09 September 2017

I Thought So

From the first track that showed Irma making a hard 90° turn north, I wanted to know why. Not one of the dozens of forecasters mentioned why. My suspicion was that a low pressure system would set up to the north and west of Florida about the time that Irma got there. The reason, as all armchair meteorologists know, is that lows spin counter-clockwise and would thus sling-shot Irma up the peninsula.

And so it is. Now, it's true that normal highs spin clockwise and lows counter-clockwise, and they interact to push each other around. Turns out, which should have been obvious but wasn't, hurricanes are very unique in terms of their internal wind and temperature vis-a-vis other types of storms.
"With a tropical cyclone, its very symmetric. The temperature differences driving it are vertical--there's warm, wet air below, and cool, dry air up above--so there's a vertical circulation, but it doesn't move in a horizontal direction," said Neal Dorst, a research meteorologist at NOAA's Hurricane Research Division.

"There's no impetus driving a tropical cyclone--it will have a tendency to stay where it is," he said. If a hurricane appeared on a globe with no other weather systems, it would drift northerly only at a couple miles per hour, thanks to the Coriolis effect. It would not move in any other way, he told me.

So that's why Irma isn't headed back to Texas.

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