Houston is a mostly flat, rapidly growing city about 35 feet above sea level that has struggled to keep up with its flood-amelioration infrastructure, but it's hard to imagine a city that could handle up to 50 inches of rain in a week, much less the average of 33 inches that Tropical Storm Harvey has already dropped on Houston's Harris County.
— NYT Graphics (@nytgraphics) August 28, 2017
Harvey will likely "produce more rain than we have ever seen before in the U.S. from a tropical system, and over the fourth-largest city in the country," Eric Fisher, chief meteorologist at Boston's WBZ-TV, tells The Washington Post. "It's hard, if not impossible, to compare this to any other storm." It's also hard to visualize the 9 trillion gallons of water that the Post calculates Harvey had already dumped on Houston and southeast Texas by Monday morning, with 5-10 trillion more gallons expected this week. Still, people are trying.
Corrected: About 9 trillion gallons had fallen on *Southeast* Texas as of noon on Sunday pic.twitter.com/v8rLWJyhpN
— Post Graphics (@PostGraphics) August 28, 2017
For example, USA Today points out, 26 inches of rain on Harris County would "provide drinking water for the entire county for roughly five years." During Hurricane Katrina, The Times-Picayune notes, New Orleans got up to 13 inches of rain, and even with the storm surge that caused most of the flooding, the total floodwater in New Orleans and surrounding parishes was an estimated 282 billion gallons. Harris County Flood Control District official Jeff Lindner says about 732 billion gallons of water have already fallen on Harris County.
Matthew Cappucci at The Washington Post says if you took the water that's fallen on Texas and dumped it onto New Orleans, instead of the 10-20 feet of flooding the city saw during Katrina, New Orleans would be buried under 128 feet of water. He has more comparisons:
The 9 trillion gallons of water dispensed so far is enough to fill the entire Great Salt Lake in Salt Lake City — twice! It would take nine days straight for the Mississippi River to drain into Houston and equal the amount of water already there. If we averaged this amount of water spread equally over the lower 48 states, that's the equivalent of about 0.17 inches of rain ... occupying every square inch of the contiguous United States. [The Washington Post]