On Friday, Hurricane Dorian was set to hit the east coast of Florida directly.
Then Saturday, forecasts showed the storm’s center likely drifting well east of the state.
Sunday, as the storm ravaged islands in the Bahamas, many forecasts showed the storm’s center probably edging a bit back to the west — ever closer to Florida’s densely populated southeastern coast.
The rapid changes to Dorian’s projected path have left many wondering what has made this particular storm so difficult to forecast. Here are three reasons Dorian is giving forecasters fits, according to Jeff Masters, of the IBM-owned Weather Underground.
1. Dorian is definitely going to turn north, we just don’t know when.
Hurricane Dorian is on a collision course with the edge of a massive, somewhat amorphous high-pressure system called the Bermuda High, Masters said. Once Dorian hits the edge of that system, which blows clockwise, it will turn north.
The trouble is, forecasters don’t have a great feel for that system, Masters said. The Bermuda High is not located over land, which means instead of collecting more reliable data from balloon soundings, meteorologists have had to rely on less concrete satellite tracking. Because forecasters have limited knowledge of the critical system — its strength, its relative shape — they are having difficulty projecting exactly how it will interact with Dorian.
“It’s all a matter of observing power,” Masters said.
2. Hurricane Dorian is moving slowly.
It seems counterintuitive that a slow-moving storm would be more difficult to forecast than a fast-moving one. However, Masters said to think about it like this: A storm moving at 5 miles per hour travels 120 miles in 24 hours. If that same storm were to speed up even one mile per hour, that means it will move 144 miles in the same amount of time.
In the case of Dorian, those 24 hypothetical miles could make all the difference. In the event that the storm moves one mile per hour faster than expected, if everything else about a forecast is correct, the storm would be 24 miles closer to Florida before it makes its inevitable turn north.
“A one mile per hour difference in its speed makes a huge difference," Masters said.
Hurricane forecasts get less accurate the further they predict into the future. Because this storm is moving so slowly, forecasters must predict its inevitable turn north — and all the complicated factors that go into it — further into the future than they’d have to if the storm were moving faster.
3. Hurricane Dorian intensified quickly.
Dorian strengthened rapidly, from a category two hurricane all the way up to a monster category five storm in just a matter of days. That rapid intensification affected how the storm will move, Masters said, because intensifying storms tend to grow in size.
That means that Dorian’s ever-changing size is yet another factor forecasters have to consider. Forecasting is a difficult business: weather models take hundreds of rapidly changing data points into consideration. For example, in addition to its growing size, the reason Dorian is moving so slowly is because it’s currently trapped between a low-pressure system to the west and the aforementioned high pressure system to the east. These two systems are effectively cancelling each other out, causing the storm to inch along the Atlantic.
Predicting Dorian’s potential path essentially means predicting when the shapeshifting storm will reach the western edge of the high pressure system, at which point it will be blown north by both the clockwise-blowing high pressure system and the counterclockwise-blowing low pressure systems. Sounds difficult, right?
But wait, there’s more. Forecasters have an extra difficult job with Dorian because although the storm is quite intense, it’s fairly compact. If models are wrong by just a few miles, they could incorrectly predict that Florida’s east coast will receive a direct hit. Worse yet, the opposite could happen.
So consider the forecaster this hurricane season. And no matter what the models look like, be prepared
2019 Tampa Bay Times Hurricane Guide
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