Triple-Digit Heat in the Nation’s Capital ?

DC residents who don’t like extreme heat should be pleased that another June has come and gone without any triple-digit heat.  This month hasn’t featured much extreme heat as DC’s highest temperature was 93°.  That’s a little unusual since Washingtonians average one to two days of at least 95° during the month of June.

The Nation’s Capital also averages one day of triple-digit heat during June, roughly once every five years.  Meanwhile, DC residents haven’t experienced 100°/+ in June since 2012.  This June will finish as a warmer than average month with a near average number of 90° days.  High temperatures will reach 90° once again today in parts of the DC Metro Area for the ninth time this month.

Triple-digit heat is relatively rare in the Nation’s Capital in any month, typically occurring only every three to four years.  The last time Washingtonians experienced triple-digit heat was in August 2016.  It’s important to distinguish between the actual air temperature and feels-like temperature.  The “heat index” is what it feels like when you combine the air temperature with the relative humidity.  On very humid days, it feels hotter and more oppressive compared to less humid days.  The heat index exceeds the century mark several times over the course of a DC summer, but the actual air temperature rarely touches 100°.

DC’s hottest temperature on record is 106° and has occurred twice, most recently on July 20, 1930.  DC’s second hottest all-time temperature is 105° and has also occurred twice, on July 7, 2012 and August 17, 1997.  Although weather records in the Nation’s Capital date back to 1871, the more significant date to remember is 1941.  That’s when National Airport opened and where weather records have been kept since.  Prior to that, weather measurements were made downtown.  Conditions are quite different between the two locations because one has the moderating influence of the Potomac River and the other has more of an urban heat island effect.

There were some very hot summers in the 1930s prior to when National Airport opened.  However, a key difference between then and now is that urbanization has increased and overnight low temperatures remain much warmer than they used to.  That’s why the Nation’s Capital has had many of its warmest months on record over the last 10 to 20 years.  For example, June 2010 was DC’s warmest on record with the highest number of 90° days (18) while July 2012 set a new record for most 100° days with seven.  August 2016 finished with 0.1° of tying the existing record for hottest August in the Nation’s Capital.  DC’s hottest temperature in both 2017 and 2018 was 98°, while the hottest temperature in all of last year was 99°.

How Does June 2020 Stack Up ?

This June has been a fairly typical weather month in the Nation’s Capital.  No extreme weather occurred and there has been a near average amount of rainfall.  Yesterday was the sixth day of 90° since June 1 and Washingtonians could experience a few more before the calendar changes to July.

June is typically DC’s third hottest month behind July and August.  However, five of DC’s 10 warmest June’s have occurred just since 2010.  This June will finish more than a degree warmer than average for an 11^th consecutive year.  The warmest temperature so far this month was 93° on June 3 and June 27.  DC’s coolest temperature so far this month was 54° on June 1. 

Rainfall was near average this month with 3.49” to date.  That’s close to DC’s June average of 3.78” with the prospect for more showers and thunderstorms this afternoon.  Two of DC’s wettest five June’s have occurred just since 2013.  Washingtonians also experienced the third driest June on record in 2017.

This month shares some similarities with June 2016.  That month also finished 1° warmer than average and had five 90° days – close to June 2020 on both measures.  The first 90° heat of 2016 didn’t occur until June 11 much like the first 90° of 2020 didn’t occur until June 3.  That’s compared to DC’s average first 90° day on or around May 18.  DC’s monthly rainfall was also within 0.1” of average in June 2016.

While DC went on to experience one of its hottest three-month periods on record from July 2016 through September 2016, it’s too early to make that comparison for the rest of the 2020 summer.  Although NOAA’s outlook for July is for a warmer and wetter than average month in the DC Metro Area, that is not based on what June’s weather has been like.  Having a warmer than average June isn’t a reliable predictor of the upcoming summer.  For instance, July 2014 was a cooler than average month in the Nation’s Capital and followed a June that was 2° warmer than average.  

The 2010s were a very warm decade in the DC Metro Area and Mid-Atlantic Region.  It will be interesting to look at NOAA’s new 30-year temperature and rainfall averages when they’re released next year.  The period NOAA uses will be updated to reflect 1991-2020, instead of the current 1981-2010 time period.

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.