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.

June: How Does 2021 Compare?

 

This is the first June since NOAA updated its climate data to reflect the 30-year period from 1991-2020.  As a result, average high and low temperatures, as well as average monthly temperatures in the Nation’s Capital are considerably higher than they were previously.  That’s because the decade of the 2010s was much warmer than the 1980s across most of the United States.

While average temperatures during the first three weeks of June were more than one degree above average, many Washingtonians wouldn’t view this as a particularly hot June.  In fact, by most standards this month has been relatively comfortable.  For four consecutive days last week (June 15 – June 18), temperatures were not only cooler than average but dewpoint temperatures on two of those days remained in the upper 40s to around 50° creating stellar, almost fall-like weather.

While conditions in the few days since then have been noticeably warmer and more humid, there have still only been four 90° days in the Nation’s Capital so far this month.  Washington, D.C. has averaged between seven and eight 90° June days over the last 30 years.  Last June finished with nine 90° days in the Nation’s Capital. 

DC’s highest total of 90° June days was the 18 that occurred in June 2010, which finished as DC’s warmest on record.  By comparison, only two 90° days occurred in Washington, D.C. in June 2009.  There are a few more 90° days in the seven-day forecast my colleagues and I on the WUSA9 Weather Team are working on, so the total of 90° days for June 2021 should rise. 

Whether or not June is a very warm month is not a good indicator of what the rest of the summer will be like, however.  For example, June 2016 finished with only five 90° days, but the 2016 summer went on to become one of the hottest on record in the Nation’s Capital.

This month has already become DC’s wettest since last November with 5” of rain as of June 21.  The majority of that rainfall occurred on three days when a combined total of 4.62” fell.  There are a few more rain chances in the Nation’s Capital so the monthly rainfall surplus should continue to grow.  That’s good news since April and May were each drier than average in Washington, D.C.