Hurricane Harvey could be the costliest natural disaster in US history — here's how we'll know the true cost (ALL, LEN, KBH, DHI)

So far, the consensus is that it won't cause that big of a hit to the overall economy's growth, although its personal impact already is and will continue to be far greater.

"We expect the effects of Harvey to be substantial and lasting," UBS economists said in a note. They added that it's difficult to forecast the full impact, even by comparing this to previous hurricanes, because each one is unique.

Here's how economists are calculating its impact:

The cost of Harvey

AccuWeather forecasts that Harvey would be the most expensive natural disaster in US history, at about $160 billion.

Other estimates are more conservative, but rising. According to data from the risk modeler Enki Research cited by Bloomberg on Wednesday, the hurricane could cause $75 billion in economic losses. That's up from $42 billion Tuesday.

Gas prices

Drivers are already paying more at the pump amid shutdowns of oil refineries in Texas.

Prices compiled by AAA showed that the average cost of a gallon of regular unleaded gas was $2.40 per gallon on Wednesday, up from $2.34 a week earlier.

That's because shutdowns have slowed the flow of gas from the epicenter of Texas' oil refinery industry, which processes nearly one-third of America's crude oil. Earlier on Wednesday, the Motiva Port Arthur refinery, the largest in the US, began shutting down as flood waters rose.

And because refiners are demanding less crude oil, prices are falling. West Texas Intermediate crude, the US benchmark, is down 4% this week.

"You've got a little less Eagle Ford production, and a little bit less Gulf of Mexico production, but it's nowhere near the demand that's been taken offline from the refiners," said Rob

Thummel said the sheer demand for gasoline would have the refineries back up and running as soon as they can. Goldman Sachs estimated that it could take "several months" for demand to return to pre-hurricane levels.

Exports

The US has become a bigger exporter of oil, and activity in Texas is strongly correlated with the national level.

According to Deutsche Bank, Houston accounted for 21% of chemicals exports in the second quarter.

"Harvey could potentially drag on H2 real GDP growth by as much as -20 basis points (bps)—mainly through disruptions to exports," said Brett Ryan, a senior US economist, in a note Monday.

Jobs

For Morgan Stanley, the first place to look in the economic data for the impact of Harvey is in initial jobless claims. That's the weekly tally of people who file for unemployment benefits for the first time.

In a note earlier this week, the firm's US economists pointed out the spike in initial claims in Louisiana after Hurricane Katrina, and in New Jersey after Hurricane Sandy.

They also suggested looking into the components of the monthly jobs report that show the number of people who were out of work or worked part time because of bad weather.

The hurricane is unlikely to have a notable impact on the headline number of jobs added, and definitely not in Friday's jobs report because the survey week that includes the 12th of the month was before the hurricane.

"Moreover, we are a few weeks away from the next survey period in September and Houston and its surrounding areas will have at least begun to climb out of the mess left in the wake of Harvey by then," said Ellen Zentner, Morgan Stanley's chief US economist, in the note. "According to the BLS, severe weather tends to have a much greater impact on hours worked, as opposed to payroll counts."

Car sales

Harvey may have a more immediate impact on the auto sector.

Up to half a million flooded cars and trucks could be scrapped, according to a Cox Automotive estimate cited by CNBC.

Auto sales, already in a slump this year, could be dragged lower when the latest monthly numbers are released Friday. Evercore ISI estimates the drag at between 100,000 and 200,000 vehicles.

Auto insurers will deal with claims for several months. For investors, now is a good time to bet against their stocks, according to David Rosenberg, the chief economist at Gluskin Sheff.

"If you are looking for short opportunities, look no further than auto insurers with heavy Texas exposure — like State Farm, Allstate, Progressive, Geico ... some of these stocks are up more than 20% so far this year," he said in a note Wednesday.

Housing

One reason why Harvey may not vastly harm the overall economy, according to Goldman Sachs, is that the rebuilding efforts in the future could offset slowdowns elsewhere.

"The result for builders in the short run is likely to be substantially delayed home closings and a dip in new orders in the region," said Carl Reichardt, an analyst at BTIG, in a note Monday.

A dozen homebuilders including D.R. Horton, Lennar, and KB Home are exposed to the South Texas disaster area, BTIG noted.