La Nina Strengthens: What are the Implications for this Winter?

La Niña, associated with cool water over the tropical Pacific, has a significant impact on Northwest weather, influencing temperature, precipitation, and snowfall.  

As described below, La Niña has strengthened recently, and some decidedly La Niña weather is now in the forecasts.

As noted in previous blogs, La Niña is associated with cooler-than-normal ocean temperatures over the central tropical Pacific, known as the Nino 3.4 area (see below)

According to the latest observations, we are about to transition from a weak to a moderate La Niña (see figure below).   This figure shows the difference from normal of the temperatures in the Nino 3.4 area, with blue colors indicating below-normal temperatures.   A moderate La Nina is associated with a cool temperature anomaly larger than .9C.   

We are now crossing this threshold to moderate La Niña conditions (see below).

The recent NOAA El Nino/La Niña forecast indicates continued La Niña conditions into mid-winter.  If anything, it's erring on the warm side.

La Niña winters tend to be associated with an upper-level ridge of high pressure over the Gulf of Alaska and associated cold northerly flow over the Northwest (see below).  La Niña winters tend to be cooler than normal over the Northwest, generally with more snow than typical.

Occasionally, the jet stream breaks through underneath the ridge with a strong cyclone/wind event.

Now, let's look ahead over the next week or so, considering forecast models' predictions.

The forecast of the upper level (500 hPa, about 18,000 ft) heights/pressures for Saturday afternoon shows a big ridge offshore and northwesterly flow over Washington.  Classic La Niña pattern.

Tuesday morning?    Very similar.  La Niña upper air pattern

With a La Niña pattern with northerly flow from Alaska in place, temperatures should be a bit colder than normal over the Northwest, as illustrated by the forecast temperatures at SeaTac over the next few days.  Normal is around 50F.... predicted temperatures are several degrees cooler.

The forecast surface (2-m above the ground) temperatures on Sunday at 7 AM indicate that much of the state will be below freezing, with most of the Cascades and eastern Washington in the 20s. (see below)

Below-freezing temperatures will also occur over the eastern part of Puget Sound country.    Keep in mind that temperatures could be even cooler at the surface.  

So be ready for the first frost of the season!