90-Day Weather Forecast? 95% Chance Of Bunk

AccuWeather, the largest for-profit forecasting company in the world, is launching a new service that will provide up to 90-days of weather predictions publicly.

It's an appealing idea. Who wouldn't want to guarantee a sunny wedding day, or a vacation with snow-covered ski slopes?

Unfortunately, despite AccuWeather's impressive claims, there's little reason to believe it can offer what it's trying to sell.

Independent meteorologists and scientists widely regard these claims as – to put it bluntly – bunk.

Claims to be able to predict the weather far in advance are not new. The Farmer’s Almanac, a publication established nearly 200 years ago, has long offered weather predictions extending over a year or more into the future.

How many days out do you trust the weather forecasts?

The almanac’s website suggests it has 80 percent to 85 percent accuracy, but meteorologists by and large reject these claims.

Of course, some long-term weather predictions are easy to make. In the Northeast, July is going to be hotter, on average, than May, without much doubt. And experts can predict with at least some level of accuracy that El Niño may make a season much hotter or wetter than it would otherwise be.

But we usually expect much more detail and accuracy from weather services.

Long and short-term predictions

How well can our best scientific methods predict the weather?

“It depends on what you mean by prediction,” said Cliff Mass, professor of atmospheric sciences at the University of Washington.

Usually, when people check the weather predictions, they're expecting a day-by-day forecast of the specific weather conditions of a given area. And as Mass explained it, scientists really only have the skill to make these kinds of predictions two weeks into the futures.

So when AccuWeather claims to believe that on July 23, 2016, in New York City, the high will be 89 degrees, the low will be 70 degrees, with sunny skies and a stray thunderstorm, it should be taken with a huge grain of salt:

Source: AccuWeather

For all we really know, it may be a balmy 68 degrees that day, with heavy cloud cover and strong winds.

According to Mass, the two-week limit isn’t even controversial among experts. “There’s a whole literature on it,” he explained. “You can just look at the statistics of the weather forecasting models, which are all available.

“There’s no doubt about it: There’s no forecast skill past two weeks of daily weather.”

But forecast accuracy over a 90-day period, six times longer than what experts believe is possible, is exactly what AccuWeather, which claims that it provides services to over a billion people worldwide, is promising.

In its press release for the new service, the website boasts, “AccuWeather's new industry-leading forecast provides 90 days of daily forecasts, a valuable tool for planning the best time of the season for road trips, vacations and outdoor activities, letting users follow the forecast and make more detailed plans as the date approaches.”

An AccuWeather spokesman was initially enthusiastic to talk about the new services (now available on moblie!) when contacted over email. After a confusing back and forth over email with the spokesman, Jon Porter, VP of innovations explained on the phone that he believes AccuWeather customers highly value their extended forecasts.

Porter acknowledged that there's little value in the day-by-day predictions over the 90-day period, but he argued that that the long-term trends are what really matters to customers. Despite what many others see as an obvious problem, Porter does not think customers could be confused by a 90-day forecast presented in a style mimicking the well-known, and far more accurate, 5-day forecasts.

Our chaotic atmosphere

Why is the weather so hard to predict? It’s because the atmosphere is what’s known in physics as a “chaotic system." It contains so many variables that interact in endlessly complex ways that accurately predicting specific events in the long-term becomes impossible.

As Mass pointed out, any weather forecast has some amount of error built into it. Present observations are prone to errors, and the models we plug those observations into are incomplete. The further you predict out in time, the more those errors build up.

The farther you look into the future, the unkown variables out number the known variables, and making predictions better than pure geuss works becomes impossible. “So eventually you get to a point where the errors wipe out your forecast,” said Mass

It’s like placing drops of red and blue dye into a glass of water and stirring it. You can guess where the drops may hit the water, and you can predict that eventually the entire glass of water will turn purple. But you can’t predict exactly how the dyes will swirl together, and you can’t predict where any particular molecule of dye will end up in the glass.

This is what an atmosphere is like, only on a much larger scale. We may know, for example, that the climate is warming, and that this means there will be more hurricanes. But this doesn't mean we can tell ahead of time when hurricanes will occur or what path they will take.

Even when a hurricane is just about to hit land, experts have only broad predictions about its potential course.

For anyone, even a highly trained meteorologist, to claim to know with any accuracy the probability of rain in Houston in say, exactly 60 days, strains credulity.

So should you believe AccuWeather’s 90-day predictions?

If you’re the kind of person who like to check your horoscope or have your palm read, feel free download the app and plan your vacation around it. But if you’re interested paying attention to weather predictions based on the best science out there, you might have to get comfortable with the idea that, over the long term, we just don’t know what’s coming.

Photo Credit: Jussi Ollila via Flickr (edited)