La Niña conditions, a harbinger of a cold and wet winter in the Northwest, have strengthened over the past month in the Pacific Ocean, the National Oceanic and Atmospheric Administration reported last week.
Below-average sea-surface temperatures along the equator in the central and eastern Pacific became increasingly prominent, according to NOAA’s monthly update on ocean conditions.
Climatologists lean heavily on ocean temperatures to make seasonal forecasts. NOAA’s Climate Prediction Center was scheduled to issue a new three-month outlook on Dec. 21.
The center already was forecasting a relatively cold and wet January and February for Washington and much of Oregon, Idaho and Northern California.
During a La Niña, temperatures are generally lower and precipitation above normal in the northern tier of the U.S. In the southern tier, temperatures are generally above average and precipitation is below normal.
The report last week confirmed that La Niña conditions are likely to persist throughout the winter. NOAA rates the chances of La Niña prevailing until the spring at 80 percent, up from 75 percent last month.
La Niña’s odds of sticking around have been improving through the fall.
Average sea-surface temperatures in the central equatorial Pacific, the region most closely watched by climatologists, were down 0.8 degrees Celsius, according to NOAA. Climatologists classified current conditions as a “weak” La Niña, but said they could strengthen into a “moderate” La Niña.
El Niño, during which sea temperature is above normal, usually foreshadows a warm Northwest winter. The unusually small snowpack that accumulated at higher elevations in Washington in the 2015-16 winter came during an El Niño. The low snowpack combined with a hot and dry spring and summer led to one of state’s most severe droughts.
From January through November nationwide, it was the third warmest on record, including the seventh warmest month of November on record. But again, the Pacific Northwest stood out from most of the rest of the country with near-average temperatures from January through November. Those conditions persisted during the month of November in the Northwest, along with above-average precipitation.
Notably, the Pacific Northwest lies outside of any drought designation, with the exception of a portion of Northwest Montana. But 26 percent of the country is considered to be in drought by the U.S. Drought Monitor.