Predicting an Active Season

Dr. William Gray is a professor of Atmospheric Science at Colorado State University and is one of the nation's preeminent hurricane forecasters. From his Rocky Mountain post, Dr. Gray produces an annual Atlantic Basin hurricane forecast that is eagerly awaited by the public, the National Hurricane Center and the National Weather Service. This year Dr. Gray is predicting nine hurricanes - four of them major - with a 54 percent probability at least one will hit the Atlantic coast. In this piece, Dr. Gray gives us a look into the factors behind his forecast.

A fan of both the study of hurricanes and the game of baseball, I'll start by quoting Yogi Berra, who said, "You can observe a lot just by watching." We tropical meteorologists have been watching hurricanes for a long time, and what we now know is that a hurricane, like all other weather on earth, is driven by the energy of the sun. A hurricane isn't a local weather system; it's the result of a confluence of global oceanic and atmospheric circulation systems.

The sun heats the entire planet, setting into motion weather that appears local but is all interlinked. Atlantic Basin hurricanes, the ones I study, have their potential start in an area of relatively low barometric pressure (called a "tropical depression") that forms in low latitudes. If this depression lies over relatively warm ocean water, and has cool, moist air above it, thunderstorms form. The storm's clouds begin to rise and move counterclockwise (in the Nothern Hempisphere), drawing more heat (energy) from the warm ocean waters. When wind speeds reach 39 miles per hour the depression becomes a tropical storm and receives a name. If wind speeds reach 74 miles per hour, the named storm officially becomes a hurricane.

While I began my career with, and continue to study, the formation and structure of hurricanes, for the past 15 years my team and I have tried to predict how many named storms, hurricanes and intense hurricanes (with wind speeds of 111 miles per hour or above) will occur in a given season (June 1-Nov. 30). We use a simple theory: if certain conditions prevailed in a past year or years and are present in the current season, then this year's storm activity is likely to be similar to the earlier one.

Aided by weather data going back a century, we've found a variety of geophysical phenomena that seem relevant. These phenomena include pressure waves off the African coast, winds in the troposphere (where weather occurs) and stratosphere (above it), barometric pressures near the Azores Islands in the eastern Atlantic and at sea level in the Caribbean, West African rainfall, West African surface temperature and barometric pressure, the equatorial El Nino and La Nina phenomena, and others. We use statistical equations to tr to fit some or all of these conditions into a forecast that will suggest what's to come.

There is one more geophysical indicator, one that makes me believe we are entering a new era marked by more hurricane activity and more landfalls by intense hurricanes along the U.S. East Coast and Florida Peninsula. That is the Atlantic Ocean thermohaline circulation system, or Atlantic conveyor belt.

This ocean circulation, a northbound current that sinks and then moves southbound, tends to go through multi-decadal changes. It was weak from 1900 to 1925 and again from 1970 to 1994. From about 1930 through the late 1960s the Atlantic conveyor belt was strong, and we had numerous active seasons and a lot of major-storm landfalls on the East Coast. Measures of increasing strength of the conveyor belt since 1995 (relatively warmer sea surface temperatures and higher salinity in the North Atlantic) suggest that we're re-entering a period characterized by strong hurricane activity and intense landfalling storms. In fact, 1995 began the most active, consecutive four years of hurricane activity on record, with 53 named storms, 33 hurricanes and 15 major hurricanes.

I believe hurricanes, in coming years, represent the greatest natural threat to this country, greater than global warming (which, incidentally, I don't believe has any effect on increasing hurricane activity), floods and other natural disasters. However, that's not advice to "head for the hills." People who want to live on our coasts should do so as long as they accept possible consequences to property.

The National Oceanic and Atmospheric Administration, the National Weather Service and other government agencies do an excellent job of tracking storms and warning coastal residents. Just follow their directions, respect the power of hurricanes and don't, as Berra is also reputed to have said, "make a wrong mistake."

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