Why do tropical cyclones slow down or stall over the South?

Another hurricane season has resulted in a cyclone coming to near a screeching halt, but to weather forecasters, the lack of movement is not surprising considering the storm system’s location.

Hurricane Debby made landfall on August 5 near Steinhatchee, Florida, as a Category 1 cyclone. 

The hurricane battered coastal communities along the Sunshine State’s Gulf Coast, but by the end of the cyclone’s life cycle, it might be known for its flooding versus any type of wind damage because of its slow forward movement.

According to the University Corporation for Atmospheric Research, tropical systems usually have an average forward speed of 15-20 mph. 

For a cyclone that is 300 miles wide, a city directly impacted by the storm could spend 14 to 24 hours within the heart of the system. 

In the heaviest bands, rain can fall at rates of around an inch per hour, which can lead to flooding even in a relatively fast-moving system. 

In slower-moving cyclones, communities may experience prolonged downpours, resulting in a significant increase in both rainfall and flooding.

SEE IT: DEBBY’S HURRICANE-FORCE WINDS SLAM FLORIDA AT LANDFALL AFTER INTENSIFYING OVER GULF OF MEXICO

Forecasters note that if there is a particular area in the basin where tropical cyclones are likely to slow their forward speed, it is around 30 degrees north latitude, give or take 100 miles. 

Cities such as Houston, New Orleans, Jacksonville and Mobile, Alabama, are situated along or near this latitude, with Charleston, South Carolina, Savannah, Georgia and Tampa not far away.

At these latitudes, a combination of weather factors can influence and dictate the future movement of a tropical cyclone. 

Generally, areas south of 30 degrees north latitude are considered subtropical, where the prevailing flow is from east to west. However, around this latitude line and further north, the mid-latitude westerlies dominate, driving systems from west to east.

As a cyclone moves from one region to another, it naturally slows down while transitioning through this zone. 

During this transition, a ridge of high pressure, such as the Bermuda High, can build up and create a wall-like feature that the cyclone must navigate around. This can cause the cyclone to slow down its forward speed or become stationary.

WHEN IS THE TYPICAL LAST HURRICANE STRIKE ON THE US COAST?

In the case of Tropical Storm Debby, it is stuck in the transition region from the subtropics to the mid-latitudes. 

Until a strong enough force moves it, the storm will likely linger along the Southeast coast for several days. 

Typically, hurricanes and other tropical weather features accelerate off to the north and east, but in this instance, a building ridge of high pressure over the Atlantic is blocking its exit path.

During this waiting period, the cyclone will likely meander or perform a loop-the-loop motion, with rainfall totals of one to two feet possible along the coasts of Georgia, South Carolina and North Carolina.

The rainfall accumulations sound impressive and will likely produce extreme flooding, but it isn’t the first time these communities have been affected by tropical rains. 

Hurricane Florence in 2018 produced similar rainfall totals over the Carolinas, after it spent days over the Southeast.

Elsewhere, communities along the same latitude remember Hurricane Harvey in 2017 and Tropical Storm Fay in 2008.

Some climatologists argue that warming oceans are increasing atmospheric moisture, which in turn is leading to more intense rainfall events.

If this is indeed the case, much of the country’s impact zone would fall within these hurricane-affected areas, leading to more disastrous flood events during tropical cyclone impacts.