The Long-, Long-, Long-Range Forecast

An understanding of weather patterns and their effects can put climatic events of the past in perspective and help predict weather over long periods. Here are some hints about how such patterns will affect future conditions:

• The North Atlantic Oscillation (NAO) varies from month to month and year to year. However, scientists are discovering long-term trends. During the 1940s, '50s, and '60s, the NAO was usually negative. In the '70s, '80s, and most of the '90s, the NAO was usually positive. It's becoming negative again. This is bad news for the Great Lakes, Midwest, and East Coast, which will have colder winters. The East Coast will be drier, the Southeast will experience drought, and both regions face an increased risk of hurricanes.

• The Pacific Decadal Oscillation (PDO) also appears to be reversing. In 1999, the PDO entered a cool phase, which has meant less rain and snow for the western United States, Saskatchewan, Alberta, and western British Columbia. If the studies are right, this dry spell will continue for decades.