A Potential La Nina for this Winter: What Does It Imply?

There is a good chance that La Nina--cooler than normal waters in the central tropical Pacific-- will return this fall.  And the reprise of La Nina has major implications for the weather of the upcoming winter.

As I have discussed many times in this blog, there is only one truly useful tool for predicting Northwest weather months in advance:  the correlation between temperatures of the central tropical Pacific and the weather circulation over the western U.S.

A rough cycle, called ENSO (El Nino Southern Oscillation), shuttles between warmer than normal water temperatures (El Nino), to near normal conditions (neutral), to colder than normal tropical waters (La Nina).  The period of this cycle is approximately three to seven years.  The figures below illustrate the changes between El Nino and La Nina.   

You can think of the cycle as analogous to water sloshing back and forth in a bathtub, with less dense, warm water on top.   When the warm water sloshes to the east,  you have El Nino.  To the west, La Nina.  In the images below, you are viewing the Pacific basin and red colors indicate warm water.

You will note that convection and thunderstorms follow the warm water--this is important. 

 The trade winds are strongly out of the east during La Nina years (white arrows) but reverse during El Nino years.    All of these changes influence the atmospheric circulation away from the equator...including over the Northwest.

So let me get to the punch line.  During La Nina years, the Northwest tends to be cooler and wetter than normal.  Typically more snowpack in the mountains.  And increased chances of lowland snow.  

La Nina works against drought in the Northwest.   But the situation is just the opposite in central and southern California, where La Nina years are typically dry.   Bad for wildfires and water supply in the Golden State.

The Prediction for this Winter

I generally refrain from blogging about La Nina/El Nino status until mid-August because such forecasts are far less reliable until the summer.   But we are far along now into the summer, so I can provide some useful guidance.

To follow El Nino/La Nina we generally monitor the temperatures in the Nino 3.4 area in the tropical Pacific (see map below)

The deviation of the temperatures in that box from normal is shown below.  Much cooler than normal last winter, when we were in a La Nina.   That is why we ended up to with a very healthy snowpack, by the way.   

But La Nina weakened during the spring and we are now in neutral conditions (temperatures roughly within .5C of normal).

Importantly, La Nina appears to be rebounding again.  Cold water is strengthening beneath the surface, as shown by the blue colors in the figure below, which provides an east-west vertical cross-section in the tropical Pacific.

And based on both simulation and statistical models, the Climate Prediction Center is predicting the resurgence of La Nina (blue colors) this fall.

Now, there is some uncertainty in the prediction, as shown by a plot of predicted central Pacific temperatures (differences from normal) below.  But most models are going for cooling waters.

The latest European Center seasonal prediction is projecting wetter than normal conditions this fall over the Northwest...but not over southern CA.  Consistent with La Nina.

For those concerned about Northwest drought and wildfires, this is generally good news.  

But one must be concerned about central and southern California, which has just come off of a dry, warm period.  Their reservoir levels are already quite low and one worries that serious water restrictions may be in their future if La Nina revs up during the next few months.  Stay tuned.