A preview of next year’s climate
If you can't get next week's weather right, how can you claim to predict anything a year in advance? It's the difference between weather and climate that makes long-lead forecasts possible. Actual storms can't be predicted a year out, but the longer-term features that affect climate across a season can sometimes be foretold with surprising accuracy, thanks in part to slow-changing ocean temperatures. In 1996the US and Canada began issuing long-lead outlooks that specify, month by month, the likelihood of above- and below-average temperature and precipitation for three-month intervals. For instance, the set of thirteen outlooks issued in December 2005 would cover January, February, March 2006; February, March, April 2006; and so on to January, February, March 2007. Each map has contours showing where the climatic odds for above, near, or below-aver-age conditions (each assumed to be 33 percent initially) have been shifted by forecasters. In an El Nifto winter, for instance, the precipitation outlook might place a 50 percent contour across southern California. This means the odds of above-average rainfall have risen from 33 percent to 50 percent. As esoteric as ail this might seem, it's the most concrete guidance available to the public on climatic conditions so far ahead. A similar system in Canada projected virtually all of the country to be warmer than average for the height of the 1997-98 El Nino. That outlook proved accurate for 88 percent of the nation. The US averages are drawn from a 30-year temperature base that was recently changed from 1961 -90 to 1971-2000. Since the latter was a warmer period across North America, the very definition of'average* has moved up a notch.