The full AccuWeather.com Winter 2011-2012 Forecast displays a larger version of this map. By Heather Buchman, Meteorologist
Oct 31, 2011; 4:12 PM ET
When AccuWeather.com released its 2011-2012 Winter Forecast, people asked how meteorologists can predict the weather so far out in advance. What are these long-range forecasts based on?
A great deal of research that goes into it.
Paul Pastelok, AccuWeather.com Expert Long-Range Meteorologist and leader of the Long-Range Forecasting Team, explained that the team looks at current trends, especially with Pacific Ocean temperatures (La Niña/El Niño), and looks for past years in which similar trends occurred.
Meteorologists call these years "analog years."
"We look at past La Niña and El Niño seasons, looking at the strength and trend of those El Niños or La Niñas," explained Pastelok. "We also look at the strength of the PDO (Pacific Decadal Oscillation Index) and what is going on currently with weather trends. Then we find past years that were very similar and look at those seasons more in depth."
A La Niña occurs when sea surface temperatures across the equatorial central and eastern Pacific are below normal. El Niño occurs when these sea surface temperatures are above normal. The greater these temperatures depart from normal, the stronger the La Niña or El Niño.
Both phenomena have a significant influence on the jet stream and overall weather patterns across the globe. While no two El Niños or La Niñas are exactly the same, general trends have been observed in the influence they have on the weather in the U.S. and elsewhere throughout the world.
Currently, we are in a weak La Niña and trending toward a moderate La Niña for this winter.
Throughout the past, there have been years in which there were similar trends with La Niña. The Long-Range Forecasting Team has highlighted 10 past seasons that match most closely to what has happened so far this year and have most significantly influenced what the team expects to happen this winter season.
Of those 10 previous years, the team expects this winter to be most similar to the winter of 2008-2009.
"From a really bad severe weather season in the spring to a record hot summer on the southern Plains to the way the tropics have been behaving this hurricane season... and Pacific Ocean temperature trends from the spring up until now... 2008-2009 fits best with this year," Pastelok said.
He added, "No year is ever the same, but you can get the general trends and patterns."
The accuracy of the long-range forecasts can be measured. We can see if temperatures were in fact above or below normal and whether precipitation and snowfall were above or below normal. General trends can be verified.
However, when it comes down to it, how the winter was perceived often carries more weight than the numbers.
"It's so difficult because it boils down to perceptions," said AccuWeather.com Expert Senior Meteorologist Bob Larson. "You can predict a milder-than-average winter with less snow than average and have it verify. But if you have a big Thanksgiving snowstorm, a Christmas blizzard and a New Year's Eve ice storm, that's what people remember. They remember those big storms, not the weather in between, and think it was a terrible winter. Their minds drift to memorable events."
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