By Deborah Zabarenko and Ayesha Rascoe
WASHINGTON (Reuters) - After a hot spring and a scorching summer, this winter is likely to continue a U.S. warming trend that could make 2012 the hottest year since modern record-keeping began, U.S. weather experts said Thursday.
Drought that ravaged much of the United States this year may spread in the coming months, said Mike Halpert at the National Oceanic and Atmospheric Administration's Climate Prediction Center.
"The large majority of that drought we expect to persist," Halpert said. "We even see drought expanding westward ... into Montana, Idaho and part of Oregon and Washington."
Dryer-than-usual winter weather is expected in much of the Pacific Northwest, with higher-than-normal precipitation predicted for the Gulf Coast, according to NOAA forecasts.
For much of the country, a three-month (December-February)winter forecast is hard to pin down. The vast majority of states have what the experts said was an equal chance of below-normal, normal or above-normal precipitation.
The densely populated East Coast, along with the southern tier of states from Texas to Florida and the upper Midwest also have an equal chance of colder, normal or warmer weather this winter, according to the forecasters.
Still, there is enough data to predict a warm winter overall, said Deke Arndt, chief of climate monitoring at NOAA's National Climatic Data Center. 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 since record-keeping began.
"The main issues facing the U.S. going into this (winter) outlook period stem from persistent heat and drought," Arndt said at a telephone briefing. "It is likely that 2012 will be the warmest of the 118-year record for the contiguous United States."
An El Nino pattern -- a recurring patch of warmer than usual water in the equatorial Pacific that can have a potent effect on U.S. weather -- gave hints of developing in September but then subsided, the first time this has happened in approximately 60 years of record keeping on this phenomenon, Halpert said.
"This is one of the most challenging outlooks we've produced in recent years because El Nino decided not to show up as expected," he said.
A record-warm winter would be in line NOAA's latest report on global temperatures, which found September 2012 tied for the hottest September in world records going back to 1880.
However, Arndt said that the signal of human-spurred climate change is less apparent now in some U.S. regions, especially in winter. This is due in part to the changing baseline forecasters use.
U.S. government experts look back 30 years to figure out baseline temperatures for the country. In the past, they used the 30-year period from 1971 through 2000; this year, they used 1981 through 2010. That latest period shows little sign of a warming trend in areas like Florida and other parts of the southeast, Arndt said.
That updated baseline helps to "mask" the signs of climate change on a regional and seasonal basis, Arndt said.
(Reporting by Ayesha Rascoe; editing by Sofina Mirza-Reid and Bob Burgdorfer)