Why Intensity Doesn't Matter Much for Tropical Rainfall Potential

Tropical cyclones that move slowly near the coast or inland are among the most feared by forecasters because rain will continue adding up in the same areas for hours, if not days, regardless of their wind speeds.

Hurricane Florence in 2018 and Hurricane Harvey in 2017 are recent reminders of the dangers caused by slow-moving storms.

(MORE: Ocean Heat Was at Record Levels in the Gulf Before Harvey)

Slow Forward Speed Means Major Flooding

The amount of rainfall a tropical depression, tropical storm or hurricane can produce is a function of its forward speed. As illustrated in the graphic below, the slower the speed, the more incredible the rainfall amounts can become.

A storm chugging along at just 5 mph has a rainfall potential upwards of 30 inches, while one moving briskly at 20 mph typically produces much smaller rain totals in any given location.

The amount of rain expected from a tropical cyclone is just as important – if not more important in some cases – than the wind speeds.

About 27% of all U.S. hurricane deaths from 1963 to 2012 were from rainfall flooding. For comparison, strong winds contributed to 8% of the deaths during that time. The majority of the U.S. hurricane deaths – 49% – were from storm surge during the 1963-2012 period.

(MORE: Water is Deadliest Factor in U.S. Hurricanes and Tropical Storms)

(MORE: Before-and-After Images Reveal Houston's Disastrous Flooding From Harvey's Rainfall)

Just over a year later, in September 2018, Florence became the nation's second-wettest storm behind Harvey after it stalled over the Carolinas for days. Preliminary state rainfall records were set in both North and South Carolina. The highest total was 35.93 inches of rain near Elizabethtown, North Carolina, about 50 miles northwest of Wilmington.

Southeastern Texas saw another soaker in June 2001 from Tropical Storm Allison. Similar to Harvey, Allison and its remnants lingered for days in the Houston area, unleashing more than 40 inches of rain. The flooding from this storm resulted in 23 deaths in Texas.

In August 2008, Tropical Storm Fay moved very slowly across Florida and caused significant flooding in parts of the state. Fay made eight total landfalls, including four in Florida, and meandered over the Sunshine State for 138 hours. The tropical storm flooded 19,000 homes and caused an estimated $560 million in damage.

In 1994, flooding from Tropical Storm Alberto killed 31 people as it moved slowly across the western Florida Panhandle, Alabama and Georgia. Americus, Georgia, recorded 21 inches of rain in just 24 hours.

While not a tropical cyclone, a slow-moving area of low pressure brought extreme rainfall and historic flooding to southeastern Louisiana in August 2016. At least four locations in Louisiana saw more than 2 feet of rain, with the highest amount being 31.39 inches near Watson, Louisiana. Record flooding was observed on at least 11 river gauges in the state.

(MORE: Historic August 2016 Flooding in Louisiana)

For comparison, the highest rainfall total in Louisiana during Harvey was 23.71 inches west of Vinton, Louisiana, according to the National Hurricane Center.