Accuracy of long-term winter forecast questionable

According to some long-range forecast predictions, large sections of the Midwest, Northeast and Rocky Mountains will face chilling temperatures and large snow accumulations this winter. For example, AccuWeather.com’s 2011-12 Winter Forecast predicts that Chicago will receive 52 inches of snow accumulation and that New York will get 33 inches.

However, weather experts and meteorologists who spoke with Consumers Digest believe that consumers should take detailed long-range forecasts, particularly of snow accumulation, with a grain of salt. Nature just doesn’t afford that kind of accuracy.

The atmospheric conditions that determine weather conditions can change rapidly and thus can’t be accurately predicted over the long-term, says Mike Halpert, who is deputy director of National Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center. Outlooks that come from NOAA’s National Weather Service (NWS) project only broad regional climate conditions, not specific temperature ranges or snow accumulation. He says NWS makes its projections that are based on the past 30 years of weather data. In contrast with AccuWeather.com’s predictions, NWS determines only whether a region’s overall climate will be above or below average in terms of temperature and precipitation levels.

NWS projects for the next 3 months that California, the Great Lakes, the Northern Plains and the Pacific Northwest will experience a climate that’s colder and wetter than average and that most of the southern United States will be warmer and drier than average.

El Nino and La Nina, which are conditions that develop in the Pacific Ocean that impact global weather patterns, also factor heavily into seasonal outlook predictions. This winter’s climate will be impacted by La Nina. NWS uses a model of prior La Nina conditions to project this year’s seasonal outlook.

However, specific predictions are uncertain because winter conditions are affected by Arctic Oscillation, a sea-level pressure anomaly. If the Arctic Oscillation is in a negative phase, it will push cold arctic air into the United States and can cause snowy conditions.

Meghan Evans, who is a meteorologist with AccuWeather.com, defends her company’s predictions and says it can make more-accurate long-range winter forecasts, because it also uses an Arctic Oscillation projection model to determine the frequency of storms. However, a report from NOAA says Arctic Oscillations can be determined, at best, about 2 weeks in advance.

Tom Skilling, who is chief meteorologist at WGN–TV in Chicago, cautions consumers against taking long-range snow-accumulation forecasts at face value. “People have to be very careful in using seasonal outlooks,” he says. “We had indications in Chicago that, while there are sure to be some snowstorms [this winter], when all is said and done the season may not be all that extreme.”

Halpert says NWS’ snow accumulation and temperature forecasts are determined no more than a week ahead, and consumers should rely on local forecasts for the most accurate information.

– K. Fanuko