Ranking this Week’s Cold

Rare Atlanta snowfall January 2017  (Courtesy: Grace Olson)

Washingtonians have earned the milder weather on tap today and the next few days after having survived the coldest January weather in several years.  As if today’s high temperatures in the 50s aren’t balmy enough, temperatures will be on either side of 60 degrees tomorrow.  

That’s in stark contrast to the last several days when DC residents experienced a truly rare occurrence.  From January 7 – 9, high temperatures not only remained below freezing, but held in the 20s.  Several times in recent winters there were consecutive days with high temperatures below freezing, but having three consecutive days with temperatures in the 20s or colder in the Nation’s Capital is rare.  While temperatures did remain below freezing for five consecutive days last February 11 – 15, only two of those days saw highs under 30 degrees.

Although the high temperatures on January 7 – 9 (27, 25, 29 degrees) didn’t set any records, they did make for the coldest three-day period in Washington, D.C. since January 22-24, 2014 (19, 28, 24 degrees).  Such cold weather is more common at more rural locations like Dulles Airport in Northern Virginia or BWI-Marshall Airport.  Those airports are away from the marine influence of the Potomac River and from a major urban area. 

Being in a more rural area reduces the “urban heat island” effect and creates more ideal conditions for a phenomena known as “radiational cooling.”  This refers to what happens on clear nights with light winds when heat at the surface (whether from human industry or the sun) that accumulates during the day is able to radiate back up into the atmosphere.  Snow cover and a dry air mass enhance this process.  

That’s why clear nights after a snowfall when winds are calm tend to get so cold.  For example, following the record snowfall that occurred last January 22-23, low temperatures plummeted on the following two nights.  Specifically, low temperatures on January 24-25, 2016 fell to 19 and 16 degrees, respectively, at National Airport, while Dulles was much colder (9 and 4 degrees).  Although significantly more snow occurred at Dulles Airport, the key difference here is the urbanization around National Airport is significantly more than around Dulles.  This helped keep temperatures warmer given similar atmospheric conditions.  Just yesterday, the low temperature at National Airport was 16 degrees, while Dulles Airport was a frigid seven degrees.

Fortunately, a significant shift in the weather pattern is underway and temperatures are poised to be warmer than average on several days during the next week.  In fact, Washingtonians could experience the first 60 degree weather of 2017 tomorrow.  The last time it reached at least 60 degrees in the Nation’s Capital was December 27 (68F).  NOAA’s temperature outlook for the next 1-2 weeks is for more above average temperatures, which is good news for everyone but those longing for snow.