Thursday, November 13, 2014

Winter 2014/15 Outlook

One of our most asked questions is "What is the winter going to be like?"  In this blog, we'll try to answer that question as best we can.  As most people know, looking this far into the future is more of an "Outlook" than a "Forecast".  

Most everyone has heard of El Nino and La Nina, especially when we're talking about the winter outlook.  We often refer to this as ENSO (El Nino Southern Oscillation)  There are other "oscillations" in the atmosphere that affect our weather.  However, ENSO is the only one that is linked to the ocean temperatures.  This is important for two reason.  First, we have some skill at forecasting ocean temperatures. Second, the ocean temperatures don't suddenly change.  These two facts allow us to predict the atmosphere several months into the future.  Most of the other oscillations are purely atmospheric.  As such, we can predict them only 1-2 weeks into the future with any skill.  What does all of this mean?  ENSO is just one of the influences on our winter weather, and the only one that we can predict with any skill.  The other oscillations are less predictable, and could alter or even override the effects of ENSO.  So let's first take a look at what is the state of ENSO.

The area of the Pacific Ocean that is monitored for El Nino is shown in the figure below.
ENSO Monitored Regions

The Nino 3.4 is the most critical area, although the other regions also play a role.  What is monitored is the Sea Surface Temperatures (SSTs) in this area.  And the main way to look at these is to observe how the temperature compares to what we "normally" observe in this area.  This comparison is referred to as an "anomaly".  For El Nino, the anomaly must be 0.5C warmer than normal for 5 months.  La Nina is defined as an anomaly of 0.5C cooler than normal.

Currently, the SST anomaly in the 3.4 region is very close to 0.5C.  It's actually been close to that value since early summer, but decline a bit in August before warming this autumn.

Sea Surface Temperature Anomalies in the 4 NINO regions

So the Nino3.4 region is already close to the 0.5C criteria for El Nino.  So what is the forecast for the SSTs?  Here's a figure that shows a number of numerical predictions from several different countries.

This graph shows that the majority of forecasts lie between the 0.5C and 1.0C lines through the winter and next spring.  This would equate to a "weak" El Nino.  There are a few that expect a somewhat stronger El Nino, as well as a few that are predicting no El Nino.  

Here's the prediction from a number of runs from the U.S. CFS model:

Again, the majority (but not all) of the forecasts are between 0.5 and 1.0C, with additional warming during the summer.  The European climate model has a similar forecast:

So the take-away from all of this is that the majority of computer forecast are expecting a weak El Nino to persist through the winter and into the Spring of 2015.  The latest NOAA Climate Prediction Center discussion states that there is a 58% chance of an El Nino.  

The affects of El Nino on North America weather are fairly well understood.  And these are reflected in the official winter outlook from NOAA:

For the Pacific Northwest, the expectation is for warmer and drier conditions than normal.  And this fits what we commonly see in our area during an El Nino winter.

Here's a bar chart showing the observed temperatures at Spokane for the winters since 1949/50.  The bars are colored in red (El Nino), blue (La Nina), and white (neutral).  The black horizontal line represents a "normal" winter.

As you can see, just about every red bar (El Nino winters) is above the black line, meaning that the El Nino winters are warmer than normal.  

A similar graph for precipitation is found below:

In this instance, you can see more variability with the red bars.  Some El Nino winters are wetter than normal, others are drier than normal.

The third graph shows the occurrence of snow.

This is a fairly clear signal.  We haven't seen an above-normal snowfall El Nino winter since 1977/78.  Most El Ninos bring below normal snowfall to the Inland Northwest.

So the outlook for this winter in the Inland Northwest is for above-normal temperatures, with below-normal snowfall.  But as was earlier stated, this is by no means a done deal.  The other atmospheric oscillations could still alter the weather beyond the affects of ENSO.  Additionally, this is an outlook for the entire winter.  So there could still be a frigid or snowy week or two.  But when the winter is all said and done, we'll probably look back at it as a milder winter.

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