2012 was a year of crazy weather in Washington, D.C. from the nearly snowless winter to the summer’s scorching heat and high energy derecho to Superstorm Sandy. But the year will have an asterisk next to it for achieving perhaps the most remarkable milestone: warmest on record.

The record will be broken by a landslide. With only a few days left in the year, D.C. should finish 1.3 degrees F warmer than the previous warmest year on record, 1991. D.C.’s weather records date back to 1871.

For the remaining 100 hours (or so) in 2012, Washington, D.C.’s temperature would have to average about 80 degrees below zero not to break the record.

2012’s exceptional warmth stands out most when you consider the previous 9 warmest years are all clustered within 0.9 degrees of one another, whereas 2012 is 1.3 degrees above 1991, the second warmest year.

Of the top 10 warmest years on record, 40 percent have occurred since 2002, and 60 percent occurred betweem 1973 and 1998. 2010, 2011, and 2012 make up three of the top 6 warmest years.

The fascinating thing about 2012’s warmth was its duration and consistency rather than so much its intensity (with the exception of March, which was intensely warm). Eleven of 12 months in 2012 finished warmer than normal and the disparity in the frequency of warm days compared to cool days was just enormous.

Annual cycle of temperatures compared to normal in 2012 in Washington, D.C. (National Weather Service)

Only November was colder-than-normal. I’d have to say that March did an impressive amount of work being outrageously warm and breaking all sorts of records nationally. (Keep in mind that the month of March has 31 days so the +10-degree warm anomaly is an average of that many data points). No other month was so warm, but four others have differences from average (or anomalies) of over 4 degrees, which is typically a pretty warm result.

What was driving this durably warm pattern? The main culprit appears to be a hyper-charged response to what is known as a negative Pacific Decadal Oscillation or - PDO.

The - PDO pattern is characterized by warm waters in the North Pacific between the Aleutians and Hawaii with cold waters extending from the Gulf of Alaska down the western coasts of Canada and the U.S. At times, that warmth in the North Pacific was stronger than anything seen before. It offers a positive feedback mechanism for big warm ridging in the jet stream pattern in the North Pacific Ocean and cool to cold troughing towards the West Coast. This see-saw effect leads to a downstream warm and dry ridge in the middle to eastern thirds of North America. It was the main driver of last summer’s super-drought in the mid-continent too (and our dry-leaning weather also).


The - PDO pattern started to weaken a bit in the autumn, which led to closer-to-seasonal or even cooler than average temperatures in September through November. It has oscillated a bit more here in December, but the - PDO is currently on track to be much weaker than last winter (meaning more snow and cold weather chances this winter vs. last winter, but you already know that since we’re already seeing it!).

In addition to the weather pattern that favored these warm temperatures, a long-term warming trend is also obvious in Washington, D.C.’s data record. The warming trend is thought to be a result of three primary factors: 1) the move of D.C.’s observing station from 24th and M St. to the warmer Reagan National Airport in the 1940s 2) urbanization 3) climate warming from increasing greenhouse gas emissions. The relative contribution of these three factors to the warming is complicated and not known with certainty.

Jason Samenow contributed to this post