The Washington PostDemocracy Dies in Darkness

Hurricane Center: Harvey’s ‘overwhelming’ rains were likely nation’s most extreme ever

Floodwaters from Harvey overflowed from the bayous around Houston in late August. (AP Photo/David J. Phillip)
Placeholder while article actions load

Hurricane Harvey unleashed a tropical deluge probably unsurpassed in U.S. history, the National Hurricane Center says. In an in-depth meteorological review of the storm released Thursday, the center said it was unable to identify any past storm that had unloaded so much rain over such a large area.

“[T]he areal extent of heavy rainfall is truly overwhelming, literally and figuratively,” the report said about the storm, whose catastrophic flooding engulfed Southeast Texas in August’s final week last year, killing 68 people.

Such extreme rain amounts  — which only have a 1 in 1,000 chance of happening in a given year — covered an enormous area, an accompanying geographic analysis showed.

“[I]t is unlikely the United States has ever seen such a sizable area of excessive tropical cyclone rainfall totals as it did from Harvey,” the report said. By one estimate, the storm dispensed more than 33 trillion gallons of water over Texas and the southern United States.

Harvey’s rains easily exceeded previous Texas landmark storms, such as Allison in 2001 and Beulah in 1967, as maps in the report show that Harvey was in a totally different league.

The report confirmed that peak rainfall totals reached record-crushing levels, just over 5 feet near Nederland and Groves, Tex., near Port Arthur. “Both of these values (and from five other stations) exceed the previously accepted United States tropical cyclone storm total rainfall record of 52.00 inches at Kanalohuluhulu Ranger Station, Hawaii, in August of 1950 from Hurricane Hiki,” the report said.

Isolated rainfall amounts might have even been more extreme. The report noted that radar estimated totals “as high as 65-70 inches in southeastern Texas.”

The excessive rainfall was caused in large part when the monster storm stalled over the Lonestar state, drawing moisture from the warm Gulf of Mexico and dumping punishing torrents over an extended duration. Additional factors helped intensify and focus some of the extreme rainfall. “While Harvey was very slow moving over Texas, not all drifting cyclones produce such torrential rain totals,” the report explained. It said Harvey interacted with a weak cold front over the region, which “hardly moved” and intensified the rainfall.

The consequence of such excessive rainfall was one of the most expensive natural disasters in U.S. history. “Harvey is the second-most costly hurricane in U.S. history, after accounting for inflation, behind only Katrina (2005),” the report said.

The report does not discuss the possibility that Harvey’s rains were affected by human-induced climate change, but independent analyses published in peer-reviewed journals found the heating of the air and sea from climate warming may have boosted Harvey’s rainfall output by at least 15 percent.

In addition to extreme rainfall, the Hurricane Center report said Harvey raked the area just northeast of Corpus Christi with Category 4-intensity sustained winds (around 130 mph) at landfall and generated a coastal surge of 8 to 10 feet above ground level.

Read more

Harvey is a 1,000-year flood event unprecedented in scale

60 inches of rain fell from Hurricane Harvey in Texas, shattering U.S. storm record

Harvey’s two-step across Texas: The peculiar meteorology behind a U.S. rain record