JACKSONVILLE, Fla. – Everybody wants to know where it's going. More than any other aspect of a hurricane, the No. 1 thing people want to know is if the storm is headed their way. Some storms are relatively easy to forecast a track for.
And then there are storms like Hurricane Elena in 1985, which I remember so vividly. This storm traveled westward along Cuba's north coast into the Gulf of Mexico. After several days of consistent movement, hurricane advisories were issued for the western Gulf Coast. Then Elena turned toward the north and looked as if it would hit the central Gulf Coast. So hurricane advisories were shifted eastward. Then Elena abruptly turned east, headed right at Florida, until it stopped dead in its tracks about 60 miles off of Cedar Key and sat there for a day or so. The storm then eventually resumed a track to the northwest and impacted the Louisiana-Mississippi coastline. If memory serves me correctly, every part of the Gulf Coast was under some sort of hurricane advisory at some point during Elena's lifecycle.
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More recently, who can forget Hurricane Irma? The forecast that called for it to first go up Florida's east coast, then the west coast, before then it ultimately turned north and traveled inland from the state's west coast, affecting much of the peninsula with hurricane and tropical storm force wind.
So what steers these monster storms? Quite simply, the wind pattern around 20,000 feet aloft is the general steering current. Sometimes, a hurricane encounters a stronger wind field aloft that picks up the storm and guides it in a well-defined direction. A nice example is a storm moving westward and approaching the coast, while an upper level trough of low pressure is moving east toward the coast. The winds on the lead edge of that trough will accelerate the hurricane northward or northeastward. And as long as we have a good handle on the trough and its movement, we usually have better than average confidence in the hurricane's track. We'll revisit this scenario later.
But sometimes, those upper level steering winds are very weak, and the computer models have a very difficult task with the cyclone's future timing and location.
Let's take a look at the big picture over the Atlantic. Every summer, a giant upper level area of high pressure, called the Bermuda High, develops.
As you a probably aware, some of our strongest hurricanes move off the west coast of Africa and track westward across the Atlantic south of the Bermuda High. These are called Cape Verde storms. When the Bermuda High is weaker, many of these storms recurve northeastward into the North Atlantic before reaching the U.S.
When the Bermuda High is stronger, then Cape Verde storms are steered more toward the eastern U.S., and pose a bigger threat.
When the Bermuda High is strongest, the storms are steered on a more southerly path toward either Florida or the Gulf of Mexico.
So how does this explain why Hurricane Dorian's exact path has been so hard to predict? Because as Dorian moves westward, it moves away from the steering currents of that Bermuda High. Yes, we've been confident for days that the storm would eventually make a turn to the north. But exactly when has been quite a challenge because the steering currents aloft are so weak. The computer models have had a field day with this, suggesting all sorts of solutions to our forecast problem. And when the models present lots of different scenarios, we meteorologists have less confidence in the forecast. But once Dorian gets a little farther northward, an approaching upper level trough will pick it up and accelerate it northeastward.
So this gives you a little behind-the-scenes understanding of why hurricanes move the way they do. And by the way, if two hurricanes approach each other and the vortices do a little "dance" around each other, this is called the Fujiwara Effect. Now you know.