Odds are increasing that an El Nino will form, the National Oceanic and Atmospheric Administration reported Thursday, bolstering earlier forecasts that winter will be mild in the Northwest.
NOAA’s Climate Prediction Center put the chances of an El Nino in December through February at more than 80 percent. One month ago, NOAA assessed the odds at 70 to 75 percent. Since then, sea-surface temperatures have risen along the equator between South America and Asia, according to NOAA.
“I’ve got to go with a warmer winter than normal and a snowpack that by the end of the winter certainly won’t be as healthy as it has been the last couple of years,” Washington State Climatologist Nick Bond said.
During an El Nino, surface temperatures warm up to at least 0.5 degree Celsius above normal. The heated ocean alters atmospheric weather patterns and pumps warmer than usual air into Alaska, western Canada and the northern U.S.
Temperatures are typically a few degrees above normal in the Northwest. The change can be enough to make the difference between rain and snow at some elevations.
The effects of an El Nino are not a sure bet. Washington suffered a snowpack drought during a “weak” El Nino in 2014-15. The poor snowpack led to a shortage of water for summer irrigation. The snowpack rebounded the following winter during a “strong” El Nino.
The Oceanic Nino Index for August through October was 0.4 degree Celsius, just below the El Nino threshold. NOAA is predicting a “weak” El Nino.
Even if the El Nino strengthens into a moderate one, it’s unlikely to lead to a winter as warm as the one four years ago, Bond said. The ocean off the West Coast is warmer than usual, but not as heated as it was then, he said.
“I don’t think it’s going to be a major player, but likely will contribute to a little bit of the warmth,” Bond said.
NOAA predicted last month that Oregon, Idaho, California and Washington will have above-average winter temperatures and normal amounts of precipitation. NOAA will update its winter outlook Nov. 15.