July Comes to a Warm and Wet Conclusion

July will finish warmer than average for a fifth consecutive year in the Nation’s Capital and for the eighth time in the last 10 years.  That’s in large part to DC’s longest heat wave in nearly three years.  It was also a wetter than average month for a third consecutive year.

The 12-day heat wave from July 11 – 22 was the longest Washingtonians experienced since August 2016.  Although no record highs were set, DC had its hottest temperature of 99° (July 21) since August 15, 2016.  There have been 21 days of 90° heat this month in Washington, D.C.  That’s helped July 2019 rank among the warmest on record in the Nation’s Capital.  DC’s average monthly temperature (combining daily high and low temperatures) through July 30 is 82.1°.

The weather pattern was favorable for a lot of warmer than average weather coupled with many very humid days.  That was due in large part to the semi-permanent summer feature known as the “Bermuda High.”  This phenomenon refers to a sprawling area of high pressure that typically sets up during the summer months off the East Coast of the United States near the island of Bermuda.  The southerly/southwesterly winds that blow around the area of high pressure help pump warm and humid air northward from the Gulf Coast toward the Mid-Atlantic Region.  

A series of fronts have also impacted the DC Metro Area during July, bringing frequent showers and thunderstorms to the Nation’s Capital.  Since July 1, a total of 6.48” of rain has fallen in Washington, D.C., well above July’s average of 3.73”. DC’s wettest day this month occurred on July 8 with a daily record 3.44” at National Airport.  However, only 1.05” of rain fell at Dulles Airport on July 8; and a below average total of 2.71” has fallen there this month.  That helps illustrate how localized heavy rain associated with summertime showers and thunderstorms can be.  

As wet as this month has been in the Nation’s Capital, the last two July’s were wetter.  More than 9” of rain occurred in Washington, D.C. in both July 2017 and July 2018.  That was the first time on record that 9” or more rain occurred in consecutive July’s in the Nation’s Capital.  The potential exists for showers and thunderstorms this afternoon as well as the next several days.  August 2019 has the potential to pick up where July left off, with frequent shower and thunderstorm chances.

Warmest July’s in Washington, D.C. by Average Temperature (Combining Daily High/Lows)

1.  84.5° (2011)
2.  84.0° (2012)
3.  83.1° (2010, 1993)
5.  83.0° (1999)
6.  82.7° (2016)
7.  82.6° (1987)
8.  82.3° (1980)
9.  82.1° (1955)
10.  81.9° (1988)

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Most 90° Days in July in Washington, D.C.

1.  25 – 2011
2.  24 – 1993, 1987
4.  23 – 2016, 1988
6.  22 – 2012
7.  21 – 2019
8.  20 – 2017, 2010

Comparing DC Weather Trends in 2019 vs. 2018

U.S. Supreme Court, Washington, D.C.

The first seven months of 2019 have featured remarkable similarities to 2018.  While not as wet as the record-setting 2018 was in the Nation’s Capital, there is a rainfall surplus over the first seven months of 2019.  Check out this new story I co-wrote for the WUSA9 website.

Comparing July 2019 and July 2018

July 2018 was DC’s fourth wettest on record with 9.73” of rain.  This month will also finish wetter than average with 6.48” of rain at National Airport as of July 26.  In fact, Washingtonians experienced a record rainfall total of 3.44” on July 8, 2019, that nearly equaled DC’s entire July average of 3.73”.

A key difference between July 2018 and July 2019 is how streaky the rain was last year compared to this year.  No measurable July rainfall occurred prior to July 17, 2018.  Then, over the final two weeks of last July, Washingtonians experienced three days with an inch or more of rain.  Those three days alone produced a total of 8.14” or more than 80% of the July 2018 total.  By comparison, this month’s rain has been spread out more evenly with measurable rainfall on 11 of the first 26 days of the month.

In addition to both July 2018 and July 2019 being wetter than average, they both were warmer than average in the Nation’s Capital.  DC’s average July temperature (combining daily high and low temperatures) is 81.9°.  So far this month is 2.1° through July 26.  That’s largely the result of the 12-day heat wave from July 11 – 22.  July 2018 also got off to a dry start and was very hot early on with highs in the 90s on eight of its first 12 days.  However, once the rain began falling on July 17, high temperatures reached the 90s only twice the rest of the month.

DC’s weather this July has been fairly streaky as well, with frequent rainfall over the first week of the month followed by the 12-day heat wave.  While eight of the last 10 July’s have been warmer than average in Washington, D.C., no triple-digit July heat has occurred since 2016.  Official weather measurements for the Nation’s Capital are made at National Airport.  It’s important to make that distinction since both Dulles Airport in Sterling, Virginia and BWI Airport in Baltimore, Maryland both experienced 100° heat earlier this month for the first time since July 25, 2016.

DC’s hottest temperature during 2018 occurred on July 16 when it was 98°.  Similarly, DC’s hottest temperature so far this year of 99° occurred on July 21.  High temperatures reached the 90° plateau yesterday for the 32^nd time this year.  That’s pushed the 2019 total for the Nation’s Capital closer to the annual average of 36 days of 90° heat.  High temperatures reached the 90s on 42 days last year.  While above average that was nowhere near DC’s annual record of 67 days of at least 90° (2010, 1980).

Weather Quiz

True or False.

If July 2019 were to end today, it would finish among DC’s ten warmest on record.

DC’s Longest Heat Wave Since 2016 Finally Ends

July 23 will finish 15° - 20° cooler than July 22  (Source: WUSA9)

DC’s longest heat wave since August 2016 came to a conclusion yesterday after temperatures reached the 90s on a twelfth consecutive day.  The heat wave began on July 11 with a high temperature of 91° at National Airport.  DC’s hottest day since then was Sunday, July 21 with a high/low temperature of 99°/80°.  As hot as the last week and a half have been, no record highs were set in the Nation’s Capital.

No triple-digit heat occurred during this memorable heat wave.  The last time Washingtonians experienced 100° heat was August 15, 2016.  That’s not to say the last several days haven’t been dangerously hot as excessive heat warnings were in effect July 19 – 21.  Official weather measurements for Washington, D.C. are made at National Airport along the Potomac River.  There was a south wind blowing for much of the last few days, which helped keep National slightly cooler than it might have otherwise been.  DC’s hottest days occur with a west wind blowing, which is often slightly drier than a southerly wind.  

Most people would say the weather the last few days was as humid as any time in recent memory.  Dewpoint temperatures reached the mid-70s and that, combined with temperatures in the mid to upper 90s, produced dangerous heat indices of 105° - 115°.  It also provided plenty of fuel for heavy rain to fall in the thunderstorms that developed.

Although National didn’t reach 100°, Baltimore (BWI Airport) did on July 20 and 21.  Dulles Airport also reached 100° on July 21.  In both cases, it was the first triple-digit heat since July 25, 2016.  However, no record highs were set at either BWI or Dulles Airports.  It is worth pointing out that Dulles had hotter high temperatures than National on five days out of the 12-day heat wave.  That underscores the impact that National being on the Potomac River can have on air temperatures.  

The Nation’s Capital has had two previous 12-day heat waves over the last decade prior to the current one.  One occurred from August 10 – 21, 2016 and the other on July 14 – 25, 2010.  The key difference between those earlier heat waves and this one is that triple-digit heat occurred in DC during the two earlier heat waves.  Consequently, the current heat wave resembles the earlier ones in length only.  As is, the final four days of the most recent heat wave featured high temperatures between 95° and 99° for the longest such streak since July 19 – 22, 2017.

Some Perspective on DC’s Dangerous Heat

Dangerous July Heat in the DC Metro Area  (Source: WUSA9)

Starting today temperatures will approach the 100° mark for three consecutive days in the Nation’s Capital.  Having one day of triple-digit heat is relatively rare in Washington, D.C., let alone on consecutive days.  Even if high temperatures remain in the upper 90s, the relative humidity will combine with the above average temperatures to make it feel dangerously hot.

The “heat index” is what people feel when air temperatures are combined with relative humidity.  It will be between 105° and 115° each afternoon through Sunday and that’s as dangerous as it gets.  The last time any triple-digit heat occurred in Washington, D.C. was in August 2016.  That was also the last time DC residents experienced triple digit heat on consecutive days (August 13 – 15).

DC’s hottest temperature in all of 2018 was 98° on July 16.  Meanwhile, so far this year the hottest observed temperature in the Nation’s Capital was 96° on June 29.  Temperatures are poised to exceed that on each of the next three days.  It’s also worth noting that having three consecutive days of high temperatures ranging from 95° to 99° is also rare in the Nation’s Capital.  The last time that occurred was July 19 – 22, 2017. 

Although weather records in Washington, D.C. date back to the early 1870s, they have been kept in two different locations.  Since National Airport opened during World War II, weather measurements have been made there.  Prior to that, they were made downtown.  This is significant since National Airport’s location on the Potomac River has a moderating influence on temperature.  For example, if a south wind is blowing at National then the wind is blowing off the water.  During the summer that keeps temperatures slightly cooler than areas away from the River.

It’s also interesting to note that DC’s hottest overall temperature of 106° occurred twice prior to National Airport opening: once on July 20, 1930 and again on August 6, 1918.  Not only was that a downtown temperature, but Washington, D.C. was much less developed then.  There wasn’t as much of an urban heat island effect at the time, making those days of triple-digit heat even more impressive.  Today, overnight low temperatures don’t fall as much as they used too.  That’s the result of a much stronger urban heat island effect as the Nation’s Capital has become more developed combined with the fact that National Airport is located on the river.

That said it will be interesting to observe how high temperatures go the next three days.  Regardless of how hot it gets and whether or not any records are set, this weekend’s weather will be dangerously hot. 

Record High / High Low Temperatures (Source: National Weather Service)

July 19:

Washington, D.C., National Airport (DCA): 102° (1930) / 81° (2013)
Dulles Airport (IAD), Sterling Virginia: 98° (1977) / 77° (2013)
Baltimore, MD (BWI Airport): 103° (1930) / 80° (1942)

July 20:

DCA: 106° (1930) – DC’s hottest July temperature on record / 82° (2015)
IAD: 101° (1980) / 75° (2015 – has also occurred in previous years)
BWI: 102° (1930) / 80° (1930)


July 21:

DCA: 104° (1926) / 82° (1987)
IAD: 101° (1991) / 77° (1987)
BWI: 104° (1930) / 80° (1930) – BWI’s hottest July “low” temperature