Mt. Vernon Register-News

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April 17, 2013

Everybody talks about the weather, but it's not easy to predict

(Continued)

When the almanacs risk testable predictions, they often fail. The Old Farmer's Almanac predicted that the Atlantic corridor would average 47.5 degrees for March 2013, off by nearly four degrees. That might seem like a reasonably good guess, except that anyone with access to historical averages — that is, anyone with an Internet connection — can usually get within a few degrees by sticking near the mean. The almanac's prediction for February was 29 degrees, nine degrees below the actual temperature, and the forecast for January was off by five degrees.

Critics of the almanacs are nearly as old as the almanacs themselves. A forecaster at the U.S. Weather Bureau complained about the almanacs' inaccuracy in 1905, and a Harvard professor did the same in a public address in 1926. But we simply can't let go of the dream of weather omniscience.

Generally speaking, forecasters who make predictions months in advance rely on analog techniques, which means they look for patterns in the current weather, then find similar patterns in prior years. Their predictions are based on what happened in the past. The problem is that this technique has never been shown to work particularly well. The atmosphere is a complicated place, and it's very difficult to say that a single past year, or even a combination of past years, is enough like this year to make accurate predictions.

The National Weather Service relies more on dynamic forecasting. The agency's experts observe the weather systems at work in the atmosphere, then use mathematical and physical models to calculate what will happen next. The technique is far more reliable, but it has its limits.

"We sustain higher accuracy out to two to three days in advance; then it starts dropping off faster at days six through eight," says Louis Uccellini, director of the National Weather Service.

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