Locally, 2012 was the warmest year on record by over eight tenths of a degree (58.9 was our average temperature compared to 58.10 from 1931). The national data will be crunched over the coming days but given last year was warmest on record through November and that December was above average for much of the country, we can easily deduce 2012 will be the warmest year on record in the United States.
Climate records on a national scale go back to 1895.
Temperatures last year averaged between three and six degrees above average in a number of cities throughout the Midwest, Northern Plains, Great Lakes, and into the Northeast. Besides Philadelphia, other cities that set records for annual warmth include Chicago, Minneapolis (tie with 1931), Detroit, Dallas, Burlington (VT), and Houston among many other locations.
The year's warmth nationally was a product of a very dry pattern in the middle of the country, set up by a ridge of high pressure that built in early over the Plains and held tight through the summer. The heat ridge effectively blocked storm systems from developing in the Plains and from tapping into Gulf moisture. The result was precipitation running below normal in much of the Plains and Midwest, with some of the driest years on record to boot in many locations. This ridge of heat built in March, with temperatures running in some places over fifteen degrees above average in the Midwest, and persisted through much of the year. Eleven states through November were experiencing one of their ten driest years on record, including Delaware...which was a part of a "dry belt" that extended from Boston southwest to Atlanta (see below).
For us locally, while we finished over five inches below average on precipitation, this was not one of our driest years on record. The final total of 35.94" was over six and a half inches above the old driest year on record in Philadelphia, set back in 1922. However, it was the driest year for us since 2001.