Winter Forecast for the Northwest

I have been under a lot of pressure to provide the winter forecast.  And the snow outlook is probably the biggest demand. 

OK, you will get what you want.

What some folks want

The skill of our forecasting models really fade after two weeks with the extended prediction models having virtually no skill months ahead of time.  The most useful tool we do have is the correlation of the temperatures of the tropical Pacific, quantified as El Nino (warm water), La  Nina (cool water) or neutral conditions (La Nada), and the weather over the Northwest.     The correlation is not perfect, but gives one an idea which way the atmospheric dice are weighted.

Let's start by look at the temperatures of the tropical Pacific, with the temperature of the Nino 3.4 area being the most closely monitored (see map).

The sea surface temperature (SST) anomaly (difference from normal) is shown below.   As you can see, the Nino 3.4 area  this spring went from colder than normal (La Nina) to weakly above normal.    Neutral or La Nada conditions area associated with the SST anomalies being between .5C and .5 C.    That is where we are now.

But it appears, we will get to at least to a weak El Nino (Nino3.4 temperature anomalies between +.5 and 1C).    One hint is that the temperature of the upper ocean is warming (see below)

And if we look at a collection of forecast models, they are pushing the temperature anomaly to about 1C above normal in the Nino 3.4 area (see below).  A weak El Nino.

 The official Climate Prediction Center prediction is for us to move from neutral to weak El Nino, with roughly a probability of 65%.

But we have an issue.  To get a significant El Nino signal, one that really changes the midlatitude weather pattern, one needs a moderate to strong El Nino.  In those circumstances, the Northwest is generally warmer than normal, the jet stream splits over the northeast Pacific and good portion of the jet stream heads south towards southern CA.

The El Nino this year appears to be a marginal one, with far more limited impacts.   Even in a weakened state, the El Nino tendency would be for warmer than normal conditions and less snow than normal.   And add to that the decadal trend to slowly increasing temperatures from global warming, and the fact that an important source of natural decadal variability, the Pacific Decadal Oscillation (PDO), recently moved into a warm phase. (see below)

So warmer than normal seems a good bet. 

Let's take a look at one of the best extended forecasting modeling systems (but still with marginal skill), the North American Multi-Model Ensemble (NMME).  I will show you the average anomalies (differences from normal) for precipitation and temperature for November through January.

Precipitation?  Drier than normal over much of the Northwest, but close to normal over western Washington.

 Temperatures?   Around .5C above normal.

The implication of all this would be a lower than normal snowpack in our mountains this winter.  Lower heating bills.  A slow fill for our regional reservoirs.   

One thing I do know...the next three days will represent the most perfect fall weather you can imagine.  Perfect day time temps, cool nights, leaves starting to turn.  My favorite season.

from Cliff Mass Weather and Climate Blog https://ift.tt/2xHIW9M