A Closer Look at DC’s 2021 Summer Weather

 

Despite the hot weather the last few days, this month’s temperatures are poised to finish within 0.5° of average.  That’s in stark contrast to last July in the Nation’s Capital that was the third warmest on record.  This month will finish slightly warmer than average with a little less rainfall than average. That follows June, which finished a bit warmer than average with a rainfall surplus of 1.31”.

There were two days with more than an inch of rain in Washington, D.C. during the first nine days of July.  By comparison, there has been a combined rainfall total of less than an inch of rain in the Nation’s Capital since July 10.  Barring any heavy rain from a thunderstorm during the next few days, this month will finish as DC’s first drier than average July since 2016.  Two of DC’s five wettest July’s on record have occurred in the last four years (2017, 2018).

High temperatures the last two days reached the upper 90s in Washington, D.C. for the first time in more than a year.  However, there have been several days this month with highs in the low-to-mid 80s and comfortable relative humidity that’s broken up the stretches of high heat and humidity.  No day was more fall-like than July 3, with a high/low temperature of 81°/63°.  As of yesterday, there have been 17 days of 90° so far this month.  However, that’s relatively tame compared to last July that had a record 28 such days. 

Because weather measurements for Washington, D.C. are made at National Airport where the Potomac River has a moderating influence, triple-digit heat is a relatively rarity.  If there’s a south wind in the Nation’s Capital, then it blows off the cooler waters of the Potomac River at National Airport.  That tends to keep temperatures down slightly compared to downtown areas away from the river.  DC’s hottest days have a west or northwesterly wind and are a little less humid than the days with a southerly wind.  No triple-digit heat has occurred in Washington, D.C. since August 15, 2016.

Last month featured a similar story with temperatures.  Washingtonians had seven days in the 90s last month, but another six days high temperatures remained in the 70s.  While 5.51” of rain fell in the Nation’s Capital, 4.62” (or roughly 84%) fell during a five-day stretch.  That’s why temperature and precipitation extremes are often hidden when looking at monthly weather averages. 

Tropical Downpours in the Nation’s Capital

 

Photo Credit: Robin Beal

The National Oceanic and Atmospheric Administration (NOAA) recently updated its running 30-year average temperature and precipitation totals for the United States.  The 30-year period of 1991-2020 was appreciably warmer and wetter than the previous 1981-2010 period in the Nation’s Capital.  That’s no more apparent than during the month of July.

Previously, July was DC’s third wettest month of the year with an average of 3.73” of rain.  However, things changed dramatically with the updated climate statistics that saw the decade of the 2010s replace the 1980s.  July is now the wettest month of the year in the Nation’s Capital with a new monthly rainfall average of 4.33” – 0.6” greater than the previous average.  If you look at how rainy the last few July’s have been in the D.C. Metro Area, it’s easy to see why there was such a significant increase in the new average.

2020:  The monthly rainfall total of 6.51” was above average.  What made this July significant was that 3.93” or 60% of the monthly rainfall occurred on just two days.  A daily rainfall record of 2.04” occurred on July 7, while another 1.89” fell on July 23.  

2019:  July got off to a wet start two years ago with measurable rainfall on five of its first eight days.  The signature rainfall event occurred on July 8 when a daily rainfall record of 3.44” occurred at National Airport.  That accounted for more than half of DC’s monthly total of 6.49”.  Most of that rain fell in less than two hours, snarling the morning commute across the D.C. Metro Region.

2018:  An astounding 4” of rain fell at National Airport on July 21.  Not only was it a new daily record, but was the wettest day in the Nation’s Capital since September 30, 2010.  Although there was no measurable rain prior to July 17, 8.52” of rain fell during the eight-day period from July 17 – July 24.  It finished among DC’s Top 5 wettest July’s with 9.73”.

2017:  This July finished with 9.15” of rain making it DC’s wettest since 1969.  During an eight-day period from July 22 – July 29, a total of 6.15” of rain fell that accounted for a little more than two-thirds of DC’s monthly total.  The wettest day of the month occurred on July 28 when a daily record 3.31” of rain was observed at National Airport.

Weather Quiz

 

July 1996  (Source: NOAA)

The remnants of which former hurricane impacted the Mid-Atlantic Region on July 13, 1996?

A.  Bertha

 

B.  Fran

 

C.  Floyd

 

D.  Isabel