It’s still too early to know Hurricane Harvey’s final classification, but experts warn that the current system of terminology needs reworking.
“When you’re talking about a 100-year storm, it is complicated and confusing,” said Hal Needham, founder and president of Marine Weather & Climate, a firm specializing in flood risk analysis, flood reconstruction, storm surge prediction and coastal climatology.
“Are you talking about wind speed, rain or storm surge? With Harvey, you’re pretty much talking about the water and the rain that fell.”
Hurricane Harvey made landfall Aug. 25 in Rockport, about 200 miles south of Galveston County, but in the 72 or so hours that followed, it dumped more than 50 inches of rain in some parts of the county, swelling creeks and bayous and flooding an estimated 20,000 homes in the county.
Expert opinions about how the storm should be classified have varied.
Officials with the Harris County Flood Control District said the numbers exceeded that of a 500-year rain event. Others have classified Harvey as everything from an 800-year storm to a 1,000-year storm.
“What people don’t understand there is that the terms indicate the probability of an occurrence for each return period,” said Scott Overpeck, a meteorologist for the National Weather Service in League City. “That is the probability it could happen in any one given year.
In other words, the different classifications — 500-year storm, 800-year storm, etc. — correspond to probabilities, Overpeck said.
“They get the idea that a 500-year storm only occurs once every 500 years,” Overpeck said. “That’s not what it means. That’s where the confusion comes in. They think that this is a once-in-a-lifetime storm and will never happen again, and that’s not necessarily true.”
A 100-year storm has a 1 percent probability of happening in any given year, while a 1,000-year storm has a 0.1 percent probability of happening in a given year, Overpeck said.
To determine the probabilities, scientists took the available data and charted the storms based on their size, Needham said.
Further complicating matters is the fact that exact measurements change from location to location, Overpeck said.
“When classifying a storm, it’s always important to say what the criteria is,” Needham said. “In Harris County, it might be a 500-year event and in Friendswood, it might be a 1,000-year storm.”
Meteorologists don’t yet have exact numbers from across the region to be able to make final determinations, Overpeck said.
Hydrologists and insurance companies like using phrases like 500-year storms, but meteorologists at the National Weather Service have shied away from such terminology, Overpeck said.
“We’re sticking with phrases like ‘extreme event,’” Overpeck said.
Part of the problem with the terminology is that it confuses people, especially when a city like Houston has four 500-year floods in a 20-year period, Needham said.
“I realized, not being cynical, that no one knows what the 100-year level is,” Needham said. “There are just different methods for estimating — models, data, observation — anything they can do to get a better estimate.
“It’s becoming more likely we underestimated the 100-year storm level. A storm we thought was a 500-year storm is really 50.”
The confusion could be because of insufficient data, or it could be because of climate change, Needham said.
“There is evidence that sea levels are rising and that they are warmer than they used to be,” Needham said. “With Harvey, you had the storm stall and blow winds onshore for days over warm water. Will we see more hurricanes? I’m not sure you can say that from a climate change perspective, but sea temperatures are rising.”