Tropical storm Dorian: Will it fall apart or head for Puerto Rico?

Tropical storm Dorian is projected to weaken over the next 24 hours, only to gain a bit of that strength back four days from now. The center of the storm is currently about 1,500 miles east of Puerto Rico.

Tropical storm Dorian, the 2013 Atlantic hurricane season's fourth tropical cyclone, appears to be heading for a close encounter with Puerto Rico, Hispaniola, and Cuba during the first half of next week, according to the latest forecast from the National Hurricane Center in Miami.

That is, if it makes it that far.

Although forecasters have projected a track out through next Wednesday, Dorian might not last that long. The storm currently is quite ragged. And over the next 24 hours or more, it will be entering a more-hostile atmospheric environment: dry air and wind shear, a change in wind speed or direction with height. Shear stunts the growth of storm clouds by robbing them of moisture and the heat that's released as water vapor condenses into cloud droplets.

For now, however, the official forecast calls for the storm to weaken over the next 24 hours, only to gain a bit of that strength back four days from now, when Dorian's center is projected to skirt the north coast of the Dominican Republic as it travels west.

On Friday at 11 a.m., Eastern time, Dorian's center was located about 1,500 miles east of Puerto Rico. The storm was packing maximum sustained winds of 50 miles an hour and traveling at about 21 miles an hour.

Dorian emerged Wednesday from a tropical depression located over the eastern tropical Atlantic, just in time to become the first Atlantic tropical cyclone to be tracked by a pair of new supercomputers and significantly upgraded hurricane-forecasting software.

The new supercomputers, one based in Reston, Va., and the other in Orlando, Fla., represent a threefold increase over the hardware they replace in the number of calculations they can perform each second.

At the same time, the National Weather Service activated its upgraded hurricane-forecasting software on the new computers, according to Ben Kyger, director of central operations at the National Weather Service's National Centers for Environmental Prediction (NCEP) in College Park, Md.

Typically, NCEP doesn't roll out new computers and new forecasting software at the same time because trouble-shooting any problems becomes more complicated. If a problem crops up, it's unclear whether the model or the computer is to blame.

"But we made an exception for the hurricane model because it was such an improvement," he says. "It went live with the computer yesterday."

The model's track and intensity forecasts for five days out represent a 15 percent improvement over its predecessor's forecasts, based on having the upgraded model forecast past tropical cyclones using the same starting conditions that the older model used.

It took about 18 months to complete the software and hardware overhauls while ensuring that upgraded and existing forecasting software played nicely with the new, Linux-based computers, Mr. Kyger says.

Thursday's start-up was met with celebration at NCEP. But in keeping with the tight fiscal times, the celebration was modest.

"We had a box of doughnuts," Kyger says, adding that Dunkin' Donuts and Krispy Kreme were well represented. "It was a big day for us, so different people brought in different vendors."