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Posted at 12:24 PM ET, 10/18/2012

NOAA winter outlook: Big uncertainty due to fickle El Nino


NOAA’s winter oulook for temperature and precipitation. A indicates “above normal”, B indicates “below normal”, whereas EC indicates “equal chances” above or below normal. (NOAA)
Since the summer, forecasters have called for El Nino to develop this fall, but so far, it has defied such predictions. El Nino’s baffling behavior has left NOAA forecasters scratching their heads and unable to make a solid call about what kind of winter to expect over large parts of the United States.

El Nino, the episodic warming of ocean temperatures in the tropical Pacific, is often linked to certain weather patterns over the U.S. Generally, during El Nino winters, it is cool and wet across the southern U.S. and warm and dry across the northern tier.

Related: El Nino may fizzle and fall short of hype

But such signals are much less prominent, if present at all, when El Nino is weak or non-existent (neutral) - the case so far this year.

Mike Halpert, deputy director of NOAA’s Climate Prediction Center, said El Nino development “abruptly halted” last month.

“This is one of the most challenging outlooks we’ve produced in recent years because El Niño decided not to show up as expected,” Halpert said.

NOAA still sees signs a weak El Nino will develop and its outlook, released today is based on that tentative assumption.

NOAA favors above normal temperatures from Texas to the central and northern Plains and westward through the intermountain West. The cool signal typically associated with El Nino across the South is muted, with the exception of south Florida. Everywhere else, including the mid-Atlantic and Northeast, it assigns “equal chances” of above or below average temperatures.

Its precipitation forecast also includes “equal chances” of above or below average rain/snow over a large portion of the U.S. The exceptions to that are in the upper Midwest and Pacific northwest where dry conditions are forecast - often the case in El Nino winters. Wet conditions are predicted across parts of the deep South - another El Nino signal.


Typical winter weather patterns associated with El Nino. (NOAA)
NOAA’s Halpert said that if El Nino suddenly strengthened, below average temperatures and above average precipitation might cover a larger region of the South, whereas dry conditions might expand beyond the north central U.S. towards the Ohio Valley.

In a press call, Halpert repeatedly stressed the difficulty in developing this year’s outlook, both due to the elusive El Nino, and broader challenges in seasonal forecasting.

“The science behind seasonal prediction is in its infancy,” Halpert said, noting such outlooks are about 20-30 percent better than a random guess, and even less than that when the El Nino signal (or conversely, its opposite phase, La Nina) is weak.

Related: NOAA participation in seasonal prediction conference in limbo due to lack of travel approval

Halpert noted computer models used as a tool in developing such outlooks were providing conflicting information.

“[There’s been] no consistency among different model forecasts - even from month to month-- models are struggling as much we are,” Halpert said.

This is the first time in 60 years of records El Nino has displayed this kind of erratic behavior, Halpert said, so the past provides few clues about what the future may bring.

“The historic record really isn’t telling us much” Halpert said.

Halpert acknowledged El Nino isn’t the only player in developing seasonal outlooks, but that one of the other key predictors of winter conditions, the Arctic Oscillation, can’t be forecast more than two weeks or so in advance.

“The Arctic Oscillation can certainly play a large a role in outcome of winter, but at this point can only forecast on weekly to two-weekly basis,” Halpert said. “At this point it still has us stumped.”

In the winter of 2009-10, the Arctic Oscillation was sharply negative, resulting in cold, stormy conditions over the Eastern U.S. Last winter, it was largely positive, resulting in the opposite conditions. It remains a big wildcard heading into this winter.

Although NOAA gives equal odds of above/below normal temperatures and precipitation along the East Coast, AccuWeather is calling for above normal snowfall along the I-95 corridor, based on stormy conditions so far this fall and an expectation that El Nino, albeit weak, will help juice up storms approaching the East Coast from the south.

NOAA acknowledged past large East Coast snowstorms have developed during El Nino winters but stopped short of predicting them this year.

By  |  12:24 PM ET, 10/18/2012

Categories:  U.S. Weather, Latest, U.S. Weather, Winter Storms

 
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