Erika: The Forecast


Even Tougher Than Usual
Hurricane forecasts have improved dramatically since I worked at the National Hurricane Center in the 1970s. Here is the proof:

The numbers show that the track error (where the storm will be) in a TWO day forecast in the 70s is about the same as the current error in their FIVE day forecast. Another way to put it: their TWO day forecast is THREE times as accurate as it was in 1970! Better computer models are the main reason for the tremendous improvement.

But not all hurricanes are alike. The ones tracking straight west across the Tropical Atlantic are the “easiest” ones to forecast. The ones that track northeastward across the North Atlantic also have high accuracy scores. The toughest forecasts are for “recurvature”-the transition between that westward track and the northeastward track. That is part of the forecast problem with Erika.

Weak Goes Left; Strong Goes Right
There are exceptions to every rule in hurricane forecasting. But there are tendencies and trends. Weaker, more disorganized storms (like Erika is now) tend to track more to the left than computer models suggest. It’s related to the structure of the storm. So, instead of getting “steered” by winds above 10,000 feet, the disorganized storms get steered by lower level winds, which in the tropics are often east to west.

So far, Erika has been weak enough to be steered straight west. As long as it stays weak and disorganized, it should track more to the left than most computer models say. It just happens time after time. But when it starts getting better organized, it can turn more to the right.

Wind Shear: Slayer of Hurricanes
Hurricanes are powerful storms, but in a way they’re kind of fragile. They need just the right combination of warm ocean AND wind patterns from the lower to upper atmosphere. Otherwise, they weaken, or don’t form at all. One big negative factor is when upper-air winds are much stronger and/or from different directions than lower-level winds. It’s called “wind shear,” and the greater the wind shear, the more hostile the conditions.

Here’s a map showing predicted wind shear in the Caribbean and Atlantic for Friday morning. The reddish colors show the strongest shear, and there’s plenty of it in Erika’s path toward Florida. (Erika is the “L” in the middle of the map). Even the green and yellow colors show unfavorable conditions:

About Those Mountains…
There are some really big mountains that are in the possible path of Erika. Puerto Rico, the Dominican Republic/Haiti, and Cuba all have at least one mountain that is at least 4000 feet high! It’s all about friction, and any type of land creates some friction. The greater the area of mountains, and the higher the mountains, the greater amount of friction there is. Hurricanes hate friction.

In the past, some already-developed hurricanes have taken the “triple island-mountain track” of Puerto Rico + Dominican Republic/Haiti +Cuba. All have weakened. Some have been crushed into a disorganized mess. Erika is already disorganized-if it tracks over the triple islands, it may not survive.

The Track Forecast Is Tough Enough, But…
So, the track of Erika is more uncertain than usual. And if you think that’s tough, look at the intensity forecasts!

The Bottom Line
The “bottom line” is there is so much uncertainty, in both track and intensity, that possibilities range from Erika moving into the Gulf of Mexico all the way to curving off the Southeast U.S. coast. The rooting interest here is for Erika to track more to the left, move near the mountainous islands from Puerto Rico to Cuba, and be so disorganized that it won’t be able to recover. There’s at least a 30-percent chance of that happening. But that also means there’s nearly a 70-percent chance that it will turn out to be a threat to the U.S. mainland next week.

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