New urgency in efforts to forecast hurricanes
For two years running, no hurricane has made landfall along the US East and Gulf coasts. Historically, however, these coasts have never gone three years without a hurricane striking somewhere along their length, federal hurricane forecasters say.
The prospect that these regions are "due" a hurricane is adding urgency to projects designed to lengthen the lead time for warnings and develop an ability to forecast a storm's intensity more accurately.
It also is prompting the National Weather Service to boost its efforts to bring inland populations up to speed on the need to prepare for the effects of hurricanes that bring with them high winds, tornadoes, and torrential rains as they move over land, weaken, and break up.
"The public hasn't seen a land-falling hurricane in two seasons, and we know from previous experience that out of sight is out of mind," notes Max Mayfield, director of the National Oceanic and Atmospheric Administration's National Hurricane Center in Miami.
Federal forecasters say they expect from nine to 13 named storms in the Atlantic and Caribbean regions this season, which began June 1 and runs through Nov. 30. Of these, six to eight could become hurricanes, with two or three becoming "major" hurricanes tropical cyclones with sustained winds exceeding 111 miles an hour. This forecast dovetails closely with that of William Gray, a Colorado State University researcher who pioneered seasonal hurricane forecasts.
Dr. Gray's team has worked recently to devise broad land-fall probabilities for the country's hurricane-prone coasts. Along the Gulf Coast, Gray's team holds that there is a 67 percent probability that a hurricane will strike land this season, while the East Coast through Florida stands a 73 percent probability of taking a hit from a hurricane. The team warns, however, that a lower likelihood shouldn't be used as a reason to skimp on preparedness.
Federal officials add that tropical storms one notch below hurricanes can pack a punch of their own. Last June, for example, tropical storm Allison struck Houston and lingered for five days, dropping nearly 37 inches of rain. By the time Allison had moved across the southeast and up the East Coast 12 days later, it had inflicted $5 billion in damage, while the death toll reached 41.
In an effort to give emergency managers more time to evacuate residents when a hurricane threatens, forecasters have developed an experimental, five-day landfall forecast that could become a staple if the results from a second year of tests this season prove "reasonable," says James Franklin, a hurricane specialist at the National Hurricane Center. Currently, he explains, the center issues these forecasts up 72 hours in advance.
Much of the impetus for the five-day forecast has come from the US Navy. When a hurricane threatens, the Navy must move ships and aircraft out of the storm's way. The current forecast barely gives the crews time to fire up their ships' boilers.
Last year, the new forecast, which hurricane specialists pull together in the background while they issue their standard forecasts, yielded "pretty good" results, Mr. Franklin says. He notes that the new product's landfall error of 315 nautical miles was smaller than the error in the 72-hour forecasts when they were first officially issued in the 1960s. Yet, he adds, if the five-day forecasts prove to be ready for prime time, his agency faces a challenge in helping emergency managers and the public at large understand its limits.
"People are accustomed to a certain level of accuracy in our forecasts," he says. "The further out you try to predict, the larger the errors are apt to be. The danger is that people won't treat the new predictions with the same respect" they do current landfall forecasts.
Over the past decade, hurricane research has helped improve the accuracy of those forecasts by 20 to 30 percent. "But the same cannot be said for intensity forecasts," Franklin says. "The pressing issue for us is on the intensity side."
Research attention has focused not only on the larger environmental factors that strengthen hurricanes, such as ocean temperatures and atmospheric circulation, but on smaller-scale developments that occur in a hurricane's "eye."
Those processes "are not well defined. In fact, we're not sure we know what we need to measure," Franklin says. In an effort to close that gap, researchers spent a month last season sending instrument-laden jets into and over the hurricanes at much higher altitudes than forecast-related aircraft fly.
One key objective was to analyze the hurricanes' ice content, which builds in the upper 45,000 feet of a storm. Ice formation releases large amounts of latent heat, and heat is a key driver for these storms. In addition, the size of the crystals helps determine the precipitation rate, explains Andrew Heymsfield, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo., and a lead investigator on the project.
If the crystals are small enough, updrafts move them up and out of the storm, where they evaporate. If the crystals are large enough, they're pulled back down into the storm to become rain.
Flying through hurricane Humberto last September, scientists detected centimeter-sized crystals at high altitudes. Updrafts in hurricanes generally are weak, Heymsfield says, so some scientists didn't expect to see such large snowflake-like crystals at such high altitudes. They would be more typical at lower, warmer altitudes.
Moreover, the large concentration of micron-size, highly reflective crystals at high altitude mirrored more of the sun's energy back into space than previously assumed, Heymsfield says. This could lead to stronger cooling at the cloud tops, generating a stronger temperature contrast between the top and bottom of a storm. That contrast could play a key role in determining a storm's intensity, with a stronger contrast leading to a more intense hurricane.
The researchers' results will be fed into forecast models to see what kind of effect the data have on intensity and track predictions. In addition, the modeling results are expected to inform decisions about another round of experiments, which could come in another two years.