La Nina - Does this mean a harsh 2020-21 winter?

After high winds and even snow already hitting parts of the region in October, many may be wondering what this winter will bring.  In this blog we dive into the Winter 2020-21 outlook, so lets get to it.

Some of you may have already heard about the upcoming La Nina winter.  What does this mean for the Inland NW?  Let’s take a look at the general pattern that often sets up during a La Nina.

Typical La Nina pattern

Typically the jet stream enters into the region from the northwest bringing in cooler temperatures and precipitation.  This is a good combination for snow lovers, with just about all locations historically receiving above normal snowfall.  But not every La Nina brings above normal snow! Here is a locally developed map showing percent of normal snowfall for all La Nina years from 1950-2015 based on historical snow data for all our COOP sites as well as airports.

% of normal snow La Nina years 1950-2015

This map is interactive, link here. Snow charts are available like the one below for all sites on the map by either clicking on the number icon on the map, or by selecting a city to the right of the map.

Spokane snowfall (inches) based on ENSO.  Red line is average snow for all years 1950-2015 

It is worth noting however that every La Nina is different, as shown by snow totals for all La Nina winters in Spokane since 1950

Spokane snowfall for each La Nina year since 1950

The reason for the difference is another oscillations in the atmosphere that impact our weather during a winter.  La Nina is only one piece of the puzzle.

So what will this La Nina bring?  Let’s take a look at the Sea Surface temperature anomalies in the Central and Eastern Equatorial Pacific from Oct 4-31st, 2020.

Wow, that is a lot of blue in the equatorial Pacific!  A large area of cooler than normal waters which continues to cool with time.  Anything colder than -1.5C that persists for a duration of three months or more in the central and eastern Equatorial Pacific (Nino 3.4 region) would be classified as a strong La Nina. 

This particular model has a good handle on the current anomalies, suggesting a strong La Nina this winter with the cold anomalies bottoming out near -2.0C.  Each line in the image is one ensemble member forecast amongst the same model.

CFS Model of Nino 3.4 SST anomalies

Past strong La Nina years

We thought it would be interesting to look back and previous Strong La Nina episodes to see what happened.  Here we will look at 3 of the past 4 events.

2010-11

The last strong La Nina year was 2010-11.  This was a harsh winter in terms of several cold air outbreaks.  Republic had four episodes where lows dropped below 0F, with the coldest low -14F. The blue bars on this graph are the observed temperatures for each day.

Republic temperatures Oct 2010-Mar 2011

Similar story for Pullman, with temps as low as -12F that winter

Pullman temperatures Oct 2010-Mar 2011

2007-08

Lots and lots of snow this winter.  Spokane had 92.6” for the season with January and February bringing frequent rounds of it along with drifting snow like this image from NWS Spokane on Feb 7th, 2008.

NWS Spokane February 7th, 2008

And Sandpoint an impressive 122.8”!

Sandpoint accumulated snowfall Oct 2007-Apr 2008

1998-99

This La Nina winter broke records in terms of mountain snow.  A very active jet stream brought frequent rounds of heavy mountain snow.  The map below shows horizontal wind speed anomalies for the winter with the enhanced jet stream aimed at the region

Observed zonal wind anomaly Nov 1, 1998 - Feb 28, 1999

Mount Baker in the Northern Cascades of Washington measured 1,140”, claiming this to be a world record!

Mt Baker snow totals with the 1998-99 noted as a world record

But the strong influx of milder air off the Pacific didn’t mean a snowy winter for many of the lower elevations.   Here is a temperature graph from Moses Lake (brown shade is the normal range while the blue bars are observed).  You can see temperatures were above normal for much of the winter, except for one cold spell just before Christmas.

Moses Lake temperatures Oct 1, 1998 - Feb 28, 1999

2020-21?

So what about this winter?  Lots of possibilities based on some of the recent strong La Nina years.  Here are the latest CPC outlooks.

CPC Dec 2020 - Feb 2021 Temperature Outlook issued Oct 15th, 2020

CPC Dec 2020 - Feb 2021 Precipitation Outlook issued Oct 15th, 2020

These outlooks suggests slightly elevated odds of cooler and wetter than normal conditions.

What are the climate models suggesting?  An active Pacific jet with above normal precipitation as noted by the green shading on the second image, and about normal or slightly above normal temperatures.

NMME forecast of temperature anomaly Dec 2020 - Feb 2021

NMME forecast of precipitation anomaly Dec 2020 - Feb 2021

It is worth noting though that the climate models can often capture the details of La Nina well, but often miss other less predictable factors that vary on the time scale of weeks. 

So what flavor of winter will this La Nina bring? La Nina winters historically bring above normal snowfall, so be prepared for an active winter.