September 01, 2017
Hurricane Harvey, now downgraded to tropical depression Harvey, dumped 50 inches of rain on parts of the Texas coast this week. This epic storm has wreaked havoc on a large swath of the southwest and left destruction and devastation in its wake. When a large low pressure system moving in from the sea runs smack dab into a high pressure system over the coast, it’s a recipe for a natural disaster. Counter-clockwise circulating air vacuums up moisture from the Gulf, and all that warm, moist air rising up must eventually come down. And come down it did. “Harvey came inland about 200 miles south of Houston, and the outer rain bands pushed into Houston on Saturday. . . Houston lies a few dozen feet above sea level, and during normal rainfall residential yards drain into streets, streets drain into bayous, and bayous carry water into Galveston Bay and the Gulf of Mexico.
But this was not normal rainfall; it was extreme tropical rainfall. Meteorologists measure rainfall rates in inches per hour at a given location. A rainfall rate of 0.5 inches per hour is heavy, while anything above 2.0 inches per hour is intense (you'd probably stop your car on a highway, pull over, and wait out the passing storm). [In the Houston area], from 11pm to 1am that night, 10.6 inches of rain fell, about as much rainfall as New York City gets from October through December. That happened in two hours. Ars Technica