Hottest Weather of the Summer ?

DC’s hottest weather of the summer will arrive later this week.  That’s typically the hottest time of the year in the Nation’s Capital.  Average high temperatures are at their pinnacle (89°) from July 7 through July 22.  In that time, all but two record high temperatures are at least 100°.  A “heat wave” is defined as any period of three or more consecutive days with high temperatures of at least 90°.
 

The current heat wave began on July 11 and will continue through at least next Monday, July 22.  A heat wave of at least 12 days would tie for DC’s fifth longest one of the last 20 years.  The Nation’s Capital has had two 12-day heat waves in the last two decades.  One was from August 10 – August 21, 2016 and the other was from July 14 – July 25, 2010.  The longest heat wave Washingtonians have experienced in the last 20 years was 18 days from July 22 – August 8, 1999.

There have been some other noteworthy heat waves in the Nation’s Capital.  Perhaps the most noteworthy was an 11-day heat wave from June 28 – July 8, 2012.  While not as long as some others, high temperatures reached or exceeded 100° five times in that 11-day span.  That included June’s hottest temperature on record (104° on June 29) in Washington, D.C., and its second hottest overall temperature (105°) on July 7.

Triple-digit heat is a rarity in Washington, D.C., typically occurring only every few years.  The last time DC residents experienced 100° was August 15, 2016.  However, the Nation’s Capital is poised to approach the century mark on Friday, Saturday and Sunday.  Even if high temperatures remain in the 90s, the increasing relative humidity will make it feel much hotter.  The “heat index” which is what people feel when you combine the air temperature with the relative humidity, will feel like 105° – 110°.  Those are dangerously hot conditions and any outdoor activities should be done early in the day when temperatures are cooler.  Air quality is also the best early in the day.

Since July 16 is my birthday, I want to mention that DC’s record high temperature today is 104° set in 1988.  High temperatures today are in the 90s which, while above average, aren’t close to record levels.  The high temperature on July 16 last year was 98°.  That turned out to be DC’s hottest temperature in all of 2018.  DC’s coolest July 16 high temperature was 75° in 1962.

Record High Temperatures (Source: National Weather Service)

July 19:

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

July 20:

DCA: 106° (1930) – DC’s hottest July temperature on record
IAD: 101° (1980)
BWI: 102° (1930)


July 21:

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

July Hurricanes Like Barry

Tropical Storm Barry Getting Stronger, July 12, 2019 (Source: NOAA)

“Barry” became a minimal hurricane shortly before its landfall in south central Louisiana earlier today.  Sustained winds around its center were 75 mph, making it a Category 1 hurricane on the Saffir-Simpson Hurricane Wind Scale.  The difference in impacts between a strong tropical storm and a minimal hurricane like Barry are rather insignificant.  

A tropical storm becomes a hurricane when sustained winds around the center of circulation reach 74 mph.  In systems like this, the major impact is flash flooding that results from a copious amount of rainfall, often 10” to 15”.  Rainfall totals from tropical systems largely depend on how quickly the system moves.  

Barry’s landfall as a Category 1 hurricane set a series of benchmarks.  Barry became the first July hurricane to make landfall in Louisiana since Cindy in 2005.  It was also the first hurricane to make landfall in the United States during the month of July since Arthur in 2014.  Overall, the last hurricane to make landfall in the state of Louisiana in any month was Nate in October 2017. 

In addition to flooding, Barry is generating isolated reports of severe weather.  Landfalling tropical systems often spawn tornadoes as the storm’s circulation interacts with a given land mass.  Fortunately, the majority of tornadoes that occur under these circumstances are relatively weak.  However, they are dangerous since sight of them is quite frequently obstructed by heavy rainfall.  It will take some time to assess the final impacts of Barry, since the destruction of landfalling tropical systems often extends well-inland.

Barry and Cindy were both minimal Category 1 hurricanes at their respective times of landfall in Louisiana.  However, Cindy made landfall earlier in the season than Barry, having come ashore on July 5, 2005.  Other significant hurricanes have developed in July.  Hurricane Dennis was a powerful Category 4 hurricane with sustained winds of 150 mph while over the Gulf of Mexico in 2005.  Fortunately, it weakened prior to making landfall in the Florida panhandle as a Category 3. 

For a short time, Dennis was the strongest July hurricane on record in the Atlantic Ocean Basin (the Caribbean Sea and Gulf of Mexico).  That was before Hurricane Emily briefly became a Category 5 hurricane on July 16, 2005.  It had sustained winds of 160 mph at peak intensity southwest of Jamaica.  Although Emily didn’t make landfall in the United States as Cindy and Dennis did, its impact was felt in the Caribbean Sea, Yucatan Peninsula of Mexico as well as mainland Mexico.

Hurricane Barry is the second named storm of 2019 and the Atlantic’s first hurricane of the season.  NOAA scientists are expecting an average hurricane season this year.  An average season has 12 tropical storms, of which 6 become hurricanes with 3 major hurricanes.  A “major” hurricane has sustained winds of at least 111 mph, making it a Category 3.

How Unusual Was Monday’s Rainfall ?

Driving on I-270 South towards Rockville on July 8

DC Area residents and tourists alike experienced a deluge of rare proportion this past Monday morning, July 8.  In less than an hour, 3.3” of rain accumulated at National Airport.  In all, a total of 3.44” of rain fell in the Nation’s Capital on July 8.  That obliterated the previous daily rainfall record of 2.16” from 1958.

Monday’s rainfall was roughly equivalent to getting a month’s worth of rain in just one day.  The month of July is DC’s wettest of the year with a monthly average of 3.73”, so Monday’s total of 3.44” is more than 90% of that total.  Moreover, 3.44” is greater than DC’s monthly average rainfall in seven other months.  In addition, Monday’s rainfall was the only instance of DC’s highest hourly precipitation report to occur in the morning, dating back to 1936.

Flooding was inevitable with such a high volume of rain in such a short period of time.   Monday’s rainfall was the latest wave of rain over an especially rainy past several weeks in the DC Metro Area.  It rained on 20 out of 30 days in June 2019, followed by rain on five of the first six days of July prior to Monday.  That left the ground saturated and unable to absorb any additional rainfall.

Consequently, the combination of saturated ground after a steady stream of rain since Memorial Day and Monday morning’s deluge produced widespread flash flooding across the DC Metro Area.  That resulted in impassable roadways, countless stranded cars and flooded buildings.  As quickly as the flash flooding occurred, the water dissipated leaving destruction in its wake.  A prime example is the sinkhole that developed on Belfast Road in Potomac, Maryland.

The majority of DC’s wettest days can be attributed to current or former tropical systems such as the combination of Hurricane Irene and Tropical Storm Lee in 2011.  However, the weather pattern in the Mid-Atlantic Region over the last 15+ months has been unique.  Washington, D.C. has had an astonishing a total of 80.93” of rain from April 1, 2018 – June 30, 2019.  That’s 30.36” above normal for the time period!  Making that rainfall surplus even more remarkable is that the Nation’s Capital averages only 39.74” of rain for an entire year. 

Monday’s rainfall guaranteed that July will finish wetter than average for a third consecutive year.  DC’s rainfall surplus will also continue to grow.

DC’s Seven Wettest July Day’s  (Source: NOAA)

1.  4.69” (July 9, 1970)
2.  4.35” (July 22, 1969)
3.  4.25” (July 30, 1878)
4.  4.12” (July 30, 1876)
5.  4.00” (July 21, 2018)
6.  3.68” (July 5, 1908)
7.  3.44” (July 8, 2019)