In La Nina Winters, The Tri-State Area
Often Receives Below Average Snowfall

Updated November 19, 2010


In October 2010, the Climate Prediction Center issued it's official temperature and precipitation outlook for the winter of 2010-2011.  You can view the complete article here.

The outlook is based heavily on the fact that La Nina conditions have been observed in the equatorial Pacific since mid-summer.  This web article will briefly explain La Nina, and will explore what has occurred here in the Tri-State area during previous La Nina winters.

 

The image above shows the temperature outlook for the United States for the period December 2010 through February 2011.  It indicates that warmer than normal temperatures are expected across much of the southern and central U.S., with the best chances of this occurring across New Mexico, Texas, Oklahoma into Louisiana.  The 50% area shown over Texas does not indicate that temperatures will be 50% above normal, it indicates there is a 50% probability of warmer than normal temperatures occurring during the 3-month period.  In contrast, climatology would indicate a 33% chance of any given area receiving below normal temperatures, a 33% chance of of near normal temperatures, and a 33% chance of above normal temperatures.

In northwest Kansas, southwest Nebraska and east-central Colorado (the "Tri-State" area), the outlook shows around a 40% proabability of above normal temperatures this winter.  If the weather pattern this winter is similar to that occurring during previous La Nina winters, our area should have a slight tendency to be warmer than normal from December to February. 

 

The image above shows the precipitation outlook for the United States for the period December 2010 through February 2011. The strongest signals lie across the southern tier of states, and across the northwest U.S. and over the Ohio Valley.  This means that during previous winters where La Nina conditions were observed, these areas often had the most noteworthy departures from normal conditions, either wet or dry as indicated above.

In the Tri-State area, the outlook indicates "Equal Chances".  This does not mean near normal precipitation is forecast for the winter in our area, it means that, based on previous La Nina winters, there is no strong signal toward either wetter or drier than normal conditions.  Note that just to the southwest of our location, there is a tendency toward a drier than normal winter.

So what exactly is a La Nina?

Ships and data buoys routinely collect the temperature of the sea surface, as well as the temperature at various depths in the water.  There is a recurring pattern of temperature swings in the ocean from colder than normal to warmer than normal that typically repeats every 3-5 years.  When the sea surface temperatures are warmer than normal (left in the image), it is called an El Nino or warm episode, and when water temperatures are colder than normal (right in the image), the term La Nina, or cold episode is used.

The image shows that sea surface temperatures will deviate from normal by as much as 3-4 degrees Celcius (5-7 degrees Fahrenheit).  If that doesn’t sound like much, realize that this is occurring across thousands of miles, and is lasting for a period of time from 6 months to several years.

What are the current sea surface temperatures in the equatorial Pacific Ocean?  You can find the latest information here. The image below shows the latest available at the time this article was written.

The top image shows the observed Sea Surface Temperature in degrees Celcius.  This is great information, but most of us have no idea if the temperatures shown are warm or cold compared to what normally occurs.  The bottom image is more useful.  It shows the departure from normal water temperature, where blue shading indicates that water temperature is colder than normal and yellow/orange shading indicates warmer than normal water temperature. 

Variations in sea surface temperature occurring on a scale this large will influence the location, persistence and intensity of thunderstorms over the Pacific Ocean, which in turn has big implications not only for ocean currents, but for atmospheric wind patterns as well.

In a typical La Nina pattern, the downstream effects on North America during the winter months can be summarized by the graphic below.  The Pacific Northwest is often colder and wetter than normal, while the southern US is often drier and warmer than normal, and the Ohio and Tennessee Valleys are typically wetter than normal.

It is important to mention that other factors will influence our weather this winter.  The weather pattern which predominates this winter does not hinge solely on ocean temperatures in the equatorial Pacific.  However, this is a strong underlying signal.

Given that a moderate to strong La Nina is now underway, it is informative to look back at previous La Nina winters and take note of what occurred in those years in terms of temperature and precipitation in our local area.  Since 1950 there have been 18 La Nina years.  The charts below show the La Nina years along the horizontal axis, and the Snowfall Departure From Normal (in inches) along the left axis.  

The chart below for Goodland, Kansas shows, for example, that during the La Nina in 1999 (third bar from the right), snowfall in Goodland was 17 inches below the normal of 37 inches (47% below normal).  During the La Nina of 1973, snowfall was 10 inches above normal.  With the exception of just a couple years, the overwhelming result is below normal snowfall in Goodland, since most of the bars appear below the zero line.  The chart below also illustrates that even though a La Nina may be occurring, other atmospheric factors can overwhelm the Pacific ocean temperature influence on our weather pattern (note the year 1983).  During all 18 La Nina winters studied, snowfall in Goodland averaged 10% below normal.

To help determine if below normal snowfall occurring at Goodland during previous La Nina years was just a fluke, additional reporting sites around the region were researched to see what trends would be revealed.  The chart below is for Scott City, Kansas, which lies about 75 miles to the southeast of Goodland.

Once again there are exceptions where snowfall was above normal during La Nina winters (notably 1973 and 1998), but in most years, snowfall at Scott City was below normal.  The average of all years is 15% below normal.

The snowfall record at Lamar, Colorado, 100 miles southwest of Goodland, showed similar results, with snowfall averaging below normal during La Nina winters. Again, there are exceptions during the La Nina's of 1983 and 2000, but overall, snowfall averaged 16% below normal.

  

Looking further north into Nebraska, the trends at Imperial are shown below.  In La Nina years, winter snowfall has been averaging 13% below normal.

 While it is interesting that snowfall shows a consistent below normal tendency during La Nina winters, this trend does not appear in all locations.  As shown above, this dry trend appears at some stations around Goodland, but not at others.  Some locations in fact show a neutral to slightly above normal snowfall trend.  Consider the snowfall record at Culbertson, Nebraska below.  Even a quick glance at the chart indicates that in some La Nina winters, snowfall is below normal, while in others it is above normal.  Overall, the average snowfall during La Nina winters at Culbertson is just about normal, meaning that ocean temperature swings occurring in the tropical Pacific appear to have less influence in this area.

  

Results for Hays, Kansas look similar to Culbertson.  During La Nina winters, snowfall is above normal about at often as it is below normal as shown below. 

Here are some results for Flagler, Colorado.  While the majority of the La Nina years have below normal snowfall, the amounts are overwhelmed by the few years where snowfall is far above normal.  Overall, snowfall at Flagler is about 2% above normal when a La Nina is occurring.  Note that snowfall for one La Nina year (1974) was incomplete, so only 17 years appear in the chart below.

When plotted on the map below (click to enlarge), the snowfall tendencies at the various stations can be more easily visualized.  There is a corridor of stations from the Texas Panhandle, north through western Kansas and into Nebraska where snowfall trends below normal during La Nina winters.  While the exact cause of the pattern is not certain, speculation is that storm systems coming east out of the Rockies into the High Plains during La Nina years may not initially have enough moisture from the Gulf of Mexico, but as the systems travel into central Kansas moisture may be more readily available.

  

The trends for winter temperature are not nearly as pronounced as snowfall, at least not for Goodland.  The chart below shows how temperatures compared to normal during La Nina winters.  The left axis gives the scale in degrees Fahrenheit.  Overall, the average temperature from October through April for La Nina winters is skewed slightly to the warm side, with the departure averaging +0.2%.

 

When researching the temperature records for Goodland, we were also curious to determine if there was any trend toward having an unusually early or late last spring freeze.  In other words, in the spring following a La Nina winter (given below normal snowfall), could the planting season get underway earlier?

The chart above compares the date of the last 32 degree temperature in spring with the normal date of May 5th.  In this chart, if the bar is below the zero line, it means the last spring freeze that year occurred before May 5th, and if above the zero line, that year saw a later last freeze.  For example, in the year 2000, the last freeze occurred 8 days later than the normal May 5th, or on May 13th.  In 1974, the last freeze occurred 13 days before May 5th, or on April April 23rd.  Since the chart shows an almost equal number of years before or after the average freeze date, there does not appear to be much of a signal in terms of when the planting season will begin.

   

 


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