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Why tornadoes are among a hurricane’s potent threats


Hurricane Dorian wasn’t the only vortex that the southeastern US had to worry about this week. The cyclone spawned tornadoes in parts of the Carolinas — typical of hurricanes that get enough of their circulation over land.

At least 24 tornado reports were made in South Carolina and North Carolina as the hurricane hit those states.
Tornadoes are common in the thunderstorm bands of hurricanes and tropical storms, especially in the right-front quadrant of the system. The tornadoes tend to be short-lived and can come with little, if any, warning.
Here are some things to know about twisters in hurricanes:

Hurricanes with circulation over land are conducive to tornadoes …


Nearly all tropical cyclones that reach the United States produce at least one tornado, “provided enough of the … cyclone’s circulation moves over land,” the National Oceanic and Atmospheric Administration says.
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) shear,” 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 300 miles from the center, though some have been spawned near the inner core, NOAA says.
Tornadoes from a tropical cyclone might be a little weaker, and dissipate faster, than a tornado spawned at, say, higher latitudes, such as in the US Central Plains.
That’s because other than vertical shear, tornadoes thrive on an unstable atmosphere — and for tropical cyclones, thermal instability happens mostly at a lower height than for storms at higher latitudes.
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.
Editor’s note: This story, first published during the 2017 hurricane season, has been updated to include references to Hurricane Dorian.
Original story from CNN

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