Another Warm June?

While DC residents can expect an average of six or seven days of 90° heat in June, there have been only four so far this month.  Nevertheless, June 2020 has been more than one degree warmer than average through June 23.  That’s largely due to the high number of warmer than average overnight low temperatures.  The Nation’s Capital has even set two record high “low” temperatures this month.  That means on two days the “low” temperatures were unusually warm.

June 2020 is the final June that NOAA will use the existing 30-year average for daily and monthly temperatures before its updated.  Starting in January, NOAA’s average will reflect the 30-year period from 1991 – 2020.  Consequently, there should be a significant difference in average temperatures.  For example, average low temperatures have only gotten warmer in recent decades as urbanization has enhanced the urban heat island effect.  Suburban sprawl has increased as well, so a tangible difference could also be expected at Dulles Airport in nearby Sterling, Virginia.

Once NOAA’s running 30-year average is updated next year, a month like this may be more of an “average” month.  That’s because low temperatures in the summer tend to be warmer than they used to be.  When a more humid air mass is in place low temperatures don’t fall as far, but at the same time the urban heat island effect has contributed to making summer nights warmer than they once were.  That’s not exclusive to the Nation’s Capital, but remains an important fact to consider.

While this month has been warmer than average with some very humid days, most Washingtonians wouldn’t consider June 2020 to be an exceptionally hot month.  Certainly not compared to some recent Junes.  Five of DC’s 10 warmest June’s have occurred just since 2010.  The last cooler than average June in the Nation’s Capital was in 2009.  This June is well-positioned to become DC’s eleventh consecutive warmer than average June.

DC’s official weather reporting station at National Airport has the unique geography of being on the Potomac River.  Being on or near a body of water has a moderating influence on temperature and, depending on wind direction, can help keep air temperatures cooler during the day and warmer at night.  There’s often a difference of a few degrees between the temperature at National Airport and other nearby areas such as the White House or the National Zoo.  That’s why some think DC’s official weather reporting station should be at a location not on the Potomac since it isn’t representative of what conditions are like downtown.  However, National Airport has been DC’s weather reporting station since it opened during World War II and isn’t likely to change.