A Strong El Nino Develops: What Does This Mean for the Pacific Northwest?

The forecasts are now emphatic:  a strong El Nino is now developing and will be in place this winter.

What does this tell us about the character of our next winter here in the Pacific Northwest?  More or less snow?  More or less storms?  Drought or wet?  Warmer than normal or not?  Let me tell you what we know.

As I have noted in previous blogs, El Ninos are associated with warmer than normal water in the central and eastern tropical Pacific.   Think of the Pacific Basin as a big bathtub with warm water near the surface.  The water is sloshing back and forth; when the warm bath water sloshes towards South America, you get an El Nino. Sloshes the other way, La Nina.

Babies and ducks have intuitive understanding of El Nino

A plot of the sea surface temperatures in the Nino 3.4 and Nino 1+2 areas in the central and eastern Pacific, respectively, shows the story.  Nino 3.4, in the central Pacific, is progressively warming-- now about 1.2C above normal. An El Nino is defined when the Nino 3.4 temps are more than .5C above normal for roughly a half year.   But look at Nino 1+2!  The eastern Pacific has warmed hugely, after being below normal late last winter.  THAT is a major sign that the situation is very different from last year.

The U.S. (NOAA) has put in a network of buoys over the tropical Pacific (the TAO array) that tells us how warm the water is below the surface.  These buoys show a huge increase in the heat content of the upper ocean in the central and eastern Pacific, as shown by the plot (below) of the  upper ocean heat anomaly (difference from normal).   The Pacific bathtub has really sloshed warm from the western to eastern Pacific!

NOAA and others have complex ocean/atmosphere models that can simulate the evolving El Nino. They are NOT perfect (as shown by their poor forecast last spring of a major El Nino that never materialized).    But we are farther along in time  and the uniformity of the predictions provides some confidence that their forecasts will be more skillful this year.

Here is the prediction from the NOAA Climate Forecast System (CFSv2) model for sea surface temperatures in the central Pacific (Nino 3.4 area) based on an ensemble of different CFS forecasts. All the forecasts are for warmer than normal conditions.  NONE are going for cooler or normal.

Based on this and other forecasting systems, the U.S. Climate Prediction Center (CPC) produced a consensus forecast for the upcoming year (see below).  90% probability for an El Nino.   They are going for it.

Strong El Ninos have a sea surface temperature anomaly (difference from normal) in the central Pacific of at least 1.5C.     There appears to be an excellent chance we will achieve that.  Keep in mind that we haven't seen a strong El Nino since 1997-1998 and that the global effects of that event were profound.  For example, California had heavy precipitation with flooding and landslides that winter.   In any case, we are sure to have at least a moderate El Nino.

So with a moderate to strong El Nino expected to be place for this coming winter, what can we expect here in the Northwest?   To put it briefly:

1.   bModestly elow normal precipitation
2.   Warmer than normal temperatures
3.   Less chance of lowland snow
4.   Below-normal snowpack in the mountains
5.   Less storminess, with reduced probabilities of major windstorms and floods.

But there are many subtleties here, including differing local impacts between moderate and strong El Ninos.  This an important point missed in many media accounts.    Let me show you the changes in impact as  the strength of an El Nino increases.  First, for precipitation,or more exactly, the precipitation anomalies (differences from normal).

Considering all El Ninos that meet the minimal requirements (.5C anomaly), western Oregon and Washington are dry---particularly the western slopes of the Cascades.   California is wetter than normal.

Strong El Ninos are similar, with greater drying for the Northwest.

 But then there are the super El Ninos, like 1997-1998 (see below) and they are different animals.  California is VERY wet and above-normal precipitation extends into the Northwest, with the sole exception of the western slopes of the Cascades.   If this happens, you can expect William Shatner to propose a big pipe to send water BACK to the Northwest.

What about temperature?   This is a simpler story.  Here are the temperature anomalies from climatology (normal) fro all El Ninos:  modestly warmer than normal from eastern Washington to Minnesota.

Moderate El Ninos?   The signal fades a bit.

Strong El Ninos enhance the warming a bit in the northern tier of states, but cooler than normal temperatures extend from California eastward.

Super strong El Ninos? Very warm from the Northwest eastward, with the warmest anomalies in the Upper Plains and Midwest.  A fine winter to visit the sites in North Dakota or northern Minnesota.

Big windstorms?  The plot show the timing of  the really big windstorms (red dots) versus tropical sea surface temperatures.  Big windstorms AVOID strong El Nino years.   Similar to vampires and garlic.  But there can be moderate storms in El Nino year and it appears that the very strongest years  (like 97-98) had plenty of coastal storms.

Clearly, warmer than normal temperatures accompanying the strongest El Nino's is bad for snow over our region.   Snowpack is generally reduced in the Cascades, but not "end of the world" reduced.  Amar Andalkar did a nice study of this issue, finding a 10 % decline in Cascade snowpack for weak El Nino years and 15-20% for strong El Nino years.   This is FAR better than what we had last winter (75% reduction).   So the Cascade snowpack may be below normal but will undoubtedly be much better than last year!  

Strong El Nino years also tend to have less major arctic outbreaks extending over western Washington and Oregon--good news for those worried about cold sensitive plants.

The bottom line is that El Nino, and particularly a strong El Nino, heavily weights the atmospheric dice for a less stormy, warmer, and a bit drier Pacific Northwest.  But nothing is certain and the impacts will depend on whether the El Nino ends up moderate or super strong.  

The real importance is not about the Northwest.   Rather, will this El Nino bring enough precipitation to California to pull them out of their historic drought?  They desperately need the water.  And El Nino years normally bring far less Atlantic hurricane activity, which is a benefit to the coastal areas of the Gulf and the Atlantic coast.  And strong El Ninos produce a net GLOBAL warming, so expect the upcoming year to break many heat global heat records.

The Implications of El Nino for Northwest Autumn and Winter

There has been a lot of speculative talk about the implications of El Nino for our winter, with some claiming a certainty of drought and other afflictions.

So let's try to determine reality from hype.

Let me begin by noting that the latest forecasts are for at least a moderate El Nino this winter: GUARANTEED!

Looking at many El Nino model predictions, all have temperatures at least 1°C above normal in the tropical Pacific.  Some more than 2°C above normal.  (see forecasts below for the sea surface temperature anomaly from normal for the Nino 3.4 area).  Folks...this is about as certain as these things get.

With a certain El Nino in store, let's find the implications of El Ninos on our weather (and the rest of the U.S.), by averaging (or compositing) the NOAA Climate Division Data for major El Ninos of the past 40 years.  

An important insight is that the connection between El Nino and our weather depends on the season.

For autumn the effects are more subdued.   Here are the differences from normal of autumn (Oct-ber-December) temperatures during El Nino years.   Modestly warmer than normal (by 0.5 to 1°F) over western Oregon and Washington, and about half that over eastern Washington. Colder than normal over much of the rest of the country

In contrast, there is very little impact on precipitation in the Northwest.  Perhaps slightly drier than normal over the western slopes of the Cascades.  So there is NO reason to expect any kind of drought over the Northwest during the fall rain season.... our big water collection period.  California and the southeast typically get a bit more than normal during El Nino falls.

To make doubly sure you aren't too worried about autumn drought from El Nino, here is the average Palmer Drought Severity Index (PDSI) for El Nino autumns.   NO signal over the Northwest.

What about after January 1?   The El Nino signal generally strengthens then.

Here is the temperature difference (anomaly) from normal for January to March.   A very clear warm signal over the Northwest, which generally decreases snowfall.

The January-March precipitation signal is weak east of the Cascade crest and is pretty localized to the western slopes of the Cascades, which gets less precipitation (down around 3 inches)--which is relatively small compared to the 60-100 inches often observed on the slopes.

So what is the bottom line for expected snow impacts of El Nino for the entire winter? 

Below is a NOAA analysis of the difference from average snowfall for the 10 strongest El Nino winter and all El Ninos.    The Northwest tends to get less snow by as much as 10 inches over the mountains.  Considering that much of our high terrain gets hundreds of inches a year, the El Nino impact is modest.

So the bottom line is that the El Nino influence should be modest, with most of the impacts after January.    And keep in mind that not all El Nino years follow the above patterns.   El Nino is only one factor influencing atmospheric evolution over the planet.

La Nina is Dead. Can El Nino Be Far Behind?

I have sad news for all of you.

La Nina is dead.   Done. Finished. Terminated.   The final word came out a week ago, provided by the august U.S. Climate Prediction Center (see below).

And it appear that we have a better than even chance of moving back into the El Nino pattern again, a configuration that leads to generally less snow over the Pacific Northwest.

To track the oscillation between La Nina, Neutral conditions, to El Nino and back again, meteorologists follow the sea surface temperatures (SST) in tropical Pacific.  As I have noted in past blogs, this variation, also called El Nino Southern Oscillation (ENSO), represents a sloshing of the warm water in tropical Pacific.  When the water sloshes towards the east we have El Nino, when it sloshes to the west, La Nina.  You can think of the tropical Pacific as a giant bathtub.

To get a handle on what the sloshing is up to, meteorologists follow the SSTs for a few areas in the tropical Pacific (the Nino 4, Nino 3.4, Nino 3 and Nino 1+2 areas).  The figure below shows their locations.

The general approach is to track the SST anomalies (differences from normal) for these areas, with Nino 3.4 being probably the most popular. When Nino 3.4 has a warm anomaly greater than +.5C we have El Nino, an anomaly of -.5C or less, La Nina.   Near zero, a neutral or La Nada period.

Here are the latest graphs of SST anomalies.  Viewing the Nino 3.4 graph, you can see we had a weak La Nina for a while, but now temperatures are slightly on the warm side---we are in neutral conditions and probably have been for a few months.

What about the future?  The Climate Prediction Center provides a nice graphic of  past and predicted SST anomalies from many different ENSO forecasting systems (see below).   Forget La Nina.  Some of the model stay in neutral territory, but half move toward El Nino.

 The main U.S. seasonal model, CFSv2, is run as an ensemble (many forecasts).  Although there is a lot of uncertainty, most runs are heading towards El Nino.

Digesting all of the this information, the Climate Prediction Center is going for neutral conditions continuing into spring, but equal chances of moving toward El  Nino by midsummer, and a tendency towards El Nino by fall.

 Let me stress that ENSO forecasts made mid-winter have been notoriously bad.  Skill for the upcoming year increases substantially by summer.

One thing you can be sure about:  with our reservoirs in decent shape and a near normal snowpack, the water situation in the NW looks excellent for this summer. And with California soaked this winter and its reservoirs filled, vegetable/fruit prices should be more modest than in past years.

Wimpy El Nino

We are now close enough to the winter season to have a fairly clear idea of the El Nino situation ahead.  Remember that El Ninos are associated with warmer than normal water in the tropical Pacific and that such anomalies can influence Northwest weather (less storms, warmer, less snow).

Originally there was a lot of talk (last spring) of the potential for a Super El Nino, with some of the global warming "advocate" sites talking about its effects on the global temperature record (global temperatures can warm substantially with strong El Ninos).

However, the sea surface temperatures in the critical  central Pacific is only modestly warmer than normal and the atmospheric circulation has not reacted in a way to reinforce the warming and push us towards a moderate or stronger El Nino.

Let's start with one of the key measures:  the sea surface temperature in the Nino 3.4 area of the tropical Pacific.  The official definition of El Nino is an anomaly  there greater than .5C.  We are not there now and in fact the SST has declined recently.

A number of groups run statistical and full-physics models to simulate El Nino.  They suggest we will have an El Nino, but a weak one.  However, verifying these models for the forecasts started earlier in the year, suggest they have been pushing warming in an unrealistic way.

The National Weather Service Climate Forecast System (CFS) model is now going for an entirely marginal event, barely reaching the .5C criterion

And the official probabilities for El Nino are now only about 65%.

So we should not expect much more than a marginal El Nino during the upcoming fall and early winter months.  And amplitude matters.   Weak El Ninos have lesser impacts.

The correlation of our weather with El Nino is not perfect to start with.  And for weak El Nino years the relationship weakens further.

Let me illustrate this for you.

Here are the precipitation patterns across the U.S. for some STRONG El Nino years.  The most consistent implication is  the wetter southeast U.S.   There is a tendency for southern/central California to be wetter.  The Northwest is less consistent.

But for weak El Nino years, the precipitation patterns are all over the place.  I would not place bets on anything.  This is the story of our upcoming winter unless El Nino revs up unexpectedly.

So based on the correlation with El Nino, we have very little guidance for the upcoming winter. Sorry.

The latest NWS Climate Forecast System forecasts for December-January-February is for warmer than average over much of the U.S.  No hint of the feared "polar vortex."

What about precipitation?  Very little signal over the Northwest.  But wow...California is much wetter than normal.  This would really help the drought.  Fingers crossed.

And this week should be a shock to some.  Showers today.  Generally dry on Sunday and Monday day.  But they the celestial spigots will turn on.

Hopefully, the rain will be over by next Saturday.  Why?   Because one of my favorite events will occur Sunday morning at 9AM:  the UW Dawg Dash fun run.  For more information, check out their site: http://depts.washington.edu/alumni/blogs/dawgdash/

El Nino: A Weather Force Awakens

El Nino's impact on the West Coast will soon awaken.  And like the Dark Side of the Force, it has planted the seeds of its own demise and a new order will arise next winter.

This year one of the strongest El Nino's of the past 50 years has developed.  A number of studies have shown that the major impacts of strong El Nino are typically not felt over the West Coast until after approximately January 1.  These impacts include wetter than normal conditions over California, a bit drier than normal over the Northwest, reduced snowpack accumulation over the NW mountains, and less chance of lowland snow.

After January 1, El Ninos tend to produce an offshore trough of low pressure (heights) over the NE Pacific and relatively higher pressure (heights) over NE Canada.  The composite below 500 hPa heights (around 18,000 ft) shows the general pattern.  This flow pattern tends to cause the Pacific jet stream to split, with flow going into California (black arrow).

The latest forecast models suggest that exactly this kind of transition is about to occur, with a major circulation shift that will dry out the NW and increase precipitation over California.

Take a look at the flow pattern anomaly (difference from normal) for 500 hPa for 4 PM Sunday (yesterday).  Higher heights than normal over the NE US,  lower heights than normal over the West Coast and higher height over the NE Pacific.  Not very El Nino like.

But here are the forecasts for 5 and 10 days out.  Huge change with low pressure developing over the Pacific.  Shades of El Nino.  A huge ridge over us on Friday in the first map...absolutely dry

Take a look at the total precipitation predicted over the next 2 weeks by the National Weather Service GFS mode.  Substantial precipitation over southern  and northern CA (red colors), but much less  over Washington State and BC.  Very much an El Nino pattern..

The latest Weather Channel ten day forecast for Seattle shows NO PRECIPITATION for over a week.

 And the latest Climate Prediction Center 8-10 day forecast predicts the classic  strong El Nino pattern: dry over the northern tier of states (including the Northwest) and wet over California.

The El Nino force is strong with this one.

But the dynamics of a strong El Nino puts in motion changes that eventually destroys this feature.    The latest central Pacific tropical sea surface temperature index (Nino 3.4) shows that this powerful El Nino has peaked and slowly declining.

And seasonal climate models (like the NOAA/NWS CFSv) indicates a rapid weakening of El Nino during the spring, with substantial confidence.

 But El Nino will remain strong enough the second half of the winter for powerful effects.   Next year will probably be a neutral or La Nina year, but it is too early to tell.

The bottom line:  enjoy your skiing NOW.    You should not expect the second half of the winter to bring bountiful snow like the first.  Our reservoirs are in great shape with huge amounts of December water.  Water managers need to be conservative from now on, saving as much water as possible.  Our huge snow pack (compared to the wimpy one of last year) should ensure that next summer is no repeat of last summer.

_______________________
2016 Northwest Weather Workshop On March 4-5, 2016

The Northwest Weather Workshop, the big regional meeting on Northwest weather will take place in Seattle on March 4-5, 2016.   At this meeting we talk about the latest advances in understanding our local weather.  It is open to all.   For more information go here.  

If you want to give a presentation, abstracts are due on February 1.