Thursday, September 07, 2017

The Models Are Beginning to Disagree with Each Other

As Hurricane Irma approaches Florida, you'd expect the models to start coming to a consensus about future movement. Instead, the models seem to be diverging from each other. There is a real question about where the storm will go next.

The NVG model now suggests the storm will move north along Florida's west coast, from Naples, straight through Tampa, and north. The GFS model also suggests the storm will move north, but will stay largely east of Florida.

Both models feature a northward storm movement, but the reasons for the movement are quite different.

The weather situation in the U.S. is quite unusual right now, with a very strong ridge in the Plains and a very strong trough along the East Coast. There is a small, weak low pressure system present in the Dakotas that is eroding the Plains ridge. NVG says it makes sense for the Dakota low and Hurricane Irma to start rotating around a common center in the lower Mississippi Valley, forming a cutoff low. That common movement, similar to dancers linking arms and swinging around each other at a square dance, is what makes Irma move north so abruptly.

Meanwhile, the less-imaginative GFS model suggests Irma will get captured by the East Coast trough in a standard manner, and that's why it moves the storm north.

And there's a third possible solution: Hurricane Irma keeps moving west without a turn, following Hurricane Katia, and ends up on the Mexican or Texan coast. That's a different way to make a cutoff low, and quite a logical solution.

Personally, I don't like the NVG solution. Hurricane Irma is strong and the Dakota low is weak. There is no compelling reason for them to work in concert. The Dakota low isn't powerful enough to fling Hurricane Irma north. The GFS solution is still the most likely one - the storm stays east of Florida and the storm slowly moves north. And it's still possible there is no northward movement at all and the storm just keeps heading west.

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