Washington Department of Ecology
La Nina conditions have emerged in the Pacific Ocean, fortifying the chances of a cold and wet winter in the Northwest, federal forecasters reported Thursday.
The National Oceanic and Atmospheric Administration assessed the odds of La Nina prevailing at approximately 75 percent through the winter, up by about 10 percent from last month’s outlook.
La Nina, a cooling of surface and sub-surface waters, became more apparent in October along the equator across most of the east-central tropics, according to NOAA’s Climate Prediction Center. A month ago, the center reported sea temperatures were edging toward La Nina, but were still neutral.
“Overall, the ocean and atmosphere system reflects the onset of La Nina conditions,” according to a statement from NOAA’s Climate Prediction Center. “La Nina is likely to affect temperature and precipitation across the United States during the upcoming months.”
La Nina tilts the odds in favor of below-average temperatures and above-median precipitation across the northern tier of the U.S. For the southern tier, the reverse is true.
La Nina doesn’t always lead to cold and wet Northwest winters, but does portend the greater accumulation of snow to supply water for summer irrigation.
“It certainly gives us the possibility of a wetter winter,” Idaho State Climatologist Russell Qualls said.
Qualls said that snowfall was heavy in recent years when ocean temperatures were warm enough to create strong La Nina conditions. This La Nina is expected to be weak, but snow totals have been above average even in those years.
La Nina conditions are now similar to one year ago, according to NOAA, An ample snowpack last winter got Washington irrigators through a record-hot August.
Washington’s snowpack drought in the winter of 2014-15 was during an El Nino, a warming of ocean temperatures.
The Climate Prediction Center planned to issue a new three-month forecast Nov. 16. The October long-range forecast called for precipitation and temperatures to be average for most of the Northwest through the end of January.
Northwest basins have an unusually large amount of snow for this time of year, according to the Natural Resources Conservation Service. Snowpacks in many basins in Oregon, Idaho and Washington were double or triple of normal for mid-November.
According to NOAA, a substantial amount of cooler-than-average water below the surface of the tropical Pacific could have an affect for months, extending La Nina into the spring. Sea-surface temperatures stayed cool in October, resisting atmospheric forces that normally have a warming effect, according to climatologists.