Three Reasons Slow-Moving Tropical Storms and Hurricanes Are the Worst

 Chris Dolce

Published: September 13,2020

Tropical storms and hurricanes that move slowly near the coast or inland are among the most feared by forecasters.

Sally is forecast to briefly slow down as it approaches the northern Gulf Coast early this week. That slow movement could prolong impacts from rainfall flooding, storm surge and strong winds in some areas.

Here's a look at what a slow speed means for rainfall potential and some other typical threats from a storm that's in no hurry to exit a region.

1. Slow Forward Speed Means Greater Flood Danger

The amount of rainfall a 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. When this occurs near mountainous terrain, rainfall totals can be further enhanced.

The amount of rain expected from a tropical storm or hurricane 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. Strong winds contributed to 8% of the deaths during that time.

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

Hurricane Harvey is the most extreme example of the major flooding caused by a storm that stalls out or moves slowly. Catastrophic damage occurred in Houston and other parts of southeastern Texas after Harvey spent days soaking the state.

Tropical Storm Allison and its remnants loitered for days in the Houston area in June 2001, unleashing more than 40 inches of rain that resulted in massive flooding and 23 deaths in Texas alone.

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

2. Persistent Winds

Tropical storms and hurricanes can down trees no matter what speed they move, but when the winds continue for hours and hours, impacts can be enhanced.

To begin with, long-lasting winds compound the stress on trees.

Making matters worse is the heavy rain from slow storms quickly saturating soil conditions. The persistent winds in combination with the soaked soil can make trees topple easier at lower wind speeds than otherwise with drier soil.

This can lead to widespread tree damage and power outages.

3. Piling Up the Water

Storms that stall or move at a snail's pace near the coastline pile up water and prolong the coastal flooding potential. In addition, large battering waves over a longer period of time can worsen damage to the coastline, including severe beach erosion.

This is because the winds blow onshore for a longer period of time.

For coastal flooding, the persistence of winds doesn't allow the water to recede from inlets, bays and rivers as it would normally with a storm that moves inland at a more progressive pace.

Family members depart after salvaging items from their flooded home in Plaquemines Parish on Sept. 3, 2012, in Braithwaite, Louisiana, after Hurricane Isaac.

One of the most recent examples was Hurricane Isaac in 2012, which moved slowly and stalled at one point near the Louisiana coast. Though Isaac's maximum strength was a Category 1, its large wind field and slow movement prolonged the storm-surge inundation.

The persistent east to southeasterly winds at Shell Beach, Louisiana, produced an 11-foot storm surge, higher than that of Hurricane Gustav in 2008.

A so-called back levee was overwhelmed by storm surge in Plaquemines Parish, Louisiana, on Aug. 29, 2012, sending water up to 12-feet high into Braithwaite, Louisiana, an area that did not flood significantly in Hurricane Katrina.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.