National Geographic : 2005 Jun
fall, and how much. Forecasting precipitation, he said, is one of his greatest challenges. At least it wasn't summer, when thunderstorms, too small for the computer models to capture, deliver most of the rain. "You'll have an inch of rain here, but five miles down the road you get nothing," said Terry. Precipitation forecasts are easier in the cool seasons, when weather systems tend to be large and well organized, like this one. He stared at the screen some more. "It's really important to be able to recognize the biases in the models," he said. Right now, comparing the rain on the screen with model forecasts, Terry sensed a bias was at work. "In the first six hours of the forecast, the models never seem to have enough precipitation." His experience told him this storm system was ripe for heavy rain. A patch of low pressure was lingering over the southern Rockies, drawing wet air from the Gulf of Mex ico and supercharging the storm with moisture. The computer had given him its best guess, 106 NATIONAL GEOGRAPHIC * JUNE 2005 but now it was time to follow his instincts. He frowned and used his computer drafting tool to sketch in corrected precipitation estimates, call ing for heavier rain. By morning, he forecast, the core of the storm would be drenching Washing ton, D.C., and the leading edge would be over New England, promising snow. The next day, commuters would find he was right. "You noticed Bruce was bald?" his colleague Pete Manousos joked. "He pulls his hair out when he does his forecasts." In fact, what the HPC staff such as Bruce Terry produce is not, officially, a forecast, but "guidance" or "advice" for the 125 local NWS Weather Forecast Offices. It's also grist for com mercial forecasters-purveyors of weather "prod ucts" such as glossy maps and satellite images for radio, newspapers, TV, and websites. Some even sell specialized forecasts to windsurfers wonder ing which beach has the best conditions, or to orchid growers needing advance warning of frost.