Thursday, September 10, 2015

What will El Nino bring this winter?

After such a dismal snow year last year, many of you may be asking what kind of winter we will have this year.  Many may have already heard that a strong El Nino is present.  El Nino often brings warmer temperatures, so does this mean another bad year for snow?  In this blog we'll try and answer this question.

First of all, what patterns tend to set up during El Nino Winters?  Here is a graphic from the Climate Prediction Center...

Idealized weather pattern during an El Nino winter.  During El Nino winters, a strong and persistent Jet Stream over the Pacific is oriented over the southern tier of states.  Cold air intrusions from Canada into the northern states are infrequent.

Typically during El Nino winters, the jet streams splits with the cold northern branch over southeast Canada while the wet Pacific Jet Stream sends weather systems to our south across California.  This tends to give us warmer winters, while precipitation tends to be more variable with some years a bit wetter than normal while other years a bit drier.

What is the official forecast from the Climate Prediction Center calling for?  The graphics below issued August 20th shows highly increased odds for a warmer-than-normal winter, with slightly elevated chances for drier-than-normal conditions.

To evaluate the current state of El Nino, we need to look at the equatorial region out in the Central and Eastern Pacific.  Here is map showing the current SST (Sea Surface Temperature) anomalies over the past month through September 5th.

Sea Surface Temperature (SST) anomalies in the Pacific for 9 Aug - 5 Sep 2015.  Red shading indicates warmer-than-normal while blue shading shows lower-than-normal SST's.

The important area to key on is along the equatorial region in the Central and Eastern Pacific (middle part of graphic).  Values warmer than 1.5C are an indicator of Strong El Nino conditions.  Currently there are values greater than 2.0C, which supports a strong El Nino.

So how does this compare to past events?  Let's take a look.

Three-month average sea surface temperatures in the Nino 3.4 region of the equatorial Pacific for 2015 and the past 6 strong El Nino winters.
The purple line on this map is this year's current and projected trace.  While some uncertainty still remains regarding the peak of this event, it should end up as one of, if not the strongest El Nino, most similar to the strength of the 1997-1998 Strong El Nino.

So what has happened in past winters during Strong El Nino episodes?  There have been six cases since 1950.  First let's look at temperature composites for December, January, and February for past strong El Nino years.

As you can see, for the Inland NW, three of the six were much warmer than normal (1957-58, 1982-83 and 1991-1992) and two were near to slightly above normal (1965-66, 1997-98).  The 1972-73 Strong El Nino was actually cooler than normal.

What about precipitation?  Let's take a look...

In this graphic the yellow and orange shaded areas indicate drier than normal while the greens and blues indicate wetter than normal.  There's even more variability than with temperatures.  Two of the strong El Nino winters were wet (1957-58 and 1982-83), two were dry (1965-66 and 1991-92), and two were near normal (1972-73 and 1997-98).

So what do we gather from all of this?  The most important point is that every El Nino year is different.  Some of the differences are related to the timing of when the typical El Nino pattern develops.  During the 1972-73 event it wasn't until mid-February before an El Nino-like pattern kicked in.  Other years El Nino conditions start by January.  The main message here is that El Nino conditions are most favored during the second half of winter.  Thus, parts of November and December are at times more prone to fall and winter storms over the region.

Strong El Nino years also differ in terms of snowfall depending on where you live.  Below are images for snowfall for past episodes with the value on the far right the current 1981-2010 normals.

As you can see, for Spokane, every strong El Nino has brought at or below normal snowfall, with three of the six having less than 20 inches.  Meanwhile for Wenatchee amounts have varied greatly with each year different.  For Republic each strong El Nino year has brought close to normal snowfall, except for a snowy 1965-1966 year.  Bonners Ferry was also very snowy during the 1965-1966 El Nino, with at or below normal snow the other years.

The strong El Nino of 1991-92 saw an early season cold snap in late October. During the 1965-66 El Nino, most areas received near to above normal snowfall.  In 1972-73, a prolonged cold snap occurred in December with high temperatures several days in the teens and lows as cold as -11F in Spokane.  This was followed by another cold snap in January that lasted a week.  Take a look at the Spokane graphic below.

Spokane temperatures in the 1972-73 winter.  Blue bars show daily observed temperatures.  Brown shading shows the normal temperature range.  Red shading indicated above-normal temperatures.  Blue shading shows below normal temperatures.

The strong El Nino of 1997-1998 saw a brief cold snap with highs in the lower teens in January.

Meanwhile the El Nino of 1957-1958 saw no such cold snaps.  Here is the Spokane trace for that winter.

Spokane temperatures in the 1957-58 winter.

So while El Nino years tend to be warmer, most have at least brief periods of very cold or snowy conditions.

For this upcoming year are there any indications of what type of El Nino year we will have?  First, persistent high pressure has been evident in the Eastern Pacific Ocean off the Washington, Oregon, and British Columbia coast.  Note the large positive anomalies (yellow, orange, red colors) in the graphic below.

500mb Height anomalies for April 1 - August 31, 2015.  Red shading indicates higher-than-normal pressure while blue shading shows lower-than-normal pressure.

The longer this persists, the more likely we will prone to early season fall and winter storms as high pressure in the Eastern Pacific results in north-northwesterly flow over the area bringing cooler and potentially wet storms into the area.  However with the current strong El Nino the tendency for upper ridging may not last long.

To summarize, despite the current strong El Nino, be prepared for winter like conditions especially for the early part of the winter.  As winter continues on, El Nino like conditions becomes increasingly favored.  Thus after this winter is over, most will look back and probably think of this being a mild winter.

What does this mean for our mountain snow pack?  We'll address this in another blog in the near future.