How unusual are Warm La Nina Winters?

                       Issued Noon CST Saturday Jan 7th 2011

     We are currently in the midst of a moderate La Nina Winter, with Nino 3.4 values around -1.0.   Looking back since 1950, there have been 14 La Nina winters with moderate values (colder than normal sea surface temperatures along the equatorial Pacific Ocean). 

1950-51, 1954-55, 1964-65, 1970-71, 1971-72, 1973-74, 1975-76, 1984-85, 1988-89, 1998-99, 1999-2000, 2007-08, and 2010-11.    This winter would be the 15th such moderate or stronger La Nina winter. 

If you average all of these La Nina winters at Milwaukee and Madison, the 3 month period from Dec 1 to Feb 28 is about 1.5 F below normal.  However, taking a closer look at the individual winters,  3 of the 14 at Milwaukee (21%) and 4 of the 14 at Madison (29%) were actually warmer than normal.

Breaking it down further, 6 of the 14 Decembers at Milwaukee (43%) and 7 of the 14 Decembers at Madison (50%) were warmer than normal.   So the warm December during a La Nina winter is actually rather common. 

Comparing December 2011 to the 14 previous moderate La Nina Decembers, the 33.3 F average temperature at Milwaukee was warmer than the rest, beating out December of 1998 which was 32.0 F.  In Madison, December 2011 was 30.8 F, finishing 2nd behind December 1998 when it was 31.3 F.

Looking ahead to the rest of the winter,  29% or 4 of the 14 moderate La Nina winter January's were warmer than normal for both Milwaukee and Madison.  That number drops to 21% or 3 of the 14 winters in February for both locations. 

So there is about a coin toss odds of getting a warmer December than normal during moderate La Nina events in southern Wisconsin, but the odds of being colder than normal increase in January and February.  Given how warm we have been so far in December and January, it is very likely that we will finish warmer than normal for the whole 3 month winter period.  Since that has happened 21% of the time in Milwaukee and 29% of the time in Madison in the past during patterns like this we don't consider this unusual.   Given the past two moderate La Nina Winters have been colder than normal (2007-08 and 2010-11), odds certainly favored us having a warm winter this time around and so far that has been the case. 

     Jeff Craven,  Science and Operations Officer

  National Weather Service Milwaukee/Sullivan WI

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