The Cloudy Predictions of Long-Term Forecasts

NEW YORK (MainStreet) – Monday is Labor Day, and a lot of people will use the day to hold the last cookout of the season. There’s just one problem: In many parts of the country, it’s forecasted to rain that day.

According to Weather.com, the New York City area is due for scattered thunderstorms on Monday, with a 40% chance of precipitation. Accuweather is even more pessimistic, predicting rain for most of the day and a thunderstorm at some point. Weather Underground, yet another service, gives a somewhat sunnier outlook, but still says that there’s a 30% chance of rain that day.

Regardless of the predictions, don’t go canceling your barbecue just yet. While the science of meteorology has come a long way, weather is still very hard to predict if you try to peer too far into the future. And when you get into the business of predicting weather conditions a week or more in advance, the crystal ball gets very hazy indeed.

“Once you get out to the seven-plus-day range, the specifics are often difficult to pin down,” says Jon Nese, a professor of meteorology at Pennsylvania State University. “In general, the farther out you go, the less certainty you have.”

To test this out, we printed out the 10-day forecast for New York from Weather.com on the Friday before last, then observed how well it matched up to actual weather conditions. While some forecasted “isolated thunderstorms” did not materialize on Saturday, the forecast was otherwise spot-on for the next four days. It wasn’t until day six that things broke down: Thursday was forecasted to be partly cloudy but was rainy, and Friday was supposed to have isolated thunderstorms but was instead a clear day. And this past weekend, forecasted a week in advance to be cloudy with a slim chance of precipitation, instead saw New York hit by Hurricane Irene.

Nese says that it’s fair to give the Weather Channel a pass on the weekend, as hurricanes are singularly unpredictable events; while hurricane tracking is also much better than it used to be, he says that even 72 hours in advance there’s still a margin of error of about 200 miles when projecting a hurricane’s path. So it’s hard to criticize the Weather Channel for not seeing it coming more than a week in advance.

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