Hurricane Harvey’s rainfall is, quite literally, off the charts. The US National Weather Service needed to add two colors to its flood maps—a dark purple hue to represent “20 to 30” inches (51-76 cm) rainfall, and a lighter purple to indicate “greater than 30 inches.”
Prior to Harvey, which has been dumping rain on Texas since the night of Aug. 25, the NWS maps stopped at “greater than 15 inches” designation, marked by a dark mauve.
The agency called Harvey’s rainfall “unprecedented.” Parts of southeastern Texas have already received more than 30 inches of rain, with some areas forecast to get as much as 50 inches by the time Harvey, which has now weakened to a tropical storm, ends this week.
That record rainfall is thanks to an unusual situation. Hurricanes usually suck water up from the ocean and release it over land. But Harvey has already dropped so much water on Texas that the flooded area is acting like a small ocean; the storm is “pulling that water back up into itself and dumping it again as more rain,” according to Scientific American.