I’m from Florida and have seen many hurricanes throughout my life, but one thing I could never fully understood are the tornado threats from them. Idalia is still passing through my area and we are all under a tornado watch till this evening. Oddly enough, I cannot ever remember a significant tornado occurring due to a hurricane, but that might just be me not paying attention.
From what I do understand, the common tornadoes we see in the US in the south and Midwest usually come from the classic formula, with warm and cold air meeting to make thunderstorm super cells, and those cells eventually forming the rotation vortex that we see touch down as a tornado. A hurricane is a tropical low pressure system that from what I understand, is not formulated the same as these super cell thunderstorms that produce tornadoes in the Midwest.
So how exactly can tornadoes come from hurricanes? Is it a reaction from being over land and meeting cold air and thus causing the thunderstorms in the hurricane to react similarly?
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When hurricanes make landfall, the air near the ground slows down while the air up top keeps its speed. This is called a “wind shear” that can lead to a column of air rotating and producing small tornados.
Great Plains thunderstorms bubble up from the warm and humid surface into crisp, dry air. Usually from Canada. This allows them to breathe *really well*, and that’s how a mesocyclone that’s maybe 10 miles across at most can generate winds far beyond a cat 5 hurricane.
It’s kinda like they have a custom exhaust kit and a turbocharger.
A hurricane recycles dry-ish air from thunderstorm outflow. (Rain falls out the bottom, air falls out the top.) This air is high enough that it can radiate heat into space, so it becomes dry-ish and cool-ish, enough for new thunderstorms to form in the rain bands. [Overall picture looks like this](https://serc.carleton.edu/eslabs/hurricanes/2c.html), the blue stuff is the dry air.
(This air is cold because of its high altitude, but a lot warmer than normal for the altitude.)
So the thunderstorms in a tropical system are limited by a choked up exhaust system. They don’t have very strong updrafts individually, but they work together to create widespread rotation and a low central pressure.
It’s like if you spin the water in a sink and allow a little bit of flow to drain out it’s pretty easy to get a vortex. The whole storm is spinning, and spinning at different rates in different places
The tropical tornado recipe is lots of surface rotation, pulled together by an updraft that’s just a bit stronger than its neighbors. A Great Plains storm needs just a little rotation but a very strong updraft. (Strong updrafts make lots of big hail, another difference between these storms.)
This means tropical tornadoes can form, weaken, and reform quickly. The updrafts are already spinning, its just that they’re below tornado strength until something random happens that allows the parent cell to strengthen. These tornadoes also don’t become very strong because the parent cell never gets the opportunity to breathe well.
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