Beware The Long-Range Weather Forecast — And Know Which Ones To Trust

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Workers clear snow from a sidewalk in the Seaport District of Boston, Thursday, Jan. 4, 2018. Forecasts for the Boston area call for about a foot of snow during the day and blizzard-like conditions.(AP Photo/Charles Krupa)

It’s an argument that pops up at the end of August like clockwork. Each year, several brands of almanacs hit the grocery checkout lines to much fanfare online. Hurry up, see how awful it’s going to be this winter! Inside every copy holds the alleged key to your weekly weather for the next year. The long-range forecast in the almanac—all the brands, I’m not just picking on one publisher here—is not the scientific bastion you may think it is. In fact, any forecast that promises to predict specific weather conditions more than a week or two in advance is steering you down a lonely path of disappointment.

Meteorology is still an inexact science. Forecasters and researchers spend their entire lives looking for ways to perfect their forecasts, but the perfect forecast is still elusive. Forecasts are so much better today than they were just a few decades ago. A three-day temperature forecast today is just as accurate as a one-day temperature forecast was almost 40 years ago. Meteorologists have gotten good at longer-range forecasts, too, but modern technology has its limits.

Most meteorologists will tell you that the outer limit for specific weather forecasting—predicting exact highs, lows, precipitation types and amounts—is around seven to ten days in advance in calm conditions and lower than that when active weather is approaching. Most predicting beyond that involves broad patterns. The technology exists to make broad generalizations more than a week out, but scientists can’t promise you more than that yet.

Unfortunately, that’s not good enough for many people. The neighborhood grill mom and soccer dad wants to know what the weather will be for the big picnic or that playoff game in six weeks. We need to plan in advance, after all. Where there’s a demand, someone will create a supply, and it sure is some supply.

The almanacs you’d find in the grocery checkout lane closely guard the methods and tricks they use to come up with specific forecasts for regions weeks and months in advance. The forecasts are written cleverly enough—“very cold with some snow late in the second week”—that it sounds like something you’d read on The Weather Channel’s local forecast. But, like a horoscope or a psychic reading, it’s just vague enough to pass as accurate for most situations if you squint real hard and let some of the details slide.

There is no sound science that allows you to create a specific weather forecast beyond a little over a week in advance. While meteorologists like to focus their ire on the yearly almanacs, the truth is that some private weather companies expose more people to questionable long-range forecasts than the reach of any almanac. AccuWeather launched their 45-day and 90-day forecasts earlier this decade to much derision from other meteorologists. The company introduced its long-range forecasts by saying they’re intended for spotting trends, but the forecasts are presented with such precise details—such as exact wind speeds and temperatures—that it could easily lead anyone to believe they’re consulting a detailed forecast rather a guide for spotting trends.

What meteorologists can predict in advance is broad atmospheric patterns, and those can tell us more than you’d think. If the models show subsiding wind shear and an influx of moist and unstable air over warm ocean waters, meteorologists can conclude that the environment will be favorable for tropical cyclones if all the right ingredients come together at that time. If models show persistent ridging in the upper atmosphere in a couple of weeks (or even a couple of months), they can get the sense that calmer weather with above-average temperatures are possible. A great example of an outlet that provides reputable long-range weather forecasts is NOAA’s Climate Prediction Center. Their long-range predictions are presented in terms of the odds that a certain area will see above- or below-normal temperatures and precipitation.

There’s a lot we can learn from long-range weather forecasts. There’s real science in reputable long-range weather forecasting and meteorologists are always working to improve their methods, models, and results. But when you see a long-range weather prediction that looks like it could pass for tomorrow’s weather forecast rather than a general trend weeks or months in the future, treat it with a healthy dose of skepticism. The science is there to spot patterns and trends, but not the specifics. We’ll just have to wait and see if that picnic or soccer game in six weeks is going to be a washout or success.

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