SALEM — The National Weather Service on Oct. 12 predicted a 30% chance that a “historically strong El Nino” will form later this fall, a climate phenomenon that’s usually ushered in warm and wet winters in the Northwest.

The updated outlook boosted the chances for the first exceptionally strong El Nino since 2015-16. Such El Ninos also took shape in 1982-83 and 1997-98.

Average temperatures in Washington and Oregon were above normal all three winters. Rain and snow totals were above average, too, with the exception of Washington in ’97-98, when the winter was the 64th driest in 128 years of record-keeping.

El Nino winters are generally warmer and drier in the Northern U.S. and cooler and wetter in the Southern tier, but it is not a sure bet. An El Nino likely will influence the climate, but local impacts may not be strong, according to the weather service.

What’s El Nino?

El Nino occurs when the Pacific Ocean along the equator warms up. The mass of warm water changes wind patterns and storm paths.

The atmosphere doesn’t always react to the warm water, but it has been this time, Washington State Climatologist Nick Bond said.

“Sometimes the atmosphere doesn’t seem to care as much,” he said. “The atmosphere is responding to the ocean. How that plays out in the coming months, we’ll have to see.”

A historically strong El Nino can form if sea temperatures are more than 2 degrees Celsius above normal, according to the weather service. Seas are currently 1.5 degrees above average.

Temperatures in the winter of 2015-16 peaked at 2.6 degrees Celsius, an all-time high, according to weather service records that date back to 1950.

What models show

Two of the 26 climate models looked at by the weather service’s Climate Prediction Center predict sea-surface temperatures will top the record.

The climate center puts the odds of a “strong El Nino” by November at 75% to 85%. The El Nino has an 80% chance of hanging on until May, according to the weather service.

The last El Nino was 2018-19 and it stayed “weak.” A La Nina has prevailed the past three winters. La Nina are triggered by below-normal sea temperatures.

A strong El Nino increases the chances of coastal flooding, Bond said.

El Nino-triggered waves hit South America and spread. The waves, storms and rising sea levels caused by climate change could cause the flooding, Bond said.

“We could learn lessons from this El Nino about what we can do about rising sea levels,” he said.

Washington had its 10th warmest and seventh wettest winter on record during the historically strong El Nino of 2015-16. The snowpack statewide April 1 was 110% of normal.

Oregon had its 17th warmest and 13th wettest winter and the April 1 snowpack was 105% of normal.

California had its 13th warmest and 50th wettest winter, while Idaho had its 22nd warmest and 52nd wettest winter.

Climatologists foresee strong El Nino