A new study in Nature this week showed results of research on the forward speed of hurricanes and how it has changed over time. A rough rule of thumb for the amount of rain dropped by a tropical system is to take 100 and divide it by the forward speed in miles per hour to get an estimate of the amount of rain from the storm. For the 1994 Tropical Storm Alberto, which stalled in southern Georgia with a forward speed of about 3 mph, the estimated rain from the storm would have been 100/3 or 33 inches of rain. Considering that Americus received 21 inches in just 24 hours, that was not a bad estimate.
Hurricane Harvey in Texas last year was also a slow-mover, although the amount of rain that fell exceeded even the rule of thumb above. The new research shows that on average, the forward speed of hurricanes has decreased by about 10 percent, which would mean an increase in rain for the storm using the rule of thumb. This has grave consequences for infrastructure, houses, and other property in the floodplains (and sometimes even outside the flood plains). You can read more about it and listen to the story at National Public Radio here.