No one knows how low temperatures may fall, but Ray Eckberg, a meteorologist with the National Weather Service in Green Bay, said historic data and La Niña indicate a 70 percent chance that temperatures will be below normal by mid-January and February. Snowfall or precipitation is not so easy to predict.
The reason national forecasters are predicting some areas may have a rough winter is due to weather patterns indicated by La Niña, a name for the cooling of Pacific waters between Hawaii and Central America. Those waters are cooler than normal, Eckberg said.
The cooler water there affects thunderstorm action across the tropics, interfering with the jet stream. For areas like Texas, it may mean more droughts and for Grand Forks, N.D., it could mean more serious flooding, he said.
In Northeastern Wisconsin, he said, it may mean more Arctic intrusions or more cold fronts coming down from Canada affecting temperatures. Snowfall, on the other hand, is "highly variable," said Eckberg.
"Those are more individual storm events," he said.
La Niña is expected to cause more activity next spring in the central plains, which is bad news for the central U.S.
In contrast to La Niña, El Niño warms the Pacific waters, making the jet stream stronger and keeping arctic air north of the Canadian border. It was El Niño that brought warmer temperatures to the region in winter 1998, when the average temperature was 27.9. Compare that to the coldest winter on record, in 1917, when average temperatures were just 12.7.
Big Midwestern cities, such as Chicago and Minneapolis, will lie in the heart of this zone.
While the worst of winter will be focused over the Midwest and Northwest, it does not mean other parts of the country are off the hook.
Above-normal snowfall is also forecast for the interior Northeast and northern New England.
A region-by-region breakdown on what to expect this winter can be found farther down on this page.
Factors behind the forecast
A weak to moderate La Niña is a key factor in the 2011-2012 Winter Forecast with more typical La Nina winter conditions expected.
"La Niña, a phenomenon that occurs when sea surface temperatures across the equatorial central and eastern Pacific are below normal, is what made last year's winter so awful for the Midwest and Northeast," AccuWeather.com Meteorologist Heather Buchman said.
La Niña winters feature a stronger Northern jet stream, an area of strong winds high above the Earth's surface. This positioning and strength of the jet stream tend to cause storms to track across the northern tier, spelling harsh winters from the Northern Plains to the Ohio Valley.
Typically, the southern tier of the U.S. ends up mild and dry in a La Niña winter. There will be some exceptions to that this year, with wet weather anticipated for parts of the Lower Mississippi Valley.
Another big factor in the winter forecast is the potential for a blocking pattern to develop with the NAO, or North Atlantic Oscillation, possibly turning negative for a time. This essentially means that a large area of high pressure could set up over Greenland, forcing cold blasts to reach the U.S.
"The lack of sea ice has been believed to contribute to the development of blocking. This past summer and early fall, sea ice reached near-record low levels," according to the National Snow and Ice Data Center.
Winter still looking brutal for Midwest
Pastelok and the Long-Range Forecasting Team still think winter will leave the biggest impression on the northern Rockies, northern Plains and the Midwest.
It will be a snowier-than-usual season for the Midwest, which will lie to the north and west of the frequent storm track, or in the "sweet spot" of the storms.
Typical of a La Niña year, the harshest cold will blast the northern Plains and northern Rockies. Although this may be the case this season, cold air masses will sink farther south and east at times.
One thing that has changed with AccuWeather.com's initial Winter 2011-2012 Forecast is the expectations for a cold December in the Midwest and Northeast. The frequency and period of arctic cold is lower than earlier expected.
Therefore, temperatures for the winter season as a whole are predicted to average slightly higher for the Midwest and Northeast, with December not being quite so cold.
Minneapolis and Chicago lie in the heart of where the worst of winter is expected, with above-normal snowfall and colder-than-normal conditions predicted. Cold blasts will be longer lasting from late December into January.
Snowfall will be well above normal for cities such as Chicago, Detroit, Cincinnati, Cleveland and possibly Pittsburgh.
Farther south, closer to the storm track, some icing events can occur along the Ohio River back to northeastern Texas. Pastelok stressed that this will be most likely during January.
Northeast not off the hook
Pastelok pointed out that this winter in the Northeast will likely be marked by a lack of long-lasting arctic cold. However, there can still be cold snaps with below-normal temperatures. Overall, temperatures are expected to average out near normal to just above normal.
With a storm track favoring more storms cutting across the interior, above-normal snowfall is more likely across the interior Northeast.
Snowfall amounts were bumped up into the above-normal range farther south and east across New England, following the recent historic late-October snowstorm.
In the updated forecast, near-normal lake-effect snow will fall around the eastern Great Lakes. However, snow amounts could still end up above normal in this zone from storms cutting away from the Northeast Coast.
With a storm track farther north and west compared to last year, snowfall will be near to below normal for the highly populated I-95 corridor, including Washington, D.C., Philadelphia, New York City and Boston. Stormy weather is still expected, but there will be more events with a wintry mix or a changeover to rain.
Southeast looking chillier
Bring out the jackets and hats, because it is looking cooler than previously thought for the interior Southeast for December. The nights will be exceptionally chilly.
Temperatures are expected to climb higher in January and February, as an area of high pressure is expected to set up along the Southeast coast and pump milder air from the Gulf of Mexico northward into the region.
In general, winter 2011-2012 is expected to be milder than last winter. There will be a low chance for a damaging freeze in the citrus area this year.
"Florida looks drier than normal," Pastelok said. "The farther north and west you travel, the wetter it gets."
"Watch for potential flooding in the lower Mississippi," warned Pastelok.
An active storm track around the northern and western periphery of high pressure dominating in the Southeast will allow above-normal precipitation to fall in the lower Mississippi Valley.
The severe weather season could begin picking up in the Mississippi Valley and perhaps eastern Texas and Oklahoma in February. In March, severe weather will become more likely in this zone, which was hit hard by tornadoes in the spring.
"Mississippi, Alabama and Tennessee, which were devastated by tornadoes in the spring, will be extremely sensitive to any severe weather outbreaks," stated Buchman in the initial Winter 2011-2012 Forecast.
Southwest, Southern Plains divided between stormy, dry
"There can be a few ice events," Pastelok warned about northeastern Texas and the mid- and lower Mississippi Valley from late December to January.
Elsewhere across the Southwest, dry and mild weather will rule. This is typical of a La Niña winter, where the storm track is much farther north across the U.S.
Unfortunately, drier-than-normal weather will persist from the interior Southwest into southwestern Texas, an area still in desperate need of rain.
The U.S. Drought Monitor release on Nov. 22, 2011, revealed that nearly 65 percent of Texas is still being gripped by severe drought conditions and nearly 40 percent of the state is suffering from exceptional drought conditions, the highest level of drought.
"The lower Rio Grande Valley of Texas and western Texas into New Mexico will run the warmest and driest for the nation through the winter season," Pastelok said.
Winter looking wetter for California, stormy for Northwest
The 2011-2012 Winter Forecast was updated to show even more rain and snow for northern and central California, with the Pacific jet stream expected to hover over this area for most of the winter season.
Farther inland, this winter is expected to be another active one for the northern Sierra and interior Northwest with above-normal snowfall forecast.
In contrast, the Pacific Northwest, including much of Washington and western Oregon, will get some breaks from the wet weather.
"Watch for a 'Pineapple Connection' during the mid- to late season that will send some areas well above-normal precipitation," Pastelok said. This means "snow in the mountains and rain and mudslides in the valleys."
Also known as the "Pineapple Express," the Pineapple Connection is a phenomenon that occurs when a strong, persistent flow of tropical moisture sets up from the Hawaiian islands to the West Coast of the U.S. This phenomenon often leads to excessive rain and incredible snow events.
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