How does Harvey measure up to Hurricane Katrina, other US flooding catastrophes?

Even before Harvey’s initial landfall over Port Aransas and Port O’ Connor, Texas, on Aug. 25, comparisons to 2001’s Tropical Storm Allison and other major United States flooding disasters had begun to emerge.

Allison remains the only tropical storm name ever to be retired and is the costliest tropical storm on record, having caused $5 billion in damage and killed 41 people along the Gulf Coast.

Much like with Harvey, Houston suffered the worst of Allison’s impacts in June of 2001. A rainfall total of 36.99 inches was reported at the Port of Houston, while several other locations reported more than 30 inches of rain, according to the National Hurricane Center (NHC).

As Harvey’s impacts worsen, from the mounting death toll to major flooding and record-breaking rainfall totals, the once-Category 4 storm is well on its way to becoming the worst flooding catastrophe the U.S. has ever endured.

“Think of the significant area that’s been affected by devastating rains, and we don’t know the extent of it,” said Dr. Joel N. Myers, AccuWeather's founder, president and chairman.

“It’s probably much worse than the news media is capturing because they can’t get in there,” Myers added.

The storm has covered a larger area than Allison and killed at least 36 people.

Harvey’s average rainfall totals have already surpassed those of Allison in half of the time. It took Harvey between two and three days to drench southeastern Texas with record-shattering rainfalls, and Allison caused immense flooding over a five-day period.

The West Gulf River Forecast Center reported rainfall from Harvey in Cedar Bayou, Texas, at 51.88 inches over five days, and the storm’s heavy rainfall in Houston led to August 2017 being the city’s wettest month on record.

“[Harvey] got locked into place; it just couldn’t move and it was a very strong circulation, which, of course, pulled in tremendous amounts of moisture,” said Bill Murray, president and weather historian for The Weather Factory.

“The development in Houston has been so intense over the last ten years and a lot of it did not follow good zoning practices,” Murray said.

Harvey and Katrina: Comparing the storms

Hurricane Katrina, a Category 3 storm that inundated parts of Louisiana and Mississippi with disastrous flooding and killed more than 1,800 people in 2005, is considered one of the most devastating hurricanes ever to hit the U.S.

Though both Harvey and Katrina caused immense flooding, it was Katrina’s storm surge of up to 28 feet that breached levees and engulfed parts of the Gulf Coast, including New Orleans.

According to the Natonal Weather Service (NWS), Katrina yielded 5 to 10 inches of rainfall in two days.

“Harvey was stronger at landfall and produced more rain over a wider area than Katrina,” said AccuWeather Meteorologist Jesse Ferrell. “However, the coastal damage [including wind and storm surge flooding] was much more limited in terms of population.”

Harvey has dropped at least 50 inches of rainfall in parts of Texas, particularly in Houston, the country’s fourth-largest city.

The storm’s rainfall has been so intense that the NWS had to update color charts on their precipitation graphics in order to map Harvey effectively.

Though Katrina is currently the costliest-ever hurricane, it is predicted that Harvey’s damage will crush that record.

“There are estimates that Katrina caused $100 billion and Sandy $60 billion [in damage],” said Myers.

“[We] estimate that this storm will be at least $190 billion, which far exceeded those,” he said.

There are also concerns that the number of fatalities resulting from Harvey will surge as the recovery progress slowly begins.

“Given the conditions that AccuWeather is analyzing, we’re fearful that the death toll may go much higher because so many people in towns are still unaccounted for,” Myers said.

Harvey’s catastrophic flooding has so far prompted more than 10,000 water rescues, according to FEMA, and it’s too early to determine whether the number will yet surpass Katrina’s 30,000 rescues, Ferrell said.

“Since the rescues haven't stopped yet and non-governmental groups are rescuing people, we could see Harvey's number approach Katrina's before it's all over,” he said.