Thursday, November 10, 2016

Changes ahead, and La Nina advisory

This has been an unusual fall so far.  First the record wet October, then the long stretch of warm and sunny days over most of the Inland Northwest so far this November.   How much longer will the warm weather continue?  And what is new with La Nina?  Will take a look at these topics in this blog.

November warmth:

November has been very warm, with most places across the Inland NW running 7-11 degrees above normal through the 9th.  The warmth has been most anomalous on the Palouse where Pullman's average temperature is running 11 degrees above normal through the 9th.  Even more impressive warmth over the Central US.  Here is a map showing the anomalies for the first nine days of November.

Pattern change:
So how much longer will this continue?  The answer is not much longer.  An unsettled weather pattern is shaping up beginning early next week with occasional rounds of rain expected.  The jet stream will shift south of the region around Wednesday, Nov 16th  as an upper trough settles over the region.   See image below...

18z/10th GFS forecast for 500mb height and 250mb wind 00z Thursday (4 PM PST Wednesday, Nov 16th)

This will result in cooler temperatures, with highs for most towns only in the 40s!  While this is much cooler, this is more typical for mid-November.

La Nina:
Now, time to talk about La Nina.  The Climate Prediction Center (CPC) issued a La Nina Advisory today (Nov 10th) with La Nina conditions favored for this winter.   Details of the advisory can be found here.

Since our October blog (found here) focused on neutral years and confidence has now increased for a La Nina, we will take a closer look at La Nina.  Here is a map showing a pattern that often sets up during La Nina winters.

Typically high pressure sets up in the Eastern Pacific resulting in the jet stream taking aim at the Pacific Northwest from the northwest bringing periods of cool and wet conditions.  However, this jet stream position fluctuates frequently, so a cooler and wetter than normal winter is not always the case.

This is expected to be a weak La Nina, so we went back and looked at what happened during past weak La Nina episodes.

Did any common themes show up?  Let's begin with temperatures

Weak La Nina years winters tend to have near or below normal temperatures in most cases for Washington and north Idaho.

What about precipitation?

It doesn't get much worse then this!  As you can see, a lot of variability with some cases dry, others wet, while others have near normal precipitation.

What about snowfall?  Here are graphs for Spokane, Wenatchee, Republic, and Bonners Ferry for weak La Nina cases.

For Spokane and Wenatchee, lots of variability!  Looking at Spokane data, most people probably remember the recording breaking snowfall in the 2008-2009 year where 97.7 inches fell.  However the weak La Nina prior to this (2005-2006) only 27.3 inches fell.   For Republic and Bonners Ferry, most weak La Nina events bring near to above normal snowfall.

Let's look at the past two cases in more detail.  The graphs below show observed temperatures at Spokane for the past two weak La Nina episodes.

The blue lines represent observed data, while the brown area represents normal temperatures.  As you can see the 2005-2006 winter started off cold.  Snow fell in late November and early December with cold and dry conditions following through the 20th.  Then mild conditions dominated the end of December through early February.

In contrast for 2008-2009 cold temperatures settled in during the middle of December followed by continued cold but fluctuating temperatures as numerous storms brought snow to the region. Snowfall at the Spokane airport totaled 74.5 inches over a three week period from Dec 17th to January 6th!  This is what downtown Spokane looked like.

Downtown Spokane streets January 3rd, 2009.
As you can see, every La Nina year is different.  Be prepared for winter weather.

So what does the latest CPC winter outlooks show?

CPC 3 month temperature outlook (Dec-Jan-Feb 2016-2017) issued Oct 20th

CPC 3 month precipitation outlook (Dec-Jan-Feb 2016-2017) issued Oct 20th

For temperature, the outlook calls for Equal Chances (no clear indication how things will turn out).  For precipitation, slightly elevated odds for wetter than normal conditions.  These forecasts overall jive with past analog years for weak La Nina, with no strong indicators of how this weak La Nina will turn out.

1 comment:

  1. Hi Rex, I'm going to use data from this post, attributed to you, in my Spokesman-Review ski column, published in the Outdoor section on Thursday, Nov. 17. Thanks for the great data!

    Bill Jennings