At the National Weather Service—one of the least appreciated services that the federal government provides—meteorologists make use of both computational power and their own judgment to arrive at forecasts. The meteorologists’ judgments, according to data from the agency, result in precipitation forecasts that are about 25 percent more accurate, and temperature forecasts that are about 10 percent more accurate, than the computer results alone. Improved forecasts have real human benefit; the National Hurricane Center can now do what it was unable to do twentyfive years ago: make predictions three days in advance of where a hurricane will make landfall that are accurate enough to enable people to evacuate in time.
But some weather forecasts are less accurate because of judgments by forecasters who have other motivations. Most Americans do not get weather forecasts directly from the National Weather Service; they receive them instead from commercial services such as AccuWeather and the Weather Channel, and also from local TV stations, which tailor the government data to their own needs. According to Silver, the forecasts of the National Weather Service are well-calibrated: when it forecasts a 40 percent chance of rain, it actually does rain 40 percent of the time. The Weather Channel, in contrast, has a slight “wet bias.” When there is a 5 percent chance of rain, it says that the odds are 20 percent because people are angry at a forecaster when they leave home without an umbrella and get soaked, whereas they are delighted when they take an umbrella and the sun shines. But it’s at the local level that forecasts get seriously distorted. According to a study of Kansas City stations, the local weathermen provide much worse forecasts than the National Weather Service and are unashamed about it. (“Accuracy is not a big deal,” one of them said.) Again, television does not reward accuracy because ratings come first.Indeed, the weather forecasting coming from local TV news outlets as well as the Weather Channel is generally held in disdain by those in university atmospheric science departments. Nate's great book, the Signal and the Noise has much more on these and other types of prediction. Note that iOS gets its weather info from the Weather Channel (via Yahoo). This is easily solved, however, as one can add a desktop icon for your local forecast from the National Weather Service.