Why do hurricanes and tropical storms cause tornadoes?

Last week, the southeastern US experienced extreme rainfall from Hurricane Debby. Even as a tropical storm, Debby caused significant flooding in Florida, the Carolinas, Georgia and further up the East Coast. Flooding, storms and damaging winds are common in tropical systems that make landfall. Tornadoes are also a common weather hazard associated with tropical landfalling systems. Tornado warnings tend to catch people in a chaotic, evolving scenario involving a storm making landfall. Debby was no different. With another storm brewing in the Atlantic Ocean at the time of writing, I wanted to take a closer look at why tropical cyclones can spawn tornadoes.

Tornadoes are often found on the “dirty side of the eye” or right front relative to the circulation around the center. This part of the storm also produces the strongest winds and largest storm surge because the speed of the storm’s forward motion combines with the rotational speed of the wind. Why are tornadoes more prevalent in this region? The National Weather Service website says, “This area typically has the best wind shear and instability.”

Most tornadoes spawned by tropical systems are relatively short-lived and fall on the lower end (EF0 or EF1) of the Enhanced Fujita Scale. From my perspective, there really is no such thing as a “weak” tornado, so always keep that patch in your back pocket. They still represent one of many risks in a rapidly changing weather scenario.

Let’s dig deeper to explore why tropical cyclone tornadoes can occur. Most explanations mention “frictional effects” so I’ll start there Weather guys blog is a legendary and informative platform managed by my colleagues Jonathan Martin and Steve Ackerman (retired), professors at the University of Wisconsin. They write, “When a hurricane makes landfall, winds near the ground slow down, while upper-level winds maintain their momentum. This change in wind speed—and sometimes direction—with height is called wind shear.” However, there is more to the story.

A 2009 study published in the journal Science Monthly Weather Review provides a robust overview of the mechanisms in tornadoes associated with hurricanes. Near the inner core of a tropical system, the frictional slowing of the wind can also cause air to gather or “converge”. This leads to vertical movement and stretching of each circulation into a thin column (think skater bringing the arms in). According to the study, this extension of “vorticity” (a measure of rotation) in a moisture-rich environment can lead to the generation of an updraft or mesocyclone. Studies have shown that storms in the outer belts over water can support rotation, and this is further enhanced by friction as the belt moves over land.

The study goes on to note that tornadoes in the outer belts have been associated with daytime warming, the presence of some type of boundary, or dry air intrusions. Other characteristics of tropical cyclone tornadoes, according to the 2009 study and more recent literature, include:

• It is usually found within 100 to 150 miles of the coast.
• 90% or more are EF0/EF1 rating.
• Cells moving north or northeast at a speed of at least 11 mph are more likely to produce tornadoes.
• Over 90% of tornadoes occur within 48 hours of landfall, but inner core tornadoes are most likely within 24 hours of landfall.
• Outer belt tornadoes tend to be the most dangerous, especially when daytime heating is at its maximum.
• Short, narrow paths are common.

Because of these characteristics, tropical cyclone tornadoes are an elusive and challenging forecasting problem. Instability (usually identified by CAPE), lifting mechanisms, and wind shear values ​​can be deceptively weaker than Great Plains tornadoes. Additionally, forecasters often face multiple threats with land-based storms. It is important to have situational awareness of all landfalling storm threats. Don’t lock yourself into a “dangerous single” state of mind.