When I try to analyze El Nino/La Nina I use this info: http://www.esrl.noaa.gov/psd/people/kla ... table.html
It was recommended to me by Seattle meteorologist (and powder junkie) Larry Schick.
With regard to impact upon snowfall there are 2 ways I've tried to look at it. First is correlating the monthly MEI index to monthly snowfall. The results are in 3 articles I wrote in the fall of 2007, headed into a strong La Nina season:http://webpages.charter.net/tcrocker818/El_Nino.htmhttp://webpages.charter.net/tcrocker818/La_Nina.htmhttp://webpages.charter.net/tcrocker818 ... _areas.htm
The methodology of those articles has the advantage of utilizing as much data as possible but does not address the issue that the El Nino/La Nina might need to be of at least moderate strength to have any effect at all. So I've recently looked at the months through 2008-09 where MEI is > 0.750 for El Nino and < -0.750 for La Nina. For Mammoth there are 42 El Nino months which average 113% of normal snowfall and 31 La Nina months which average 90% of normal snowfall. For Southern California there are 37 El Nino months which average 125% of normal snowfall and 20 La Nina months which average 82% of normal snowfall.
There is a legitimate question if this is enough data to draw firm conclusions. I'm guessing that many of you would be surprised that for Kirkwood there are 37 El Nino months which average 103% of normal snowfall and 22 La Nina months which average 107% of normal snowfall. Actually, the Tahoe areas don't correlate well in my articles because they have just as many good La Nina seasons as bad ones. However, most of the Tahoe areas have higher percentages than Kirkwood for the El Nino months.
The problem with the "look at El Nino/La Nina seasons" method is that there have been only 4 La Nina seasons since 1977, and not many areas have snow data before then. For those places there are only 16 La Nina months (I'm just looking at December-March for the most credible snow data), and it doesn't take much in the way of unusual weather to move those stats. The classic example is southern and western Colorado, which by popular opinion is considered favored by El Nino. Several of those places (Aspen, Crested Butte, Telluride) set their all time snowfall records in the strong La Nina year of 2007-08. Since that's 1/4 of the La Nina data naturally those areas now favor La Nina by the "look at El Nino/La Nina seasons" method. By the correlation method those areas, like Tahoe, are close to neutral, and I believe that's the best assumption to make for them.
The question of "what kind of season follows an El Nino" is subject to the same question of sufficient data. The records go back to the 1950's instead of the late 1970's, but now we've subdividing the data between "normal El Ninos" and "Modoki El Ninos." Another hypothesis would be that the stronger the El Nino, the more likely a swing to a strong La Nina. That works well for 1972-73 and 1997-98 but not for 1982-83 or 1991-92. FYI 2009-10 was a fairly strong El Nino, 7th highest of the 60 years of MEI data and just short of 1972-73.