Why Slow-Moving Tropical Storms And Hurricanes Are The Worst

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

Sometimes these tropical cyclones may not have powerful wind speeds, but the threats posed can be amplified due to their sluggish pace.

Hurricanes Florence and Harvey are recent reminders of the dangers caused by slow-moving storms since they both caused massive flooding

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.

Slow forward speed means major flooding. 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.

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.

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.

Slow-moving storms prolong coastal flooding, too. 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.

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.

Chris Dolce has been a senior meteorologist with weather.com for over 10 years after beginning his career with The Weather Channel television network in the early 2000s.