On Wednesday it was a tropical storm. By Friday it had been supercharged from a Category 1 hurricane to Category 4. That’s because Harvey passed over an . . extremely warm eddy, 1 to 2 degrees F warmer than the surrounding Gulf of Mexico . . [it] has become wedged between two areas of high-pressure, one system over the SE U.S, the other over the SW U.S. . .
Harvey is still producing so much rain despite being mostly over land [because] it has dropped so much water over such a large area of SE Texas that the storm is pulling that water back up into itself and dumping it again as more rain . . [The] flooding in Houston is so severe [because] water swollen rivers heading to the sea is meeting a storm surge coming inland. In Galveston the sea surge was about 3’ but the actual water surge was about 9’ . .
A low-pressure trough system has been forming north of Harvey and could begin to pull it northward by the end of the week.
[www.theguardian.com/us-news/live/2017/aug/28/ex-hurricane-harvey-houston-flooded-as-catastrophe-unfolds-in-texas-latest-updates] 1740 BST