La Nina and This Winter's Weather

I am always nervous about predicting the character of the upcoming winter's weather for a number of reasons.  Seasonal forecasting skill is not good, with our long-range numerical models having very little skill past three weeks. Furthermore, our main seasonal forecasting tool with any skill, the relationship between El Nino/La Nina and local weather, only explains some of the interannual (between years) variation.  In addition, the state of the tropical Pacific (which determines whether we are in El Nino, La Nina or La Nada) often changes during the spring/summer.

Earlier in the year it appeared that we would have a neutral (or La Nada) winter, but recently the waters of the tropical Pacific have cooled and the National Weather Service has released a La Nina Watch (see below).

La Nina is associated with cooler than normal waters in the central and eastern tropical Pacific Ocean, with enhanced easterly trade winds near the equator and the shifting of convection (thunderstorms) westward in the Pacific.

Moving the convection to the west has huge impact on the rest of the atmosphere, even outside of the tropics.  This is the source of long-range forecast skill with El Nino/La Nina.

Let's look at the change in tropical Pacific sea surface temperatures for the Nino 3.4 region (see map below)

Examining a map of anomalies (differences from normal) reveals that during the past few months, the Nino 3.4 area ocean temps have dropped below normal (blue color).

Just as important, the temperatures BELOW the surface have also cooled. Here are a series of views below the surface at the equator (the x axis in longitude across the Pacific, and the y axis is depth below the surface) for July through now.  Cooler than normal temperatures (blue) have developed.

 Atmosphere/ocean coupled models, such as the National Weather Service CFS, are predicting the tropical Pacific will cool further this winter (see graphic below of several of their predictions in time).

 And with all this input, the official NOAA/NWS Climate Prediction Center forecast for this winter is that La Nina will be observed (see below).

 OK, what does all this imply for the Northwest winter?  Generally cooler and wetter than normal.  More snow than average in the mountains.

Here are some statistics from the National Weather Service for the region encompassing western Washington and the western slopes of the Cascades. The eastern side of the State is similar.

For precipitation, two plots are shown, one for fall (OND) and the other for mid-winter (JFM).  The red line is the mean and 50% of the years are within the blue boxes. The extremes are shown by the "whiskers".    La Nina years tend to be wetter than normal (neutral) and El Nino years.

 What about temperatures?  A bit cooler in the autumn, but much cooler during mid-winter.

 The implications for snow is clear, especially after January 1.... a higher probability of the white stuff, particularly in the mountains.  Yes... a reasonable year to get an annual pass at your favorite ski area.

The strength of this relationship depends on the amplitude of La Nina, and at this point the models are only going for a modest one.  And the La Nina/El Nino connection is not dominant, with natural variability being larger.  Finally, one should NOT expect more precipitation than last winter, which was the wettest on record by several measures.

But after the smoke and heat of last summer, I suspect many Northwesteners are breathing a sigh of relief.  And the upcoming week promises plenty of clouds and rain to get us in the mood.