Putting a Dent in DC’s Rainfall Deficit

Warmer than Average weather Expect in August 2017  Source: NOAA

Washingtonians are on the verge of reaching a significant milestone as we enter the final week of this month.  August 1 will be the second anniversary of what has been a very warm and dry period in the Nation’s Capital.  As of this past Friday, Washington, D.C.’s rainfall deficit had swelled to 14.3” dating back to August 1, 2015.  That shortfall represents about one-third of the 39.74” of rain that DC averages in an entire year.

Prior to this month, only four months have been wetter than average in Washington, D.C. during the last two years.  In that time, the Nation’s Capital has set multiple record high and record high-low temperatures that contributed to the warmest December on record (2015) and a series of months finishing among the warmest on record.  Washingtonians also experienced the first 100° temperature in four years last summer, including a rare streak of three consecutive 100° days.  

The warmer than average temperatures combined with the below average rainfall contributed to moderate drought conditions for parts of the Mid-Atlantic Region earlier this year.  Fortunately, a wetter than average second half of April and month of May eliminated the drought conditions, but not the rainfall deficit.  This month has featured several days with significant rainfall in the DC Metro Area that have ensured this month will finish as only the fifth wetter than average month in the last two years.

July has also been a warmer than average month in the Nation’s Capital with 18 of the 22 days so far this month in the 90s.  High temperatures today and tomorrow are expected to reach the 90s, with several more chances during the upcoming week to pad that total.  The record for highest number of 90° days in Washington, D.C. during the month of July is 25 from 2011.  By comparison, following a cooler than average start, July 2016 finished with 23 of its 27 final days at 90°/+.  

There are several more rain chances the next few days that could provide some relief from the heat and make a dent in the ongoing rainfall deficit.  However, it will take several consecutive wetter than average months to make up the deficit entirely.  NOAA’s Climate Prediction Center is expecting near average precipitation for the Mid-Atlantic Region in August 2017.

The Hottest Time of the Year

The second half of July and first half of August is traditionally the hottest time of the year for much of the United States.  Washington, D.C. hottest temperature on record (106°) has occurred twice: once on July 20, 1930 and August 6, 1918.  The National Weather Service is responsible for keeping weather records for the United States.

The Nation’s Capital averages 36 days of 90°/+ temperatures in a given year, with the majority coming during meteorological summer from June 1 through August 31.  So far this year, Washingtonians have sweat through 27 days of 90° heat.  The record for highest number of 90° days in a given year is 67 that occurred twice (1980, 2010), while the lowest such total was seven most recently in 1905.  Although there were 58 days that reached or exceeded 90° in Washington, D.C. last year, only five such days occurred through July 4.

Residents of the DC Metro Area are well-positioned to experience another summer with a higher than average number of 90° days.  A “heat wave” is defined as any stretch of three or more consecutive days with temperatures of at least 90°.  The longest heat wave so far this summer has been six days.  However, the current heat wave, that began July 17, is poised to continue through at least next Monday, which would make it the longest heat wave of 2017.  The longest heat wave of 2016 was 13 days from July 21 through August 2.  

Last summer was the third hottest on record in the Nation’s Capital.  This summer has seen a significantly higher number of 90° days to this point (26) than last year (16).  However, there were also four days of triple-digit heat in the Nation’s Capital last year and there haven’t been any so far this year.

While no 100° heat has occurred yet this summer, there will be several chances to reach the century mark during the next several days.  Whether or not this happens, this summer has already had the tenth hottest June on record (average monthly temperature of 77.3°) and the sixth warmest July (82.8°) through July 19.    

Record Highs (Degrees Fahrenheit)  Source: NWS

July 20

DCA: 106° (1930) --> Tied with August 6, 1918 for hottest DC temperature on record
IAD: 101° (1980)
BWI: 102° (1930)

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July 21

DCA: 104° (1926)
IAD: 101° (1991)
BWI: 104° (1930)

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July 22

DCA: 103° (1926)
IAD: 105° (2011)  --> Hottest temperature on record at Dulles Airport
BWI: 106° (2011)

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July 23

DCA: 102° (2011)
IAD: 99° (2011)*
BWI: 102° (2011)*

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July 24

DCA: 101° (2010)
IAD: 99° (2010)
BWI: 101° (2010)

*= also occurred in previous years

Summer’s Midway Point

Nationals Park, Washington, D.C.

Now that we are at the unofficial halfway point of the 2017 summer season, we can reflect upon how this summer compares to average so far.  Meteorological summer runs from June 1 through August 31.  True to form, the month of June and the first half of July have featured plenty of above average temperatures.

June 2017 finished 2.1° warmer than average in the Nation’s Capital and included 8 days of at least 90°.  Last month was also exceptionally dry in Washington, D.C. with only 1.13” of rain at National Airport.  That made for the third driest June on record in the Nation’s Capital.  The dry month of June that Washingtonians experienced has been part of a larger weather pattern with 19 of the last 23 months being drier than average since August 2015.  Had May not been appreciably wetter than average, DC residents would have endured a Top 15 driest first half of the year.

July has gotten off to a hotter and wetter than average start in Washington, D.C. with 12 days of 90° heat through July 16.  There have been a total of 23 days of 90° so far this year.  By comparison, the Nation’s Capital didn’t experience day 23 day of at least 90° until July 26 last year.  There has already been 3” of rain so far this month in the Nation’s Capital and that’s just under the total of 3.13” that occurred in all of last July.

The relative humidity has made it feel much worse.  Wind direction plays a critical role in both temperature and relative humidity in the Nation’s Capital.  Winds on the hottest days in Washington, D.C. tend to be westerly, meaning they blow from west to east, while the more humid days have southerly winds.  

There was a good example of just how big a difference that a change in wind direction can make in temperatures and relative humidity this past Friday.  On July 14, the 1 PM temperature at National Airport was 92° with a dewpoint temperature of 78° and a south wind of 10 mph.   However, at 2 PM, the winds shifted to W at 16 mph with gusts to 23 mph.  The change in wind direction helped temperatures jump to 97° and the dewpoint drop to 71°.