A La Nina weather pattern is likely to cause a cold, wet winter, a forecaster says.

Odds increasingly favor La Nina prevailing for a third straight winter, the National Weather Service says.

The weather service’s Climate Prediction Center pegged the chances for another La Nina winter at 61%, up from 54% last month.

La Nina winters in the northern tier of the U.S. are generally cooler and wetter than normal. Three La Nina winters in a row have occurred only twice since 1950.

Although ample summer water supplies usually follow a La Nina, it’s not a sure bet, Washington State Climatologist Nick Bond said Friday.

The last third-straight La Nina winter, 2000-2001, yielded a poor water supply for summer irrigation and led to a drought.

“There’s no guarantee there’s going to be a huge snowpack,” Bond said.

Below-normal sea-surface temperatures in the Pacific Ocean trigger a La Nina. While the northern U.S. tends to be cold and wet, the southern tier leans toward being hot and dry. Another La Nina winter could worsen the drought in the Southwest.

La Nina and its opposite, El Nino, are climatologists’ main guides for forecasting weather months in advance. The climate center puts the odds of an El Nino forming next winter at only 5%. The odds that sea-surface temperatures will be normal, or neutral, are 34%.

A La Nina has formed the past two winters and is still in place. The climate center said ocean temperatures this past week were “quite negative” for this time of year.

Normally, La Nina has its most influence on Northwest weather in the late winter. This year, it’s effects may be lasting into the spring. “I’m prepared to believe that,” Bond said.

Washington just had its third coldest April in 128 years of record-keeping. The snowpack rallied during the month from below average to finish above average.

Natural Resources Conservation Service water supply specialist Scott Pattee said May 13 the snowpack has peaked, but is melting slowly because of low temperatures.

La Nina likely will stay through the summer, according to the climate center. Bond said he then expects its influence to wane and that the state will follow a trend toward warmer and drier summers.

A La Nina prevailed for 38 months from April 1973 to May 1976, and for 34 months from June 1998 to March 2001.