The El Nino/Southern Oscillation (ENSO) has been studied extensively for its impacts upon climate.  The shorthand definition of El Nino is an abnormal warming of the normally cold ocean waters of the Humboldt Current off the coast of Peru.  A more formal measure used by many meteorologists is The Multivariate ENSO Index , which tracks both sea and air temperatures along with wind, pressure and cloud cover across the tropical Pacific.  The data is combined and normalized to a monthly value where +1 = one standard deviation above normal (El Nino condition) and -1 =  one standard deviation below normal (La Nina condition).  I'll refer from now on to the MEI index, which is graphed below since 1950:

As most of us have observed, skilled weather forecasting has become very accurate in the short term, very useful for storm-chasing 3-5 days out, but more speculative beyond a week.  Beyond 2 weeks I find it more useful to rely on historical records.  El Nino/La Nina is the exception to this rule of thumb because the condition tends to persist for several months at a time.  I have examined the monthly MEI table, which is used in the graph above and is updated with about a 1.5 month lag.  With over 700 monthly values since 1950, each month's MEI index can be correlated with the index 1, 2, 3, etc. months in the future to demonstrate how likely an El Nino or La Nina condition might persist.  Results are shown here:

When I first wrote this article on 11/21/07 the last posted value of MEI was -1.117 for SEP/OCT 2007, and I expressed strong confidence that the observed La Nina condition would be in effect through the end of 2007, and some confidence that it would persist for the entire 2007-08 ski season.  Notice that there is a very mild tendency for El Nino/La Nina to reverse itself 15-30 months in the future (think of a bathtub sloshing one side to the other), and only after 3 years do all effects of prior conditions disappear. It turned out that La Nina remained strong (2007-08 was 4th highest La Nina in the past 42 years) through FEB/MAR 2008, then dissipated over the next 3 months to a neutral value of +0.050 as of MAY/JUN 2008.

While we have established that El Nino/La Nina are persistent weather events, their effects upon ski area snowfall are less clear-cut.  I have correlated the monthly MEI table with all of the monthly snowfall data I have collected through 2007, and the list of ski areas with statistically significant snowfall sensitivity to El Nino/La Nina is much shorter than most people think.

The monthly correlations are not large enough to have much predictive value. But by combining 6 consecutive months together to form seasonal data, the correlations for some areas get into the 50% range. This fits with observed experience that in big El Nino or La Nina years the expected effects occur from time to time but not consistently. So I considered the seasonal correlations to be the main criteria in classifying areas. Not all areas provide complete November to April data, and I like to have 20+ seasons to draw conclusions.

For areas without enough complete seasons I looked at the monthly correlations, but also at the seasonal ones for nearby areas with many complete seasons. For example Sugar Bowl is likely to be affected similarly to nearby Donner Summit and Alpine Meadows.  It is important to realize that season correlations based upon 15-40 data points have much more uncertainty than the El Nino/La Nina persistence correlations based upon over 700 data points.

The list of ski areas favored by El Nino, along with their monthly and season correlations to the MEI index, is shown below:

El Nino strongly favors only Southern California, Arizona and far southern Utah, with milder effects extending to the southern Sierra and New Mexico.  In El Nino years the only big destination resorts that are favored are Mammoth and Taos, with both of those in the mild category.  In La Nina years Mammoth is likely still good for mid and late season, but earlier bookings should wait until snow is on the ground. Taos takes until nearly February to get fully covered in normal years, and skiers should be more wary during La Nina years.  The data I acquired for Las Lenas in 2005 and Portillo in 2007 support the prevailing view that the high Andes are strongly favored by El Nino. Advance bookings to South American ski areas should be avoided in La Nina years until snow is on the ground.

I have constructed graphs to illustrate the variability of the snowfall correlations to El Nino/La Nina.  The one below is for selected areas favorable to El Nino.  Since the 2 strongest La Nina years were 1973-74 and 1970-71, I select areas with data that goes back that far.

The horizontal axis lists the past 41 ski seasons in order of strong El Nino at left to strong La Nina at right.  The vertical axis is percent deviation from normal snowfall.  The blue line is the sum of MEI indicies from OCT/NOV to APR/MAY, scaled to fit the graph.

The purple line shows the dramatic boost to Southern California snowfall from El Nino, with the 2 biggest snow years correponding to the 2 big El Nino of 1982-83 and 1997-98.  5 of the top 6 El Ninos produced at least 170% of normal snow.  There are no guarantees even here, as the other season 1986-87 was a real stinker at only 59%.  The top 8 La Nina seasons were all below average in Southern California, though only one was lower than 77%.

Moving to Taos (yellow line) the effect is less dramatic.  The 2 big El Ninos were 116% and 118% of average, while Taos' record 1972-73 season at 174% was in the 5th highest El Nino year.  But only 1991-92 of the top 7 EL Nino years was below average at Taos, and that one was still 94% of average.  For the top 8 La Nina years, Taos is missing data for 1970-71, 1974-75 was a good year at 138% and 2007-08 above average. 1999-2000 was bad at 58% and the other 4 were below average in the 85% range.

In the Sierra the picture is mixed.  Everyone remembers the record Sierra snow during the record El Nino of 1982-83. But the 3rd and 4th strongest El Ninos (1991-92 and 1986-87) were severe drought years at Tahoe, and 1986-87 was Mammoth's second worst season ever at 42%.  Nonetheless 4 of the top 8 El Nino years (1982-83, 1992-93, 1994-95 and 1997-98) were at least 145% at Mammoth (orange line) and those same years were at least 125% at Donner Summit (light blue line) and Lake Tahoe.

At Mammoth the top 6 La Nina seasons were all below average, 3 of them by 30% or more.  At Donner Summit it's a different story, as the top 2 La Ninas and 5 of the top 8 are above average.  Lake Tahoe La Nina seasons are probably assisted by colder temperatures minimizing low elevation rain.  This relatively good La Nina track record is the reason that the MEI correlations are low and statistically insignificant for areas like Kirkwood, Squaw Valley and Alpine Meadows.

Ski Areas Favored by La Nina

Ski Areas Indifferent to El Nino/La Nina