News•October 18, 2012

Demise of El Niño Throws a Wrench in Winter Outlook

The upcoming winter is likely to be a mild and dry one for the West and the Upper Midwest, while parts of the Southeast may see cooler and wetter than average conditions, according to the official U.S. winter outlook issued Thursday by the National Oceanic and Atmospheric Administration (NOAA). In making its seasonal prediction, forecasters cautioned that there are large portions of the country for which there are no clear indications whether it will be a warmer, colder, wetter, or drier than average winter, largely due to a fickle El Niño event that may have petered out too early to have much of an impact on North American winter weather.

Given that the majority of the Lower 48 states are still mired in a major drought, with the latest drought monitor showing a continuous swath of drought stretching from California to Illinois, the forecast offers little hope for significant relief anytime soon. In fact, Mike Halpert, the deputy director of NOAA’s Climate Prediction Center in College Park, Md., said the drought is likely to expand from the Upper Midwest to the Pacific Northwest, and may also intensify all along the West Coast.

Winter precipitation outlook for the U.S. “EC” means equal chances of below average, average, or above average precipitation.
Click to enlarge the image. Credit: NOAA/CPC.

“My biggest concern this year is really the West Coast,” Halpert said, noting that most of the computer models used to guide forecasters consistently show that this region will experience a dry winter.

During the summer and early fall it appeared that an El Niño event, which is characterized by warmer-than-average water temperatures in the tropical Pacific Ocean, was developing. Such events can have significant impacts on North American winters, because they tend to favor particular weather patterns, and are known to help reconfigure the jet stream, which is a high altitude river of air that helps steer storm systems and establish boundaries between air masses.

In recent weeks, though, water temperatures have cooled in the tropical Pacific, and there are fewer signs of El Niño, Halpert said. The winter weather outlook incorporates the possibility that El Niño will stage a comeback, but it’s apparent collapse threw a wrench in the forecasting process.

“This year’s winter outlook has proven to be quite challenging largely due to an indecisive El Niño,” Halpert said. The demise of El Niño stands out when looking at the historical record that stretches back 60 years. During that time there has never been a similar case in which water temperatures warmed so much during August, and yet El Niño conditions failed to take hold, Halpert said.

“When we reach a certain threshold by August, in September we have always proceeded right into El Niño,” he said.

Winter temperature outlook for the U.S. “EC” means equal chances of below average, average, or above average temperatures.
Click to enlarge the image. Credit: NOAA/CPC.

Because of the lack of a clear El Niño signal, as well as the potential influence of other climate cycles, there is a large swath of the eastern U.S. where NOAA’s outlook lacks specificity. According to the outlook, the East (with the exception of Florida, which is forecast to be wetter and cooler than average) has equal chances of seeing below-average, average, or above-average temperatures and precipitation.

While it can be affected by El Niño, snowfall in the Northeast, for example, is also closely linked to the Arctic Oscillation, which is an atmospheric pressure pattern over the North Atlantic that can vary from one week to the next. During the winters of 2009-10 and 2010-11 the Arctic Oscillation was often in an extremely “negative” phase, which helped drive cold and snow into the Northeast, contributing to the famous “Snowmageddon” storm of February 2010, for example.

Forecasters lack the ability to predict its behavior more than two weeks in advance. “It is obviously critical for making an accurate winter forecast . . . but right now it’s got us stumped,” Halpert said of the Arctic Oscillation.

Recent research has suggested that the abrupt decline in Arctic sea ice may influence the Arctic Oscillation, and could paradoxically raise the odds of colder, snowier winters in the U.S. and Europe.

“It might be one piece of the puzzle . . . my own thought is there’s probably multiple pieces to put this all together,” Halpert said, regarding the sea ice research findings.

This summer, Arctic sea ice plummeted to a record low, breaking the record established in 2007. The ice loss means that Alaska may see milder-than-average conditions during this upcoming winter. In fact, the highest odds for above-average temperatures anywhere in the country can be found in northern Alaska, due to the absence of sea ice offshore of the North Slope.

“Right now there is no ice at all there. We would certainly expect the early winter to be above average [temperature-wise],” Halpert said.