A leading climate forecaster estimates the economic cost of Hurricane Irma on the United States alone will come in at around $100 billion US, which would bring the economic toll combined with predecessor Harvey to nearly $300 billion US.

After battering Florida on Sunday as a Category 4 hurricane and working its way north, the storm has passed through much of the state and on to the panhandle and Georgia. It's expected to bring dangerous winds and flooding to areas as far north as Kentucky and West Virginia, even though its intensity is predicted to be downgraded to a tropical depression on Tuesday.

Preliminary estimates of the storm's damage are just starting to come in, and it's already clear the economic toll will be significant. More than four million Floridians remained without power Monday, and much of the state's infrastructure will need repairing or replacing.

Florida Power and Light, the state's main utility, says the storm will require an "extensive rebuild" of the electrical grid. "Our West Coast customers will likely be without power for a much longer period of time," CEO Eric Silagy said.

Construction activity like that would likely add to GDP over the long run, but costs a lot in the short term.

Much of the losses will be uninsured.

Stocks in insurance companies had been hit hard last week as the storm approached, and recovered on Monday after it became clear that pricey Miami real estate had been largely spared the worst of the storm.

Citibank analyst James Naklicki estimates U.S. insured loses to be about $20 billion US. But the storm's overall tally will be much more.

"We believe the damage estimate from Irma to be about $100 billion, among the costliest hurricanes of all time," said Joel N. Myers, president of forecasting firm AccuWeather.

Trees down everywhere in downtown <a href="https://twitter.com/hashtag/Miami?src=hash">#Miami</a>. Crews already working to clear debris. 70% of city without power still. <a href="https://twitter.com/hashtag/irma?src=hash">#irma</a> <a href="https://t.co/peTOX3D3vK">pic.twitter.com/peTOX3D3vK</a> —@JWagstaffe

Hurricane Harvey brought even costlier damage to parts of Texas, giving the U.S. economy a double shot that could be large enough to show up in national numbers. The 2017 hurricane season marks the first time since record-keeping began that two Category 4 storms have made U.S. landfall in the same year, AccuWeather said.

"We estimated that Hurricane Harvey is to be the costliest weather disaster in U.S. history at $190 billion or one full percentage point of the GDP," Myers said. "Together, AccuWeather predicts these two disasters amount to 1.5 of a percentage point of the GDP, which will about equal and therefore counter the natural growth of the economy for the period of mid-August through the end of the fourth quarter."

3,800 flights cancelled

The storm is still playing havoc with airlines. Some were criticized as fares for flights out of Florida rose before the storm. Now the industry faces thousands of flight cancellations to stay away from the massive storm.

More than 3,800 U.S. flights scheduled for Monday were cancelled by late morning and more than 9,000 have been dropped since Saturday, according to tracking service FlightAware.