The western half of the U.S. can expect a warmer-than-average winter, but the eastern half is a big question mark, government forecasters said Thursday. They blamed the murky crystal ball on an "indecisive El Nino" -- the fact that it has not formed even though their models said it would. 

"We really haven't seen that before," Mike Halpert, deputy director of the U.S. Climate Prediction Center, told reporters. "Development abruptly halted last month."

El Ninos have been tracked for 60 years, during which 20 have formed, according to the center, which is part of the National Oceanic and Atmospheric Administration.

Experts rely on monitoring El Nino, and its counterpart La Nina, because of their influence on the jet stream and storms across the U.S. An El Nino is a periodic weather cycle that warms part of the Pacific Ocean, thus shifting rainfall and influencing the strength and course of the jet stream.

Why it did not form is a mystery, Halpert said, adding that while an El Nino "could still develop," for now the chances are slim.

If an El Nino does develop, it could mean below-average temperatures across the South, Halpert said.

NOAA

Data also suggest that this winter will push 2012 over the top as the warmest year in the U.S. since recordkeeping began in 1895.

The first nine months of 2012 were the warmest of any year on record in the contiguous United States, and this has been the third-hottest summer on record.

"It is likely that 2012 will be the warmest of the 118-year record for the contiguous United States," Deke Arndt of NOAA's Climatic Data Center told reporters. 

In the winter outlook, the East Coast falls under the "equal chance" category -- it has an equal chance for above-, near-, or below-normal temperatures and precipitation, NOAA said in a statement announcing the outlook.

An exception is Florida, where chances are that all but the Panhandle will be colder than normal.

NOAA

As for the ongoing drought, most of those areas "are unlikely to see much relief," NOAA stated. 

El Nino and La Nina are not the only factors at play, but they are the easiest to forecast several months out since the others are much shorter term.

"El Nino and La Nina can play a key role in winter weather patterns in North America," Weather Channel meteorologist Stu Ostro noted, "but their importance can also be overstated, and in each of the past three winters other factors have overwhelmed their influence."

The biggest influence in those years has been the so-called North Atlantic Oscillation.  A change in the pattern caused it to push the jet stream farther south, bringing more cold air than normal into the northern U.S. from the Arctic.

Skiiers and snowboarders will be among those hoping for more help from the oscillation this year. The ski season did start this month in two places -- a rope tow at Minnesota's Wild Mountain on Oct. 7 and a lift at Colorado's Arapahoe Basin on Wednesday -- but both areas relied on manmade snow.

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