome. They typically rebound once insurers and risk modelers get a clearer picture of actual losses and whether individual companies have enough capital in place to cover them, Newsome said.