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WATCH LIVE: Aerial video of flooded Houston areas

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With its relentless rain and refusal to go away — having already made three landfalls — Harvey may end up being one of the costliest natural disasters in U.S. history.

But pinning an exact price tag on the storm is more of an art than a science.

Experts have already begun to offer damage estimates, even as the historic floods keep rising. The range, however, is wide: Hannover Re, one of the largest re-insurers in the world, predicted a price tag of $3 billion on insured losses, while Accuweather projected it to have a $190 billion impact on the economy.

Part of the reason for the discrepancy is it's not an apples-to-apples comparison: The "insured losses" category only incorporates what insurance companies will be on the hook for, while the cumulative economic impact is much broader and includes losses for businesses.

Yet, even accounting for those differences, it's still difficult to nail down how much any hurricane, especially one as complex as Harvey, will cost, said Chuck Watson, a geophysical hazards modeler with Enki Research. Watson's firm calculates risks and costs of hurricanes, tsunamis, and other natural disasters. They give a middle-of-the-road estimate for Harvey costs at anywhere from $48 to $75 billion.

“Harvey has just been a miserable storm to try to forecast because the traditional forecast models just don't work for this storm.”

"Harvey has just been a miserable storm to try to forecast because the traditional forecast models just don't work for this storm," Watson said. "There's only a couple of computer models in the world that do a halfway decent job, and depending on the assumptions that you use, you can get a wide range."

And there are a lot of assumptions to choose from when estimating damage costs. When experts input data into simulation models, they must incorporate everything from the latest path a hurricane is forecast to travel to seemingly minute details such as whether wind-blown trash might clog drain systems, Watson said.

He added: "The problem is with a big system like Harvey, you're so dependent on things you can't predict," like where the rain bands are going to hover and for how long — or how the Corps of Engineers might stave the flow of water after an unprecedented event, such as a reservoir topping a major dam in Houston for the first time ever.

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