The 2010/11 winter is predicted to be one of extremes.
A colder and wetter than average winter is in store for the Pacific Northwest, while the southern United States and California are expected to see a warmer and drier winter, government forecasters announced today (Oct. 21).
Overall, forecasters expect another winter of extremes ahead based on the strengthening La Niña, a climate phenomenon that is associated with cooler-than-normal water temperatures in the equatorial Pacific Ocean.
This includes increased chances of storms and flooding in the northern plains and the Ohio and Tennessee valleys, while drought conditions continue in Hawaii.
La Niña has the opposite effect as its cousin El Niño, but both occur every two to five years and drive extreme weather around the globe. Last winter's El Niño contributed to record-breaking precipitation and flooding in some parts of the United States, while other areas experienced record heat and drought.
"La Niña is in place and will strengthen and persist through the winter months, giving us a better understanding of what to expect between December and February," said Mike Halpert, deputy director of the Climate Prediction Center – a division of the National Weather Service.
Other factors influencing the winter weather to come are difficult to predict more than one to two weeks in advance, Halpert said. This is particularly true of the northeastern and mid-Atlantic states, which are affected by the North Atlantic Oscillation, which consists of opposing variations of barometric pressure near Iceland and near the Azores.
"The northeast is always a tricky region for these forecasts," Halpert said during a press briefing. "It's often dominated by phenomena that's not predictable on the seasonal timescale."
Regional highlights of the winter weather outlook include:
- Pacific Northwest: Colder and wetter than average. La Niña often brings lower than average temperatures and increased mountain snow to the Pacific Northwest and western Montana during the winter months, which is good for the replenishment of water resources and winter recreation but can also lead to greater flooding and avalanche concerns.
- California and the Southwest: Warmer and drier than average. This will likely exacerbate drought conditions in these areas. All southern states are at risk of having above normal wildfire conditions starting this winter and lasting into the spring.
- Northern Plains: Colder and wetter than average. Likely to see increased storminess and flooding.
- Southern Plains, Gulf Coast States & Southeast: Warmer and drier than average. This will likely exacerbate drought conditions in these areas.
- Florida: Drier than average, with an equal chance for above-, near-, or below-normal temperatures. Above normal wildfire conditions.
- Ohio and Tennessee Valleys: Warmer and wetter than average. Likely to see increased storminess and flooding.
- Northeast and Mid-Atlantic: Equal chances for above-, near-, or below-normal temperatures and precipitation. Winter weather for these regions is often driven not by La Niña but by weather patterns over the northern Atlantic Ocean and Arctic. These are often more short-term, and are generally predictable only a week or so in advance. If enough cold air and moisture are in place, areas north of the Ohio Valley and into the Northeast could see above-average snow.
- Central United States: Equal chances of above-near-or below-normal temperatures and precipitation.
- Hawaii: Drier than normal through November, then wetter than normal December through February. Statewide, the current drought is expected to continue through the winter, with several locations remaining on track to see the driest year on record. Drought recovery is more likely on the smaller islands of Kauai and Molokai, and over the windward slopes of the Big Island and Maui.
- Alaska: Odds favor colder-than-average temperatures with equal chances of above- or below-normal precipitation. The interior and southern portions of the state are currently drier than normal. A dry winter may set Alaska up for a greater chance of above-normal wildfire conditions in the spring.
While last year saw some unusually large amounts of snow in the eastern United States, with the "Snowpocalypse" that buried Washington, D.C., Philadelphia and New York in large drifts of snow, any major snows this year would be more likely to happen in the western portion of the country, Halpert said.
He cautioned though that snow forecasts are dependent on winter storms, which are generally not predictable more than several days in advance.