Tornadoes a secondary but dangerous offshoot of hurricane season

North Carolina is prone to violent tornadoes and the widespread destruction from hurricanes, but sometimes those spinning threats can come at the same time.

Tornadoes spawned by hurricanes and tropical storms may not make headlines like flooding and beach erosion, but they are not necessarily rare. One out of every six tornado touchdowns in North Carolina is caused by a tropical system. Days after a hurricane has weakened over land, it can strike inland towns with tornadoes that spin up and vanish in a matter of minutes.

Early on the morning of August 13, 2004, a tornado with winds of at least 110 mph ripped through dozens of homes in the small town of Rocky Point, just north of Wilmington. This tornado that injured 29 people and claimed three lives was caused by Tropical Depression Bonnie –­ a relatively weak system that moved northeast out of the Gulf of Mexico. That is the only recorded instance of a fatal tornado in North Carolina with tropical origins. (Tropical Storm Bonnie is not to be confused with the strong 1998 hurricane of the same name which made landfall in southeast North Carolina and caused several tornadoes of its own).

The hurricane season in 2004 wasn’t very active for the North Carolina coastline, but large storms like Charley, Frances, Ivan and Jeanne pounded Florida and the Gulf Coast and curved towards North Carolina as they weakened. This parade of tropical activity made 2004 North Carolina’s most active year for tornadoes, even ahead of years with large springtime outbreaks like 2011 and 1984.

North Carolina has experienced tornadoes caused by major hurricanes that made national headlines, like Hurricanes Floyd, Ivan and Hugo. Even tropical storms with weak winds still have the necessary spinning pattern for tornado outbreaks. Storms with less memorable names – like Beryl, Chris, Earl and Cindy – have also been responsible for damaging twisters.

The ingredients that form these tropical tornadoes – and their behavior on the ground –­ are very different from the typical springtime storm. Tornadoes won’t form in any season without the right balance of moisture and heat, and a parent storm carved by twisting and shearing winds. The ideal atmosphere for a springtime tornado outbreak is inhospitable for a hurricane and vice versa. But once a hurricane or tropical storm reaches land, the balance of ingredients can change.

A hurricane’s winds are slowed by friction after reaching shore, and the spiral shape can be distorted when it encounters a cold front. This weakening in the hurricane actually makes more of the wind shear necessary for a tornado.

Coastal residents know that the right front quadrant of a hurricane has a reputation for being the most dangerous. This holds true for tornadoes as well. Almost all of the tropical–­related tornado touchdowns in North Carolina were to the right of the storm’s track, which is often to the northeast, east or southeast of the center of low pressure.

The individual storm cells responsible for the tornadoes are embedded in the spiral rain bands that extend from the center of a tropical storm or hurricane. Most are found between 60 to 300 miles from the circulation center or eye. Tropical systems can hit land at any time of day or night, but the heat of the afternoon is the most favorable time for tornadoes to develop in the rain bands. Swirls of wind can tighten into a tornado in minutes and dissipate just as quickly, usually staying on the ground for a mile or less.

Tornadoes embedded in the rain bands of a hurricane don’t necessarily follow the familiar southwest­-to-­northeast motion seen in springtime tornadoes. They can be steered towards the north, northwest or even west by the large counterclockwise motion. A tropical system is very unlikely to cause a violent, long-­lived tornado like the kind that ripped through Central North Carolina in April 2011. It doesn’t take a high-­end tornado, however, to knock over trees and damage homes, especially after tropical rainfall has soaked the ground for hours on end.

Hurricanes that track directly into the North Carolina coast or take a parallel path up the Atlantic Coast are responsible for many tornadoes east of Interstate 95. Central and Western North Carolina are vulnerable to tornadoes from hurricanes that make landfall in the Gulf of Mexico.

Even two or three days after landfall, the remnants can track along the Appalachian Mountains and spawn rotating storms as far away as the Triangle and Piedmont. 2011’s Tropical Storm Lee were still spinning through southwest Alabama when it led to a tornado 450 miles away in the foothills of northwest North Carolina.

The low clouds and blinding rain from a tropical system makes these tornadoes nearly impossible to see. Before the advent of Doppler radar in the mid-1990s, meteorologists didn’t have the ability to detect a tornado embedded in heavy rain.

Today, forecasters can anticipate the ingredients and closely monitor radar signatures when tropical systems hit land.