Why Irma is so strong and other questions about hurricanes

WASHINGTON (AP) — A powerful Hurricane Irma is threatening millions of people in the Caribbean and Florida. Some answers to questions about Irma and hurricanes:

WHERE DO THESE STORMS COME FROM?

Irma is a classic Cape Verde storm, which begin near the islands off the west coast of Africa. Some of the worst hurricanes start as puffs of unstable air and storminess there and chug west, gaining strength over the warm open Atlantic. Another storm, Jose, has followed in Irma’s footsteps. Some of those storms fizzle from wind shear or other weather conditions. Still others curve harmlessly north into the mid-North Atlantic and are called “fish storms.” Storms also start in the Gulf of Mexico, like Tropical Storm Katia which formed off Mexico on Wednesday.

WHY ARE STORMS HAPPENING NOW?

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Hurricane season starts June 1 and ends Nov. 30. That’s usually when the water is warm enough and other weather conditions are conducive to storm formation. Hurricanes need water that’s at least 79 degrees (26 degrees Celsius). Peak hurricane season is from mid-August to mid-October with the peak of the peak being Sept. 10 or 11.

WHAT’S AN AVERAGE SEASON LIKE?

An average season produces 12 named storms, according to the National Weather Service. Wednesday’s Katia is the 11th this season. Storms get names when winds reach 39 mph. The average season produces six hurricanes and three of those become major at 111 mph winds or higher. So far this year, there have been four hurricanes — Franklin, Gert, and two major ones — Harvey and Irma.

DID FORECASTERS SEE THIS BUSY YEAR COMING?

Yes. In May, the weather service predicted a 70 percent likelihood of 11 to 17 named storms with 5 to 9 becoming hurricanes. They predicted 2 to 4 major hurricanes. In early August, it was changed to a 60 percent chance of 14 to 19 named storms, 5 to 9 hurricanes and 2 to 5 major hurricanes.

ARE BACK-TO-BACK BIG HURRICANES UNUSUAL?

Major storms can and do form back-to-back and did so last year with Matthew and Nicole, but having more than one hit the U.S. in a season is strange. If Irma hits Florida as a category 4 or 5 storm, it will be the first time in historical record that the U.S. was hit by two category 4 or 5 storms in one year, said Colorado State University meteorology professor Phil Klotzbach.

WHY IS IRMA SO STRONG?

Hurricanes use warm water as fuel. Irma has been over water that…

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