For our area, the likelihood of a warmer-than-normal December is in the range of 70 percent, and the federal forecasters see only a 3 percent chance that December will come in colder-than-normal.
The outooks is based significantly on the El Niño warming in the tropical Pacific. As you no doubt have heard by now, a monster one is brewing in the tropical Pacific.
During the last three El Niños of comparable strength, Philadelphia temperatures in December – 1972, 1982, and 1997 –were significantly above long-term averages.
But El Niño or no, our years have tended to end on warm notes. For whatever reasons, warming in December has outpaced warming during the rest of the year around here.
In the 21st Century, the December average through 2014 was 38.7 degrees. That's 2.5 degrees above the 20th Century average for December. Annually, the difference has been 1.8 degrees.
Why warming would be concentrated in December is a mystery, but the monthly average has reached into the 40s in three of the last four years.
As for what it all means for the winter, winters with colder Decembers tended to be snowier, but several notable exceptions pop up on the list.