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Science & Technology
Hurricane Season 2000


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From Diana J. Choyce
May 22 - 28, 2000

Hurricane forecasts for the US are out and it looks like it will be an interesting season. "We do not anticipate a season as active as those of the years 1995, 1996, 1998 or 1999. Still, we believe we are entering a new era for increased storm activity and for East Coast landfalls by major storms," Bill Gray said. "There is a strong likelihood that in coming years we'll see more major storms as we did during the 1940s, 1950s and 1960s." Mr. Gray is a hurricane forecaster with an excellent record. The normal average for the years 1950 to 1990 was 9 named storms, 6 hurricanes, and 2 intense hurricanes. Gray's forecast for 1990 was 14 named storms, 9 hurricanes, and 4 major hurricanes. The actual count was 14 storms, 9 hurricanes, and 5 major hurricanes. Gray has been studying hurricane patterns in history and says the trend of more and fewer storms is cyclical and not related to pollution or global warming. A major difference between now and the busy storm period in the early part of this century, Gray observed, is that many more people now live in harmís way. According to Census figures, the population in Gulf and Atlantic coast states from Texas to Virginia rose from a little more than 24 million in 1930 to about 64 million in 1990.

"As we see it now, we think things are progressing about as we thought they would in our early December forecast," Gray said. "We do not believe that an El Nino will occur this year. However, the very cold [La Nina] water that's been out in the eastern equatorial tropical Pacific for the last two years we think will modify some and not be quite as cold. "That is a bit of an enhancing factor for this year's activity."Gray says another "climate signal" is the "Quasi-Biennial Oscillation," stratospheric, equatorial east-west winds, ranging from 16 to 35 kilometers in altitude, that oscillate. The direction changes every 26-30 months, typically blowing for 12-16 months from the east, then reversing and blowing 12-16 months from the west, then back to easterly again. The winds are expected to blow from the east, usually promoting hurricane formation. But "this year the winds have failed to drop as low as we expected, somewhat neutralizing their effect." Gray said that North Atlantic sea surface temperatures continue to be relatively warm, indicating that the Atlantic Ocean thermohaline circulation system, or Atlantic conveyor belt, remains strong. A strong Atlantic conveyor belt, Gray and colleagues believe, contributes to the formation of greater numbers of major or intense (Saffir-Simpson category 3-5) storms. It increases the probability of major hurricane landfall on the U.S. East Coast and in the Caribbean. Gray has frequently noted that the Atlantic conveyor belt, as measured by relatively high sea surface temperatures and high salinity in the North Atlantic, was strong during the period from the 1930s through the late 1960s, when major storms lashed the Eastern Seaboard.

Similar predictions are coming from the National Oceanic and Atmospheric Administration. The La NiŇa cooling of water in the Pacific Ocean is still having an impact on weather around the world and when that happens there tend to be more hurricanes, said D. James Baker, head of the National Oceanic and Atmospheric Administration. The same conditions prevailed last year, he noted, producing several destructive storms including Floyd, which inundated much of North Carolina. "It's not just about the number" of storms, added Max Mayfield, director of the National Hurricane Center in Miami. "What really counts is where they make landfall and how strong they are." And during La NiŇa years they tend to be stronger and longer-lasting, added Baker. Mayfield, who was named last week to replace Jerry Jarrell, will have a new eye in the sky to help him quarterback the forecast team. Last week a new weather satellite was launched that can be pressed into service this summer if needed. The $220 million GOES-L satellite had been delayed a year. The forecasters rely on two Geostationary Operational Environmental Satellites to monitor tropical storms and other severe weather. The East Coastís GOES-8, however, is a year beyond its design lifetime. Although still working, it could break down at any moment. GOES-L is a crucial backup. Over the last three decades, forecasters have gotten better at predicting where the storms will go, researchers Colin McAdie and Miles Lawrence report in the May issue of the Bulletin of the American Meteorological Society. They found that from 1970 to 1998 track forecast errors decreased by about 1 percent per year for the 24-hour forecast. "On a yearly basis, it's a small improvement, but one that yields a cumulative benefit," said McAdie, a research meteorologist at NOAA's Tropical Prediction Center in Miami. "The 24-hour forecast error expected 30 years ago of about 140 miles has been reduced to about 100 miles today." Forecasters and emergency managers hope to avoid a repeat of the destruction from last year's Hurricane Floyd, which inundated North Carolina.