Thursday, August 30, 2007

Loop Current: A Hurricane Supercharger

What is that Bermuda High doing now? It is a question I heard frequently during the hurricane season.

When I spoke at weather seminars, met with residents at local malls during our hurricane expos, or was just stopped on the street, I was asked repeatedly about the Bermuda High. Hey, Mike, where's that Bermuda High? Is the Bermuda High going to be strong again this year?

The Bermuda High, a large area of high pressure centered over the island of Bermuda in the western Atlantic Ocean, became infamous during the 2004 hurricane season for steering Charley, Frances, Ivan and Jeanne into Florida. The Bermuda High was unusually strong that year and in a perfect position to literally grab all four storms and drive them into the Sunshine State.

The exact location and strength of the Bermuda High changes from week to week and season to season. This year the high has been quite strong steering Hurricane Dean well south of the United States and into the Yucatan Peninsula.

While Floridians are often fixated on the Bermuda High, residents of the Gulf coast states are more interested in another weather phenomenon: the loop current. In 2005, the Loop Current helped create two of the most powerful and destructive hurricanes in history: Katrina and Rita.

The Loop Current is a river of very warm (over 85 degrees), very deep (2,400 feet in some areas) water that flows from the Caribbean Sea into the Gulf of Mexico, loops around to Louisiana and then exits the Gulf between Cuba and the Florida Keys.

For decades, the Loop Current has been a favorite location for fishing fleets seeking tuna and swordfish since the fish like to hang out in the warm waters. But, in recent years, meteorologists have been focusing on the Loop Current for its ability to turn an average tropical storm into a powerful monster.

A hurricane gets its energy from warm water, and therefore, the warmer the water, the higher the content of energy. When a topical system moves over a body of very warm, very deep water (like the Loop Current) its like a turbo charger for hurricanes. Suddenly, a category one hurricane can grow into a category five and do so very quickly.

Researchers at the University of Colorado traced Katrina and Rita's path through the Gulf of Mexico and across the Loop Current. In both instances, the storm's passage over the Loop Current's warm, deep waters transformed the systems into category five monsters.

In the case of Rita, the hurricane had sustained winds of 90 mph before entering the Loop Current. Over the next 24 hours, as Rita passed over the Loop Current, her wind speed increased to 175 mph. Rita's barometric pressure fell to 897 millibars, the third lowest on record in the Atlantic Basin.

When Rita moved away from the Loop Current, over somewhat cooler waters in the northern Gulf of Mexico, the storm weakened before making landfall on the Texas-Louisiana border as a still formidable category three hurricane with sustained winds of 120 mph.

Scientists say the Loop Current waters have been warmer than average during the 2005 hurricane season. They are not sure why, but they know, especially in the case of Katrina and Rita, that the Loop Current has played a crucial role in the incredible 2005 hurricane season.

Posted at 10:13 AM

Thursday, August 23, 2007

Forecasters Mark 15th Anniversary Of Hurricane Andrew

It is hard to believe that is has been 15 years since Hurricane Andrew slammed into South Florida. It was Monday morning, August 24, 1992 when the category 5 hurricane hit southern Miami-Dade County.

Andrew produced more than $25 billion in damages, making it the costliest hurricane in United States history (at least, until Hurricane Katrina came along in 2005). While it was accepted that Andrew was a powerful hurricane, researchers and scientists debated for years the exact intensity of the storm. Officially, Andrew was listed as a category 4 hurricane on the Saffir-Simpson scale, with sustained winds of 145 mph.

Still, many researchers believed Andrew's winds might have topped 155 mph, making Andrew a category five hurricane. But there was no way to confirm that in 1992 since Andrew destroyed nearly every anemometer recording device.

"Every wind instrument failed, was broke or pegged out during the storm," said Stanley Goldenberg, a research meteorologist with NOAA's Hurricane Research Division in Miami. "We didn't have direct measurements, so determining the wind speed was like detective work."

Ed Rappaport, now the acting director of the National Hurricane Center, was the detective who decided to take on the case. Rappaport didn't have many clues to solve his mystery, just a few wind and air pressure readings, along with the damage the storm inflicted on Miami-Dade County.

"We had to piece together an estimate of what winds were which led to our existing estimate of winds around 145 mph, gusts to 175 mph," Rappaport said.

Still, Rappaport did have one key piece of evidence. As Andrew was making landfall, a reconnaissance aircraft recorded sustained winds of 186 mph. That recording was made as the plane flew at an altitude of 10,000 feet.

Standard procedure in 1992 was to assume that surface winds were usually around 80 percent of the wind speed recorded at 10,000 feet. That would put Andrew's surface winds at 148 mph.

Case closed, right? Well, not quite.

In 1997, researcher James Franklin, now a hurricane specialist at the National Hurricane Center, created a new instrument called a GPS dropsonde. Released into a hurricane from a reconnaissance aircraft, the new device provided researchers with some surprising data.

The data suggested that a hurricane's surface winds are usually 90 percent of the wind speed recorded at 10,000 feet, not 80 percent as originally thought. If that formula was applied to Andrew, it would mean Andrew's surface winds were 165 mph. That would make Andrew a category 5 storm.

"I looked at a lot of this dropsonde data," said research meteorologist Mike Black. "If we applied the rules that we are applying to hurricanes today, Andrew would have been a category 5."

Other scientists, especially Franklin, agree. They are convinced that Andrew was a category 5 hurricane. Yet, other researchers at the Hurricane Research Division are not as certain.

A committee from the National Hurricane Center ultimately agreed with Franklin and Black. On the ten year anniversary of Andrew's landfall, the National Hurricane Center announced that Andrew had gained strength, and was now to be listed as a category 5 hurricane.

The committee concluded that when Andrew made landfall along Florida's southeastern coast the storm was producing sustained winds of 165 mph. It means Andrew was the third strongest hurricane to strike the United States and one of only three hurricanes to make landfall as a category five. The other two category five storms are the Labor Day hurricane of 1935 that hit the Keys and Camille which struck Mississippi in 1969.

Andrew is now part of a very small club.

Posted at 1:54 PM