As Hurricane Isaias worked its way through the Mid-Atlantic states Tuesday, its winds steadily diminishing, a new hazard arose: tornadoes.

It is not uncommon for hurricanes to spawn tornadoes, and they are similar to those that arise out of large thunderstorms in the Central Plains, said Jana Houser, an associate professor of meteorology at Ohio University.

When they form, tornadoes are created in the outer rain bands of hurricanes, Dr. Houser said, which contain convective cells — thunderstorms — of their own.

But as long as a hurricane is over water, tornadoes will not form, Dr. Houser said. That’s because the surface of the water is relatively smooth, and in the formation of tornadoes “the biggest culprit is surface friction,” she said.

But once the rain bands reach land, surface friction greatly increases. That slows the storm’s winds close to the ground.

“You suddenly create a situation where you have a change in wind speed and often direction” compared with winds higher aloft, Dr. Houser said. This is called wind shear, and it can induce a spinning movement in the air.

At first this creates a spinning cylinder of air that is parallel to the surface. But as with any thunderstorm, the convective cells in a hurricane create strong updrafts. These can tilt the spinning air upright; a tornado is born.

If the updraft is very strong, the spinning air will be packed tighter, with a smaller diameter. When this occurs the tornado can intensify, like a figure skater pulling her arms in to increase her rate of spin.

This can lead to a powerful, destructive tornado. “But typically they are not as long lived, and typically a little weaker, than those formed from supercells in the Central Plains,” she said. And the threat diminishes over time as the hurricane breaks up and weakens further.

source: nytimes.com

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