Fall & Winter Outlook: La Niña Likely Again

As we begin September and fall approaches, one thing on the mind of many Pacific Northwest residents is what to expect during the storm season. Fortunately, by this time of the year, we have a good idea of the general conditions and trends to expect this fall and winter. 

The weather patterns across parts of the Northern Hemisphere are influenced by the sea surface temperatures (SST) of the equatorial Pacific Ocean. These patterns are El Niño (warmer than average SST), neutral (normal SST), and La Niña (cooler than average SST).

This winter will most likely be a La Niña winter, which generally means above average rain, above average mountain snow, and a higher potential for lowland snow.

The forecast graphic below from the NWS Climate Prediction Center shows the probability of La Niña, Neutral, or El Niño in 3-month periods.

The blue bars on the graph shows the probability of La Niña increasing by August and September. La Niña probability is over 60% from approximately fall to midwinter, which has prompted a La Niña Watch to be issued by the Climate Prediction Center earlier this summer. Current forecasts show neutral conditions increasing around March, though time will tell how this evolves.

Another way to show the La Niña forecast is through a long-range forecast run by many forecast models, showing departure from average sea surface temperatures.

This forecast plot is best described as a forecast model ensemble, like we use for snow predictions, etc. during the winter. This forecast confirms the previous one, and shows that a majority of the forecast models are predicting SSTs in the equatorial Pacific to be 0.3-1.0° Celsius below average. 

The bottom line is that it’s safe to expect a La Niña winter. We’re sure to see the effects of La Niña beginning this fall.

Now for a quick look at our upcoming forecast…below is the Weather Underground forecast for Federal Way.

From Saturday through Tuesday, temperatures will be a bit warmer than they have been, with highs in the mid 70s to low 80s, warmest on Saturday, Monday, and Tuesday. Expect lows in the low-mid 50s except in the upper 50s-low 60s on Saturday night.

Below is the rain forecast from the UW model through Monday morning.

Expect 0.01-0.05 inches of rain south of Seattle, bands of 0.1-0.2” totals between Seattle and Everett, and 0.2-0.5 inches on the coast and from Arlington northward.

Enjoy our warm & mostly dry weather while it lasts!