Hurricane Harvey's Record Rain: How a Stalled Storm Flooded Texas in 2017

Hurricane Harvey, the first major hurricane (winds above 110 mph) to make landfall in the United States since 2005, will go down as one of the wettest storms in the nation's history. Rain measured in feet fell across coastal Texas, affecting millions of people.

Hurricane Harvey on August 25, 2017, from the VIIRS instrument on the Suomi NPP satellite. NOAA Climate.gov.

Landfall

Harvey began as a disturbance that left the coast of West Africa in mid-August but did not organize until it reached the Bay of Campeche. Over the warm Gulf of Mexico, with little wind shear (change in wind speed or direction with height) to disrupt it, it strengthened fast. It came ashore near Rockport and Port Aransas as a still-strengthening Category 4 hurricane with 130 mph winds, the strongest to hit Texas since Hurricane Carla in 1961. It flattened buildings across Aransas County, left Victoria without power or water and destroyed whole blocks of Rockport. And that was only the beginning.

The storm that would not leave

The upper-level winds that usually steer tropical storms were weak, and high pressure to the north blocked Harvey's way out, so it stalled over Texas for days, drawing endless moisture from the warm Gulf and wringing it out over the coast from Corpus Christi to Houston to Beaumont.

• 48.20 inches fell at a gauge on Clear Creek at I-45 near Houston, the most from a single storm anywhere in the continental United States.
• More than 6.7 million people across 29,000 square miles, an area the size of West Virginia, got at least 20 inches in seven days, and a large area east of Houston got 40 inches.
• Houston had two of its five wettest days ever back to back: more than 8 inches on August 26 and about 16 on August 27.

Estimated rainfall, August 25–30, 2017: over 20 inches across a large area, nearly 50 in places. NOAA Climate.gov, from the Advanced Hydrologic Prediction Service.

At Houston's Hobby Airport, the rain of August 26–28 exceeded the previous record for the wettest 65-day period. Three of the airport's five wettest days since 1930 fell during Harvey, and August 26–29 brought 35.6 inches, nearly double the old four-day record. August was the city's wettest month on record, 39.11 inches, more than twice the previous record of 19.21.

The 50 wettest four-day periods at Houston’s Hobby Airport; the top four all involve Harvey. NOAA Climate.gov, from NCEI data.

As the storm crept east, Beaumont broke its wettest-day record on August 29 with more than 26 inches, over twice the old record, and finished August with 54.73 inches, more than three times its previous wettest August and almost 32 inches above its wettest month ever, November 1902.

How rare?

Meteorologists describe extremes by how likely they are in a given year, which requires an up-to-date historical record, and NOAA was updating its Texas precipitation frequency data when Harvey struck, so an official verdict had to wait. Preliminary estimates from the National Weather Service suggested some places received rain with a 0.1 percent chance in any year, a "1,000-year" event. A University of Wisconsin analysis of one-day totals reached the same figure, and a private firm's analysis put the chance of five-day totals like Harvey's at 0.004% to 0.0002% a year.

Before: the Houston area from NASA’s MODIS. NASA Worldview.

After: flooding along the Brazos River south of Houston. NASA Worldview.

The floods

Rivers and creeks rose to records across the region. The Harris County Flood Control District estimated that one trillion gallons of rain fell on the county in four days, and that about 70% of the county was under at least 1.5 feet of water. Thousands of people had to be rescued from their homes, and flooding spread east to Beaumont and Port Arthur. Refineries and contaminated Superfund sites were swamped too. The full cost was not yet known, but Harvey seemed likely to rank among the worst weather disasters in U.S. history.

Climate change

Asking whether climate change caused Harvey is the wrong question. Climate change alters the background in which all weather happens. It does not create hurricanes, but it can make them worse: by warming the ocean that fuels them, by warming the air so it holds and drops more water, and by raising sea level through melting ice and expanding seawater, so coasts flood more easily. Studies concluded that global warming contributed to Harvey's heavy rain.

Sources

• NOAA Climate.gov, "Reviewing Hurricane Harvey's catastrophic rain and flooding"; rewritten in hubnx's own words. Images: NOAA Climate.gov and NASA.