Meteorologists examining the conditions that spawned hurricanes Rita and Katrina say there is a strong likelihood that another intense hurricane will occur in October.
And while late-season storms tend to track eastward toward Florida or don’t make landfall at all, the experts don’t rule out the possibility of another major storm targeting the battered Gulf Coast.
Researchers also warn that the country should brace for 10 to 40 more years of powerful storms because of a natural ocean cycle in the midst of the most active hurricane period on record.
“This has been the seventh hyperactive year since 1995,” Stan Goldenberg, a meteorologist with the Hurricane Research Division of the National Oceanic and Atmospheric Administration, said this month. “Not every year is going to be like this one, but there’s going to be plenty of active years to come.”
The hurricane season does not end until Nov. 30, and a forecast group is predicting that October will see two hurricanes, one of them reaching Category 3, 4 or 5. The chance of that storm making landfall in the United States is estimated at 21%, said Philip J. Klotzbach, a member of the tropical storm forecasting team led by William M. Gray of Colorado State University.
Klotzbach’s forecast does not address where hurricanes make landfall or whether the Gulf Coast could be hit again. “It’s a tricky business tracking where these storms are going to go,” he said. “That’s governed a lot more by day-to-day weather.”
Goldenberg said he “would not be surprised” if the Gulf Coast was hit again, because the same conditions that nudged Rita and Katrina toward the region are in place. Goldenberg, who helps develop NOAA’s early-season forecasts, said he expected at least one to three more storms, including a major hurricane. Hurricane forecasters have their eye on a weather disturbance in the tropics that “could be Hurricane Stan,” he added.
“This season is not over,” said Goldenberg, whose Florida home was destroyed by Hurricane Andrew in 1992. “If I was in the Gulf Coast right now, I’d prepare. Even a tropical storm could do a lot of damage.”
Historical patterns show it would be unusual but not impossible for the Gulf Coast to be hit with a major October storm. In the fall, most tropical storms that form near the Bahamas, as Rita and Katrina did, are steered north by weather patterns that deflect them harmlessly out to sea, toward the Bahamas or either coast of Florida, said Christopher W. Landsea, a hurricane researcher with the National Hurricane Center in Miami.
“Texas and Louisiana are at much less risk later in the season,” he said.
The Colorado State team bases its forecast on an amalgam of pressures, wind speeds and ocean temperatures from around the globe. The weather experts also rely on a simple rule: “When September is active, October tends to be active,” Klotzbach said.
Peak hurricane activity ends by Oct. 10, according to the National Hurricane Center, but big storms can occur later in the season. Hurricane Mitch, a Category 5 storm, caused an estimated 9,000 deaths and left 9,000 people missing when it struck Central America in late October 1998; it hit southern Florida as a tropical storm on Nov. 5 and caused an estimated $40 million in damage.
Even Atlantic hurricanes that morph into monsters, like Katrina, start out as weaklings: mere waves in the atmosphere or feeble weather systems trailing small rainstorms as they drift west across the ocean.
When they hit deep, warm pockets of water, moist ocean air is pulled upward, condensing into clouds and cooling. This movement of air produces gusty winds and thunderstorms. The energy released by the rain is then pumped back up into the clouds, making them rise taller, spin faster and grow into the large cyclonic systems that have become so familiar in recent weeks.
Large patches of warm water are needed to sustain and strengthen hurricanes. The area also must be free of wind shear -- differences in wind speeds at high and low levels of the atmosphere -- which can shred the storm.
Forecasters say the most dreaded storms are “Cape Verde hurricanes.” These storms, which begin as atmospheric disturbances flowing off western Africa, form near Cape Verde and often grow massive as they travel across the Atlantic, unimpeded by dry land or cool water. Cape Verde hurricanes usually account for a season’s most intense storms; 85% of major Atlantic hurricanes have been of this type.
What was unusual about Rita and Katrina was that they formed close to U.S. shores, near the Bahamas. This means they did not have much time to grow powerful before first hitting land. Both storms swelled to Category 5 in the Gulf of Mexico, where waters are 2 to 3 degrees warmer than normal.
Gerry Bell, the lead scientist for NOAA’s hurricane forecast program, said large-scale weather patterns, including a high-pressure system off the eastern United States, created an area of favorable hurricane formation farther west this year. Once the storms formed off the Bahamas, weather patterns that act as “steering currents” pushed them farther west into the Gulf of Mexico and toward Texas and Louisiana. “They really had nowhere else to go,” Bell said.
“We saw a similar thing last year when several hurricanes hit Florida,” he said. “That was the same thing: a focused steering current.”
Bell and fellow forecasters predict that ferocious storms will occur for the next several decades.
They cite a natural ocean cycle called the Atlantic Multi-Decadal scale, which causes weather in the tropical Atlantic to seesaw between cool, windy phases and warm periods with slack winds, spawning frequent, strong hurricanes.
These phases are driven by two massive weather patterns that control monsoon rains over the Amazon and Africa, said Bell.
The continent-sized patterns last for decades and “are so dominant, they control ocean temperature and wind conditions,” Bell said.
The historical record shows an active hurricane period during the 1950s and ‘60s and a lull between 1970 and 1994. Since 1995, hurricane activity has once again been high.
“This is a long-term, active hurricane era,” Bell said.
The active period coincides with a global rise in sea temperatures of about 1 degree -- a change most scientists attribute to global warming caused by mankind’s production of greenhouse gases such as carbon dioxide.
Whether global warming is contributing to stronger hurricanes is a subject of intense debate within the scientific community.
Experts on both sides of the debate agree that it will take years to determine what effect global warming may have on future hurricanes.
In the meantime, they are bracing for more storms.