Pattern Behind the December Warmth

Posted: January 6, 2016, 11:46 am by bmarmo

To say that December 2015 broke a few records across the Midwest and the East Coast would not be doing it justice. In fact, for some places it wasn’t even close. While a mild month was expected, nobody could have anticipated the magnitude and consistency of the warm temperatures we experienced. So let’s take a closer look at why December was an historic one for the Northeast, Mid-Atlantic and Midwest.

Believe or not, one of the principal reasons why December was so warm traces its origins back to the end of the Winter of 2015. This of course is El Niño, which blossomed during the Spring and Summer into one of the most potent events on record. In the end, it doesn’t matter if this El Niño goes down as strongest ever, as it has made quite an impression. With temperatures finishing 10 to 15 degrees above normal, December shattered records. For example, Philadelphia topped it previous warmest December record by nearly 7 degrees and many locations from Ohio to Illinois ranked December 2015 in the top 5 warmest. For more stats on December 2015, see our blog.

Overall, the dominant pattern during December was reminiscent of the classic strong El Niño signal. Contrary to the set-up we were accustomed to the last couple of winters, the overarching pattern this December featured an upper level trough of low pressure positioned over the Gulf of Alaska and the Pacific Northwest. In turn, a ridge of high pressure extended from the Northern Plains all the way to the Eastern Seaboard. This allowed mild (and sometimes even warm) air to consistently infiltrate our region.

While El Niño’s influence can’t be questioned, it certainly wasn’t the only culprit. In addition, a strong “polar vortex” and a positive Arctic Oscillation played a significant role. In other words, winds in the upper reaches of the atmosphere around the North Pole were abnormally fast. As a result, the frigid arctic air remained bottled up in northern parts of the hemisphere. Thus, any “cold” shots in December were not actually that cold. Not surprisingly, the dearth of cold air caused many to struggle in the snowfall department. The only real event of note was a storm on December 28th and 29th. Most locations in the Northeast only saw an inch or two of wintry mix, though some spots in Massachusetts and New Hampshire did measure between 4 and 6 inches of snow and sleet. Chicagoland was also affected by this storm, with 2 – 5 inches of mostly sleet!

While December was all about the warmth, it has been a different story so far in January. A quick pattern change has resulted in a shift of the upper level ridge from our region into Western Canada, allowing colder and more seasonable air to return to the Eastern US. And with more frequent cold shots expected, we anticipate an increase in snow threats as we move through the rest of this month and into February.

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