Why tornadoes are among a hurricane’s potent threats
Hurricane Irma wasn’t the only vortex that the southeastern US had to worry about Sunday. The cyclone spawned tornadoes in parts of Florida in the morning and afternoon and could produce more — typical of hurricanes that get enough of their circulation over land.
Most of Florida and parts of Georgia were under tornado watches Sunday as the hurricane’s destructive core powered its way into Florida’s Gulf Coast, and tornado warnings followed.
One tornado damaged homes in east-central Florida’s Palm Bay late Sunday morning, the National Weather Service said — while Irma’s eye wall was spinning off the Everglades, more than 150 miles to the southwest.
That tornado’s location — well away from the hurricane’s core, and in the storm’s front-right quadrant (relative to the hurricane’s direction) — is typical for cyclone-produced tornadoes. Here are some things to know about twisters in hurricanes:
Land-striking hurricanes are conducive to tornadoes …
Nearly all tropical cyclones that hit the United States produce at least one tornado, “provided enough of the … cyclone’s circulation moves over land,” the National Oceanic and Atmospheric Administration said.
Why? Tornadoes thrive in part on strong vertical shear, which means a difference in horizontal winds’ direction and speed at different heights. And tropical cyclones offer a lot of vertical shear.
… especially in the front-right quadrant (in the Northern Hemisphere)
In the Northern Hemisphere, this vertical shear is especially pronounced in a tropical cyclone’s front-right (generally the northeast) quadrant.
That’s because, in this hemisphere, a cyclone rotates counterclockwise, and on the east side of the storm, the “outer bands come on shore at an angle typically bringing winds from the southeast,” CNN meteorologist Taylor Ward said.
Meanwhile, winds in higher levels of the atmosphere generally come from the west or southwest in the United States.
“So, usually the eastern side of the storm has winds opposite of the winds aloft, so that’s where you have high levels of (vertical) sheer,” Ward said.
They’re often well away from the cyclone’s eye …
Most tornadoes happen in a tropical cyclone’s outer rain bands, about 50 to 200 miles from the center, though some have been spawned near the inner core, NOAA said.
These tornadoes might be a little weaker, and dissipate faster, than a tornado spawned in, say, the US Central Plains. Besides vertical sheer, tornadoes thrive on an unstable atmosphere — and most instability happens at lower altitudes for a tropical cyclone than for a storm at higher latitudes, NOAA said.
So, the tornado-producing storm cells from a hurricane “tend to be smaller and shallower,” according to NOAA.
… but they pop up with relatively little warning
The combination of shear and instability that a hurricane offers still produces small supercell storms that are more likely to spawn tornadoes than ordinary thunderstorm cells, NOAA said.
And hurricane-produced tornadoes can form quickly and dissipate quickly. “There’s not a ton of warning to them,” Ward said.
In a hurricane’s outer bands, tornadoes represent a burst of concentrated destruction in an area that otherwise might not see the devastating levels of wind produced by the hurricane’s core.