La Nina - are you coming back?

Will we have another La Nina winter?  Many of you remember last year's La Nina winter that brought cold and snowy weather with several bouts of ice.   The Climate Prediction Center issued a La Nina Watch on September 14th and the next update will be issued on October 12th.  The latest updates on La Nina can be found here.

So why does La Nina impact the Inland NW?  Colder water in the Pacific Ocean along the equator from the west coast of South America to the dateline suppresses thunderstorms while over Indonesia warmer water results in more thunderstorms.  The wind and convection patterns in these areas impact the jet stream in the mid latitudes.   How is the jet stream different?  See map below

Typically, a common feature of a La Nina is amplified high pressure in the eastern Pacific resulting in a colder northerly flow over western Canada into the northwest.  Meanwhile the pacific jet stream brings in storms from the west.  Colder air plus the moisture equals increased odds for above normal snowfall in the lower elevations.  The NMME (National Multi-Model Ensemble) that was issued in September agrees with this:

NMME Temperature anomaly forecast for Dec 2017-Feb 2018

NMME Precipitation anomaly forecast for Dec 2017-Feb 2018

So since the models are showing a wet winter and possibly colder than normal, can we bank on it?  Unfortunately climate models aren't good enough yet to confidently believe them months out.  But still we can look back at previous La Nina events to get an idea of what historically happens.  If we average snowfall for all La Nina years, this is what we get...

Average snowfall for La Nina (inches)

If you are interested in more snowfall maps related to La Nina and how these numbers compare to normal, check out our La Nina briefing page found here.

So should we expect 59" in Spokane, 29" in Wenatchee, and 22" in Lewiston this winter?  While these are the averages, it doesn't show the variability that occurs with each La Nina.  Let's examine 3 cases of La Nina since 2005 and see what happened

2016-2017 La Nina
Many remember last winter.  It was a cold and brutal winter, and snowy with several episodes of freezing rain for many.  It was the 6th coldest winter on record for Ephrata, WA.  Blowing snow on January 10th created very high snow drifts for portions of the Waterville Plateau into Grant County in the Columbia Basin leading to road closures.  See picture below.

Photo courtesy of Grant County Sheriff Tom Jones

Spokane recorded 14 days of freezing rain or freezing drizzle.  The classic La Nina pattern set up with our area influenced by both the Polar Jet Stream and the Pacific Jet Stream for much of the winter.  Although the Pacific Jet Stream was aimed a bit further south compared to normal across southern Oregon and northern California where the wettest conditions were observed.  Here is how last winter finished.

The winter was especially colder than normal across the Columbia Basin where snow stayed on the ground for most of the winter.  For Lewiston, 39.2" fell making it the snowiest winter recorded since 1968-69.  For Spokane 61.5" fell, close to the La Nina average.

2008-2009 La Nina

This was an unusual winter. A little colder than normal in the Columbia Basin with close to normal precipitation for most.  So why was it unusual?  Who remembers?  Well most of the winter wasn't that bad, except for a three week period which was historic for Spokane and nearby communities.  If you were in Spokane from December 17th through January 5th, I'm sure you didn't forget.  A total of 74" fell in 3 weeks with several roof collapses.  An impressive 2 feet fell in Spokane in about 30 hours on Dec 17th and 18th.   Here are a couple snow pictures during this three week period, including one from downtown.

Downtown Spokane - Photo courtesy of Kerry Jones

Photo courtesy of Spokesman Review

Here is how the final maps ended up.

Except for below normal temperatures in the valleys, these maps don't look that bad.  What is the screaming message you ask?  Even if the winter as a whole finishes close to normal, you can't rule out a brief period of extreme winter weather.  Be prepared!

2005-2006 La Nina
This La Nina was slightly warmer than normal (about 1 degree) but was very wet with precipitation 130-150% of average for most.  With the milder temperatures this meant less snow for most lower elevations.  Wenatchee only got 6.9" for the winter with Spokane at 27.3". 

As you can see, every La Nina is different.  There is a delicate balance between the polar jet and the pacific jet with each of these impacting our weather.  Precisely how these set up over this winter is unknown at this time.  But historically, La Nina brings increased odds for above normal snowfall especially in the valleys.

Since this may be the second La Nina winter following a strong El Nino winter, we went back and looked at five analog years since 1950 where this occurred.  What happened in these cases with the second La Nina?

As you can see, a tendency for cooler and wetter conditions, typical of La Nina.

Finally, we'll leave you with the latest winter outlook from the Climate Prediction Center

CPC winter outlook for December 2017-February 2018 issued September 21st, 2017

Equal chances for temperatures and slightly elevated odds for wetter than normal conditions.