Hurricane Harvey is "almost certain" to be at least the third worst storm in U.S. history when it comes to damage costs, according to Enki Research.
The firm's current estimate ranges from $48 billion to $75 billion, with an average of $57 billion. But those numbers are subject to change with each passing day.
"The computer models we normally use to do estimates for hurricanes don't really work that well for this kind of a storm," Chuck Watson, the director of research and development at Enki Research, said in an interview with "Power Lunch."
"This is more like a big, broad flooding low-pressure type thing and it's over such a large area that a lot of the normal assumptions that you build into the system start to break down."
There is also uncertainty due to the human factor, such as decisions on whether to do controlled releases from some of the reservoirs, he explained.
Harvey, now a tropical storm, bore down on eastern Texas and Louisiana on Wednesday, bringing the catastrophic downpours that paralyzed Houston.
The storm first came ashore on Friday near Corpus Christi, Texas. It was the most powerful hurricane to hit Texas in more than 50 years, killing at least 17 people and forcing 30,000 people to flee to emergency shelters.
JPMorgan predicts the eventual insured losses from Harvey could be as much as $10 billion to $20 billion.
However, Watson pointed out that he's currently estimating about 20 percent of the damage will be covered by insurance.
"It's going to be a huge hit to the flood insurance program, but we're currently thinking as much as 70 or 75 percent of the losses and impact from this storm will not be insured," he said.
— CNBC's Stefanie Kratter and Reuters contributed to this report.