Tropical cyclone · Texas, United States
Harvey: the hurricane that stood still
Hurricane Harvey made landfall near Rockport, Texas, on the night of 25 August 2017 as a category 4 storm. Then the winds that should have carried it inland died away. For four days it drifted over south-east Texas, drawing moisture from the Gulf of Mexico and wringing it out over Houston. A gauge near Nederland collected 1,539 mm, the most rain from a tropical cyclone ever measured in the United States.
25–30 August 2017 · Houston and south-east Texas · ERA5
1,539mm
Highest storm total
60.58 inches near Nederland, Texas: the US record for a tropical cyclone.S1
458mm
ERA5 over Houston, 25–30 Aug
The Lab's grid point nearest the city. A grid cell averages; gauges in the cell measured far more.
×3
More likely with today's climate
Rain like Harvey's, and about 15 % more intense, than in a climate without global warming.S2
What happened
Harvey had been a weak storm in the Caribbean. Over the warm Gulf of Mexico it grew from a tropical depression to a category 4 hurricane in about two days and struck the Texas coast at Rockport.S1 Usually a hurricane weakens and moves away after landfall. Harvey did neither: it slowed to a crawl, its centre wandering near the coast, while its rain bands trained over Houston day after day.
Bayous and reservoirs overflowed, much of Harris County flooded and tens of thousands of people were rescued from their homes. Harvey caused about $125 billion of damage, second only to Katrina among US hurricanes.N1
Four days over Houston
The daily maps show why: the heaviest rain hardly moves. At the grid point over Houston, ERA5 gives about 100 mm a day for four days running.
| Day | Rain mm |
|---|---|
| Aug 25 | 18 |
| Aug 26 | 68 |
| Aug 27 | 108 |
| Aug 28 | 93 |
| Aug 29 | 126 |
| Aug 30 | 45 |
| 25–30 Aug | 458 |
Why it stalled
A tropical cyclone is carried by the winds around it, mostly those of the middle troposphere. In late August 2017 Harvey sat between two areas of high pressure, one over the south-western United States and one over the south-east. Between them the 500 hPa flow was weak and without direction. With nothing to steer it, the storm drifted, and the Gulf kept feeding it moisture.
What the grid sees
458mm
ERA5's six-day total at Houston, against 1,000 mm and more at many gauges around the city and 1,539 mm at the wettest. A 28 km grid cell spreads the heaviest bands over its whole area, and the reanalysis model makes less rain in tropical cyclones than falls.S1
Studies of the rainfall found that global warming made rain like Harvey's about three times more likely and about 15 % more intense: warmer air and a warmer Gulf hold and supply more water.S2
Explore in the Lab
Open the event in the Climate mode
Each button opens the Lab on that view, with the date, map layer and data set already chosen.
References
News
- N1KHOU 11. NHC: Harvey is second-most costly hurricane in US; 68 deaths in Texas. 25 January 2018.Deaths and damage from the National Hurricane Center's report.
Studies and guidelines
- S1Blake, E. S., Zelinsky, D. A. (2018). Hurricane Harvey (AL092017). National Hurricane Center Tropical Cyclone Report. NOAA National Hurricane Center.The official account of Harvey: track, intensity, rainfall totals.
- S2van Oldenborgh, G. J., et al. (2017). Attribution of extreme rainfall from Hurricane Harvey, August 2017. Environmental Research Letters, 12, 124009.How much global warming changed the likelihood and intensity of Harvey's rain.