Here’s an excerpt [I’ve added some paragraph breaks for readability] from Leffler’s note:
While December 2012 experienced well above normal temperatures over the entire DC area (about +5 to +6 F) , table 1 [to the right] shows how unrepresentative Washington National Airport minimum temperatures are for the Washington DC “Area”.
Note as usual, DCA’s number of freezing or lower minimums are totally out of line with other published National Weather Service stations in the area that are not located next to tidal estuaries at sea level. Also note DCA recorded the same number of freezing or lower mornings as Jacksonville, FL, Savannah, GA, Atlanta, GA, and Baton Rouge, LA, far to the south.
Charlotte, NC’s southern airport even received 50% more freezing or lower mornings than DCA! In fact, the only other published NWS station available through the local NWS Weather Forecast Office within the District of Columbia (National Arboretum) reported 14 freezing or lower mornings, in line with other surrounding “DC area” stations! Even using DCA’s minimums to represent DC proper can be misleading!!!
This is not new earth shattering news. It’s not that the observations at DCA are “bad”. They are indeed representative (most of the time much too mild) of a very limited micro-climate found at sea level directly adjacent to large bodies of water on the east coast. It’s just that its extremely mild micro-climate is totally unrepresentative of the DC “area” as a whole.
Leffler’s note is a good reminder to keep DCA’s temperatures (especially the lows) in perspective.