Tropical Storm Irma picked up strength Wednesday and is expected to become a hurricane by Thursday or Friday, making it the fourth hurricane of an increasingly busy season.
In their latest advisory, National Hurricane Center forecasters said sustained winds had reached 60 mph as the storm rolls west at 15 mph. While it's too early to tell what threat Irma may pose to Florida or the U.S. coast, it has the potential to gain significant strength as it crosses warm tropical Atlantic waters sea surface temperatures are more than 2 degrees Fahrenheit above normal and encounters weak wind shear.
Irma is moving along the southern edge of a high pressure system that's helping steer it, forecasters said. During the next three days, it's expected to slow and begin turning toward the southwest. However, after that, the models diverge on which path the storm will take.
At seven days, the ensemble European forecast model shows Irma in a location where historically hurricanes have impacted the U.S.
What worries meteorologists is that the storm will track very close to a latitude and longitude in the tropical Atlantic that historically have proved a turning point for threats to the islands and the U.S. coast. It's also not yet clear whether the high pressure system, or a low pressure trough, will win out in a tug of war over which direction it takes.
"A little too close for comfort," Colorado State University hurricane researcher Phil Klotzbach said in an email.
However models forecasting so far in the future can be notoriously unreliable, he said.
"Beyond seven days, I really don't much put stock in computer model forecasts," he said.
Hurricane season entered its busiest time earlier this month, just after Franklin became the first hurricane of the season on Aug. 10 and made landfall the next day in Mexico as a Cat 1 storm. On Aug. 14, Hurricane Gert formed and remained well off the East Coast of the U.S. Harvey became a hurricane last Thursday off the coast of Texas, before making landfall and stalling over Houston.
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