Falling into La Nina...

Summary

It's a beautiful warm weekend with lots of sun. We are going to take a quick look at the forecast over the next 2 weeks. Then some more insights into the upcoming Winter.

Short Term Forecast

The warm weather this weekend will turn cooler as we head into the upcoming week.  A cold trough dropping into the West will bring colder weather.  The trough is far enough East that we are not expecting precipitation, but high temperatures will drop from the 60's and 70's down into the 50's starting Monday.

Here is a look at the forecast temperature anamoly for the upcoming week showing the colder air in the West.

The cooler airmass may stayin the West week 2 through the 16th.

Extended Forecast

The sea surface temps along the equator in the ENSO regions have continued to cool this week.  Right now NOAA is forecasting a weak La Nina this Winter, but some forecast models show a moderate La Nina event possible.

Here is a look at the historic numbers for a weak La Nina for % of average precip in Northern CA.

image courtesy of golden gate weather

Here is another map looking at the weak La Nina precipitation historically.

Moderate La Nina seasons have averaged more precipitation.

The big snowfall 2010-11 season is officially listed as a moderate La Nina event.  But that calculation is based on a 3 month running average of ENSO 3.4 being at least 1.5 degrees below average for 5 consecutive 3 month averages.  2010-11 had 4 consecutive at 1.5 degrees below average with the 3 month means before and after that at 1.4 degrees below average.  So it just missed being officially a strong event, and had strong La Nina conditions during the hear of the winter.

I believe the sea surface temperatures in the Northern Pacific are just as important as along the equator for influencing the Winter storm pattern for the West Coast.  We have seen evidence of that over the years.  Not all El Ninos are warm and wet and not all La Ninas are cold and snowy.  Looking at the Northern Pacific the SST's look similar to 1995.

1995 and then 2011 were weak La Nina seasons that brought around 80-85% of average snowfall to the Snow Lab at 7000' on Donner Summit.

We will have to keep watching as we go through the fall season to see if this La Nina looks stronger, and continue to watch the other signals as we get closer to Winter.

Stay tuned...BA

   

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