The slow-moving storm has been pulling in tropical moisture.
Nicholas only reached hurricane strength for a few hours, but like two other storms in the North Atlantic basin this season, it showed that a tropical cyclone need not have fierce winds to wreck havoc. Like Elsa and Henri earlier this summer, slow-moving Tropical Storm Nicholas has dropped flooding rainfall that has not yet subsided along the Gulf Coast of the United States.
Since landfall early on September 14, 2021, Nicholas has brought torrential rainfall to Galveston, Houston, and other parts of southeastern Texas and southwestern Louisiana. As of the afternoon of September 15, the remnant tropical depression was lingering over Louisiana, Mississippi, Alabama, and the Florida Panhandle, dropping heavy rain on soils still saturated and waterways still full to the brim after Hurricane Ida. More than half of Louisiana has been under a flood watch, and some isolated areas were expected to see rainfall totals measuring as much as 10 to 20 inches (25 to 50 centimeters).
As has long been predicted by climate scientists, the atmosphere now generally holds more moisture due to global warming.
This has led to stronger and more frequent downpours—even within everyday rainstorms—and more stalling storms that drop excessive amounts of rain in focused areas. So far, Nicholas seems to be following this sort of pattern. Hurricane Harvey performed a devastating, long-lasting dance along the Gulf Coast in 2017.
As pointed out by scientists at the Cooperative Institute for Meteorological Satellite Studies, Nicholas also had an extra fuel source: an atmospheric flow of moisture from the tropics. Earth has a persistent band of rain-producing clouds that hover around the tropics due to the way trade winds meet near the equator and push moisture up in the atmosphere. Scientists refer to this area as the intertropical convergence zone. Data and models suggest that Nicholas may be pulling in moisture from this region.