SAN FRANCISCO — Brrrrrrr! That might be the sound on the lips of everyone in the eastern United States, if a new weather prediction model's forecast for a cold winter proves accurate.
And why will it likely be cold? Blame snow in Siberia. The extent of Siberia's October snow cover affects weather patterns in the Arctic, enough to shift winter temperatures in the United States, Europe and East Asia, said Judah Cohen, director of seasonal forecasting for Atmospheric and Environmental Research (AER), a Verisk Analytics company.
Called the snow advance index, or SAI, the model measures the rate of change of Siberia's snow cover. When snow cover advances rapidly in October, the upcoming winter will be more severe in the eastern United States, Europe and central Asia. Conversely, a slow-moving advance of snow cover means a mild winter for the same regions. The index model also incorporates sea level pressure anomalies and sea surface temperatures from the equatorial Pacific Ocean, where the El Niño/Southern Oscillation arises.
With normal snow cover this October in Siberia, and an SAI of 2.19 (a high value on the 0 to 4 SAI scale), the model forecasts cold weather through February for the United States from the Rockies to the East Coast, Cohen said here last week at the annual meeting of the American Geophysical Union. The West Coast will have higher temperatures, as will Alaska.
But even cold-hardy Alaskans are complaining about the unusually bitter weather so far, counter to the model's prediction. An omega block, a center of atmospheric pressure sitting over the Bering Sea, is keeping the state at below-freezing temperatures. According to the National Weather Service, November 2012 was the sixth-coldest November for Fairbanks on record.
"When the Arctic Oscillation is negative, as it is now, Alaska is supposed to be warm, but it's not," Cohen told OurAmazingPlanet. "The North Pacific seems to be doing something separate from what would normally happen," he said.
Predicting Arctic weather patterns
The SAI helps predict the Arctic Oscillation, a large-scale weather pattern that affects circulation in the atmosphere at northern latitudes. When the Arctic Oscillation is in a positive phase, air pressure is higher over the United States, keeping cold air in the Arctic. A negative Arctic Oscillation draws frigid Arctic air into the United States, bringing lower temperatures.
The model's forecast is holding up better for Europe, Cohen said. Northern Europe, including Britain, is facing a cold, snowy season, but southern Europe should be warmer than average, according to the model's forecast.
With the forecast for winter of 2010-2011, AER successfully predicted colder than normal temperatures for the eastern United States, with warmer than normal temperatures in the high Arctic latitudes.
Historically, scientists have considered the Arctic Oscillation to be a result of chaotic behavior and therefore unpredictable, Cohen said. "No forecast is ever going to be 100 percent right, but there is a useful fraction of the Arctic Oscillation that is predictable, and it would be a huge improvement for seasonal forecasting in large parts of the hemisphere," he said.
Editor's note: This article has been updated to clarify the earlier AER forecast was for winter 2010-2011.