Thursday, December 1, 2011

AccuWeather predicts wet winter ahead

From AccuWeather, which created the accompanying map:

AccuWeather reports the AccuWeather.com Long-Range Forecasting Team still expects a brutal winter for the U.S. with the worst in terms of snow and cold targeting the Midwest and interior Northwest.

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 stated in the initial release of the 2011-2012 Winter Forecast.

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 tends 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 Looking Wetter for California, Still 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.

Despite the on again, off again stormy weather for the Pacific Northwest rather than a constant storm train, the region will not be spared winter's blow. Pastelok added that the bitterly cold blasts will invade northern areas, especially during the mid- to late season.

After being active early in the season, there will be less precipitation in Seattle and Portland, Ore., later on. However, both of these cities could get cold for a time with arctic air penetrating from the north and east.

For what it's worth. I have my doubts about a pineapple express, which I understand to be more of a fixture of El Nino than La Nina. But hey, what do I know?

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