Large temperature variations from year to year have significant implications, most obviously for farmers and gardeners but also for utility companies estimating energy use, city managers budgeting for snow and sports teams worrying about scheduling. Are we getting any better at predicting the weather weeks or months in advance?
Before getting to the science, it’s important to recognize that there have been false starts and inflated claims in the business of long-term weather forecasting.
Consider the most famous American weather prognosticators, the Farmers’ Almanac, published in Lewiston, Maine, and the Old Farmer’s Almanac, produced in Dublin, N.H. The writers of these venerable books claim to use top-secret formulas, and the New Hampshire version says readers find it to be 80 percent accurate.
It’s impossible, however, to fully assess the books’ accuracy, because many of their predictions read like a meteorological fortune cookie: vague enough to accommodate a wide range of weather. Both publications, for example, tend to make such predictions as “sunny, cool” in four- or five-day chunks. In any given workweek, there are usually periods of sun and some temperature variation. Does that make the prediction correct?
When the almanacs risk testable predictions, they often fail. The Old Farmer’s Almanac predicted that the Atlantic corridor (including the Washington metro area) 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.