DC’s Hottest Month

 

July is the hottest month of the year in the Nation’s Capital.  Earlier this year, NOAA updated its 30-year climate data set to reflect the period of 1991-2020, replacing 1981-2010.  Consequently, DC’s average temperatures and rainfall are both higher than they were previously.  That shouldn’t surprise many, however, as the decade of the 2010s were considerably warmer and wetter than the 1980s.

DC’s average July temperature (combining daily high/low temperatures) for 1991-2020 was 81° compared to the 1981-2010 average of 79.8°.  Using the old statistic, four of the nine July’s between 2001 and 2009 would have been warmer than average in the Nation’s Capital, rather than being nine consecutive cooler than average July’s. 

Triple-digit heat has occurred in four July’s since 2010 and most recently in 2016.  Six of DC’s 10 hottest July’s have also occurred over the last eleven years.  July is such a hot month in Washington, D.C. that 28 of the 31 daily record highs during the month are at least 100°.  By comparison, that number drops to 19 in August.

July is also a much wetter month than it used to be based on NOAA’s updated climate statistics.  The Nation’s Capital now averages 4.33” of July rainfall, compared to 3.73” in the 1981-2010 data set.  Not only have seven of the last 10 July’s been wetter than average, but Washingtonians experienced two of the five wettest July’s on record in 2017 and 2018.  Those months helped boost July’s average monthly rainfall by 0.6” when NOAA updated its climate averages.

Washingtonians have gotten used to heavy rain and record heat during the month of July over the last decade.  Something D.C. residents haven’t seen much of in recent years has been record cool temperatures.  The last time a record low occurred in Washington, D.C. during July was in 1988.  The warmer temperatures and higher average July rainfall are not unrelated as warmer temperatures can hold more water vapor.  Not since 2016 have Washingtonians gone through the entire month of July without at least one daily rainfall total of at 2”/+. 

According to NOAA’s Climate Prediction Center, there is an above average probability for July 2021 to be a warmer and wetter than average month.  That would make this month DC’s sixth warmer than average July in the last seven years and eighth wetter than average July in the last nine years.

 

June’s Sultry Conclusion in the Nation’s Capital

Today will likely become DC’s hottest day of the year with highs in the mid to upper 90s.  If today’s temperature reaches 95° at National Airport, it will not only be the hottest day of 2021, but will also be DC’s first 95°/+ temperature in almost a year (July 27, 2020). 

Today will also be DC’s third consecutive 90° day – giving Washingtonians their second heat wave of the month.  June 2021 will be DC’s third consecutive June to end with a heat wave.  The 90° heat in the D.C. Metro Area today will be particularly oppressive as dewpoint temperatures have reached the low to mid-70s.  Hot temperatures and uncomfortable relative humidity levels combine to produce dangerous heat index temperatures.  The “heat index” is the feels-like temperature when combining air temperature with relative humidity.

June 2021 will finish with seven days in the 90s.  That’s compared to the six days that high temperatures remained in the 70s.  What stood out the most in terms of DC’s June weather has been the stretches of almost fall-like weather.  From June 15 – June 18, temperatures were not only cooler than average with highs in the low to mid-80s, but dewpoints were also uncommonly low in the 40s and 50s.  There was an encore performance from June 22 – 25, when highs remained in the upper 70s and low 80s with similarly low dewpoints.

These stretches of comfortable weather were largely offset by DC’s two heat waves this month.  Average monthly temperatures will finish roughly 0.5° warmer than average.  That’s not surprising given the near even split of comfortable weather days and DC’s two heat waves since June 1. 

Finally, June 2021 will finish with 5.48” of rain, more than one inch above the June average of 4.20”.  The warmer the air temperature, the higher its capacity is to hold water vapor.  That’s a key reason why heavier rain events tend to occur during the summer and early fall when temperatures are warmest in the Nation’s Capital.  The majority of this month’s rainfall (4.62”) occurred on just three days between June 10 – June 14.

D.C.'s Unusual June Weather

 

 

This is the first June that DC’s average temperatures and rainfall are based on NOAA’s updated climate data for the 30-year period of 1991-2020 that replaced the previous data set based on 1981-2010.  Since that change was made, DC’s average temperatures are warmer and the average rainfall is higher. 

At National Airport, the official weather reporting site for Washington, D.C., this June has been a relatively mild month.  There has only been one heat wave so far this year from June 5 – June 7.  Through June 26, there have only been four days with highs in the 90s.  That’s compared to six days this month high temperatures remained in the 70s.

That’s unlike some recent June’s that have featured a significant amount of extreme heat.  As recently as 2015, Washingtonians experienced 12 days of 90° June heat and the month finished nearly 2° warmer than average.  By comparison, June 2009 had only two 90° days and finished nearly 3° cooler than average.  While temperatures have been near average so far this month, June 2021 should finish as a warmer than average month overall.  My colleagues and I on the WUSA9 Weather Team expect DC’s second heat wave of 2021 to get underway today with a high of 90°.  This June will finish with a heat wave in the Nation’s Capital for a third consecutive year.

This month is also the wettest month of 2021 in Washington, D.C. with 5.48” of rain through June 26.  While more than one inch above DC’s June average of 4.20”, it doesn’t come close to some extremely wet June’s in the Nation’s Capital.  For example, three of DC’s five wettest June’s on record occurred in 2006, 2013 and 2015.  June can also be a very dry and hot month in Washington, D.C.  Such was the case in 2017 when only 1.13” of rain fell and the month finished one degree warmer than average.

June 2021 will likely finish as a warmer and wetter than average month in the Nation’s Capital.   Is that a harbinger of what the rest of the summer’s weather might be like?  The short answer is no.  It only takes one significant heat wave to produce a warmer than average summer month, or one tropical system to make the month wetter than average.  Conversely, the absence of those things could lead to near average temperatures and rainfall.  NOAA’s Climate Prediction Center expects a higher than average probability for a warmer and wetter than average July in the D.C. Metro Region.