Winter outlook: Warm and dry, or cold and wet?

Making a seasonal forecast for the winter is always a tricky and difficult task. There are many variables that may have a significant impact on a region’s outlook.

For the Northern Hemisphere, and the Pacific Northwest specifically, there are two major conditions that could shape the season.

For this year’s winter, the two biggest contributing factors are ENSO—more commonly known as El Nio and La Nia—as well as the Pacific Decadal Oscillation. Each of these is based on sea surface temperatures throughout our oceans. Variations in temperatures in specific locations can alter how air moves across the planet, which in turn will shift the direction storms take across the United States.

ENSO is more commonly known, and receives the most media attention every winter — and for good reason. El Nio and La Nia conditions have the most impact on the U.S. Warm ocean waters along the western coast of South America spawns thunderstorms in Peru and Bolivia.

Like a rock thrown into a moving stream, these thunderstorms cause ripples throughout the atmosphere and change how storms move across the Pacific Northwest.

This coming winter, ENSO will be in a weak El Nio phase. This typically means the Pacific Northwest is drier, as storms track to the south over California. Our skies remain tend to stay clear as well with a semi-permanent high pressure center off the Washington coast. This extra sunshine helps to keep the winter warmer than average.

The Pacific Decadal Oscillation (also referred to as PDO) is all about temperatures off the West Coast of the U.S. Currently, sea surface temperatures in the Pacific Ocean are about six degrees warmer than average.

This is a textbook example of positive PDO, which usually keeps warm and wet southwesterly winds over the northwest. In a positive PDO scenario, the Pacific Northwest is usually kept warmer, with above-average snow and rain.

So considering that both El Nio and the PDO warm the Northwest, it is safe to say that this winter will be warmer than most.

Regarding moisture, El Nio keeps the region dry, but a positive PDO tends to leave the Northwest wetter. Comparing the two against one another, our precipitation outlook is spot on for average over the winter months.