The full AccuWeather.com Winter 2011-2012 Forecast displays a larger version of this map.
How the Heck Can Meteorologists Predict a Winter?
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."
By AlexSosnowski, Expert Senior Meteorologist
Dec 2, 2010; 11:14 AM ET
Lake-effect events can unload a great deal of snow in a hurry on unsuspecting areas downwind during a cold air outbreak. Similarly, flurries can pull a sneak attack well away from the Great Lakes.
Seasoned residents around the Great Lakes know all too well about the nature of lake-effect snow.
Expert Senior Meteorologist and snow-chaser Dale Mohler commented, "It is not uncommon to be driving along downwind of the lake shore in clear conditions, then minutes later, a few snowflakes can suddenly turn into a raging blizzard and snow-covered roads."
"While the snow bands tend to lose intensity after a couple dozen miles away from the lake, they can still bring surprise, sudden snowfall well inland," Mohler added.
The bands of snow can extend 100 miles or more inland during the right conditions.
Hilly areas, such as in the Appalachians, often enhance the snow.
People often comment, "Where did this come from? You guys were not forecasting this!"
Such is the challenge of pinpointing shifting lake-effect snow and pop-up flurries during a cold air outbreak in the Northeast.
In the case of lake-effect snow and flurries, the lake water adds moisture and warmth to the air, making it unstable.
The result is often "streets" of snow (lake-effect) originating from the lakes.
Sometimes, break-off flurries and heavier snow squalls can occur well away from the lakes in the Appalachians.
While bands of lake-effect snow tend to line up with the wind, flurries and snow squalls can show up out of nowhere many miles away from the Great Lakes.
The snow squalls are the winter cousins to summertime thunderstorms, while flurries would be similar to summertime showers.
Snow squalls and flurries can hit one part of a town and not the other, just like a summertime downpour. They can bring gusty winds and low visibility for a time.
Unfortunately in the winter, paved and concrete surfaces can become snow covered and even icy in a matter of seconds.
Such phenomena have contributed to multiple vehicle accidents on highways in the region more than once, such as on Interstate 80 in Pennsylvania.
Incidents of this sort have been investigated in great detail by AccuWeather.com's forensic meteorologists.
Solar Winter is Over
Feb 8, 2012; 12:10 PM ET
Feb. 5 marked the end of solar winter. Solar winter is the quarter of the year with the least amount of daylight for the Northern Hemisphere.
"We're entering the time of year where the sun's rays are getting stronger in the Northern Hemisphere," said AccuWeather.com Meteorologist Andy Mussoline. "Through the summer solstice, which occurs on June 20, we'll see the days get longer and longer. On the summer solstice, the sun's rays will go as far north as the Tropic of Cancer."
Before reaching the summer solstice, we will pass through the spring equinox, which occurs this year at 1:40 a.m. EDT on March 20.
Between now and April, day length across the Northern Hemisphere will grow at its quickest rate.
Solar Winter Over, But It's Still Winter
"There is a difference between 'solar winter' and what we traditionally refer to as winter," continued Mussoline. "It's due to a phenomenon called 'seasonal lag.' The warmth from the summer and fall months carries into the first part of winter, just like the cold from winter will carry into the first part of spring."
The phenomenon can be compared to ocean temperatures. When air temperatures begin to get warmer at the start of warmer seasons, ocean waters remain colder. Inversely, when air temperatures start to drop as winter approaches, ocean waters still remain warm for a time. There is a lag between the rates of change of seasonal daylight and seasonal temperatures much the same way.
Winter Still Coming on Strong in Some Spots
According to the AccuWeather Long Range Forecasting Team, the longest stretch of cold this winter is still yet to come for the Great Lakes and the Northeast.
The cold snap is expect to hit these regions during the second week of February and last into the third week. Temperatures may drop 3-6 degrees below normal at the harshest points of the cold snap. New England, specifically will feel the chill the most.
Beyond just cold temperatures, the chance for snow also arrives. There could be one to two big snow storms in store for the East in February. Temperatures should return to normal by the end of February.
Solar Winter is Over
March is often referred to as the windy month, though other months can be just as windy. Strong, gusty winds occur when energy from aloft is transferred to the ground. This vertical mixing occurs when warmed surface air rises into colder air aloft. This is referred to as an unstable atmosphere.
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