Tropical Storm Andrea has spawned as many as six tornadoes as of 1 p.m. EDT today (June 6), with many more tornado warnings being issued, according to various news reports. There is currently a tornado watch for most of the state of Florida until 9 p.m. EDT, and those watches could start to extend further north as the storm moves inland.
Hurricanes and tornadoes are typically thought of as separate phenomena, with tornadoes conjuring up images of the flat prairie and hurricanes associated with the warm, coastal tropics. Hurricanes are much, much larger than tornadoes, but tornadoes are capable of producing much faster winds than hurricanes. However, some tropical storms and hurricanes are capable of spinning up tornadoes, as Tropical Storm Andrea is doing.
But how do hurricanes and tropical storms create tornadoes?
Hurricanes and tropical storms, collectively known as tropical cyclones, provide all the necessary ingredients to form tornadoes. First, most hurricanes carry with them individual supercells, which are rotating, well-organized thunderstorms. (These are typically the storms that spin up monster twisters in the Plains. All tornadoes need thunderstorms to form, said Brian McNoldy, a researcher at the University of Miami.
Second, hurricanes bring with them warm, moist air, which acts as their fuel. This creates instability in the atmosphere — namely, a layer of warm air with slightly colder and less-moist air above. This arrangement is unstable because the warm air wants to rise, since it is less dense. [50 Amazing Hurricane Facts]
Finally, hurricanes create wind shear, or an abrupt change in wind speed and direction over a short change in height. These alternating winds can create swirling air, called rolls. These vortices may then be flipped vertically — creating tornadoes — by thunderstorm updrafts, which are basically currents of warm, rising air, McNoldy told LiveScience's OurAmazingPlanet.
Most hurricanes that make landfall create tornadoes, McNoldy said. "It's pretty uncommon to not have tornadoes with these," he said, adding that so far, the number of tornadoes spawned by Tropical Storm Andrea is not unusually high. Tornadoes mostly form over land, instead of over water, because the land slows down surface-level winds, creating even more wind shear, McNoldy said. Tornadoes form wherever these pre-existing supercells happen to be, he added, but meteorologists are still unable to predict exactly where tornados will strike.
These twisters usually form in the swirling bands of rain outside the cyclone, typically in the "front-right quadrant" of the storm, McNoldy said. In other words, if the storm is moving north, you're most likely to find tornadoes to the northeast of the cyclone's eye, he said.
Cyclone-spawned tornadoes are not fundamentally different from the tornadoes that form in the Great Plains. One difference is that the former tend to be less powerful, usually not exceeding a rating of EF2 on the Enhanced Fujita scale. Secondly, twisters that form in the Plains, like the tornado that struck Moore, Okla., get all of their ingredients from separate places. In the case of the Oklahoma tornado outbreak, for example, the warm air came north from the Gulf of Mexico, while the cold air came south from Canada. In the case of hurricanes, however, they provide all the required components for twisters themselves.