Weather Quiz

True or False.

August is the hottest month of the year in the Nation’s Capital.

Late August Heat

For the first time since April 30, DC residents will have a sunset this evening before 8:00 PM.  Despite the decreasing amount of daylight in Washington, D.C., blistering heat is a frequent occurrence during the second half of August and much of September.  In fact, today will be the second consecutive day of 90° heat with a third on tap tomorrow that will make the current stretch of hot weather an official “heat wave.”

DC’s average high/low temperatures today are 87°/70° according to the National Weather Service.  That’s down from DC’s hottest average daily temperatures of 89°/71° from July 7 – 22.  However, there isn’t much tangible difference between those subtle differences in average high/low temperatures. 

Last August, the Nation’s Capital experienced a five-day heat wave from August 27 – 31 and again from September 3 – 7.  DC’s most significant recent August heat wave occurred in 2016 with 12 consecutive days of high temperatures of at least 90° from August 10 – 21.  During that span was also the last time triple-digit heat occurred in the Nation’s Capital.  DC’s hottest and second hottest temperatures on record, 106° and 105°, respectively, have occurred in both July and August.  August is second only to July in terms of being the hottest month of the year in Washington, D.C.

DC’s hottest temperature on record, 106°, happened twice (August 6, 1918 and July 20, 1930).  Both instances occurred before National Airport opened in the early 1940s, where weather measurements have been ever since.  Prior to that, DC weather observations were made downtown away from the moderating influence of the Potomac River.  The Nation’s Capital was also much less developed than it is now, so the urban heat island effect wasn’t the issue it is nowadays.  

DC’s second hottest temperature has occurred three times, twice in July (July 7, 2012 and July 10, 1936) and once in August (August 17, 1997).  The latest in the year 100° heat was recorded in Washington, D.C. was on September 8, 1939 (100°).  An honorable mention goes to September 24, 2010, that featured a record high of 99°.  That was the 67^th and final 90° temperature of 2010 that tied it with 1980 for DC’s highest annual total of 90° days on record.

Triple-digit heat is a relatively rare occurrence in Washington, D.C., typically only happening every three to four years.  However, August temperatures of less than 60° have been even rarer in the Nation’s Capital in recent decades.  There have been only nine days with low temperatures in the 50s during August in the Nation’s Capital, dating back to the early 1990s.  By comparison, there have been 10 days of triple-digit heat in Washington, D.C. since 1993.  Although those totals are close, the key difference is the last time triple-digit heat occurred in DC during August was in 2016, while there have not been any <60° August temperatures since 2004.

August 19 Record Highs / Record High Lows (Source: NOAA)

Washington, D.C. (National Airport – DCA): 98° (2002) / 78° (2002)
Dulles Airport (IAD), Sterling, VA: 95° (2002) / 74° (2002)
Baltimore, MD (BWI Airport): 97° (1914) / 78° (1937)

August 20

DCA: 101° (1983) / 78° (2002)
IAD: 104° (1983 – hottest August temperature on record) / 74° (2002)
BWI: 105° (1983 – tied for hottest August temperature) / 78° (1914)

August Reaches Its Midway Point

The first half of August 2019 has been unremarkable in the Nation’s Capital.  It has been warmer and drier than average month.  Measurable rainfall occurred on three days so far this month, as compared to eight of the first 15 days of August 2018.  There has been 1.22” of rain so far this month in Washington, D.C. (0.28” below normal). 

What’s accentuated these dry conditions are the warmer than average temperatures.  There have been nine days with high temperatures in the 90s since August 1.  Only two days so far this month have been cooler than average.  That was this past weekend when high/low temperatures downtown were in the mid-80s and mid-60s, respectively.  Washingtonians looking forward to fall were thrilled with the sneak peek of more September-like weather.

While temperatures could touch 90° in the Nation’s Capital this afternoon, the 90s will certainly return tomorrow.  Whether the 90s return today or tomorrow, it will start the next heat wave for Washingtonians.  A “heat wave” is three or more consecutive days of high temperatures that meet or exceed 90°.  There have been 45 days of 90° heat so far in 2019.  That’s already greater than the annual average of 36.  Last year finished with 42 days of 90° heat.  DC’s highest annual total of 90° heat is 67 days that occurred twice: once in 1980 and, more recently, in 2010.

That’s not to say Washingtonians will challenge that record – but approaching 60 days of 90° heat is a possibility.  DC residents should reach 50 such days by the middle of next week.  A significant number of DC’s 90° days this summer occurred in July (22).  What’s made 2019 unusual is while DC has already had more 90° days than average for an entire year, no triple-digit temperatures have occurred.  The hottest temperature so far this year was 99° on July 21.  

Although average high/low temperatures decrease in August and September, 90° temperatures are common in both months.  DC’s last 90° temperature of 2018 didn’t occur until October 4, while the latest 90°/+ on record was on October 11, 1919.  Washingtonians could see 90° heat well into the fall season.  NOAA’s Climate Prediction Center expects above average temperatures to dominate local weather headlines in the DC Metro Area for much of the next six weeks through the end of September.  That said, it will be interesting to see how many 90° days the Nation’s Capital finishes with in 2019.

Summer-like Heat and Humidity Return

Residents of the DC Metro Area have enjoyed a nice stretch of warm and dry weather this past weekend with lower relative humidity and high temperatures in the mid-80s.  However, the weather pattern changed with a high of 92° yesterday and a noticeable increase in relative humidity today.  The area of high pressure that brought the comfortable temperatures and low relative humidity to the Nation’s Capital has shifted eastward.

The clockwise flow around the area of high pressure has helped usher more humid weather conditions into the Mid-Atlantic Region due to southerly winds from the Deep South.  That’s helped create favorable conditions for the prospect of showers and thunderstorms the next few days with the approach of a slow moving cold front.  Although extensive cloud cover will prevent widespread temperatures in the 90s today, DC’s already had 44 days in the 90s in 2019.  That’s nine more than the annual average of 36 days of 90° heat.  Last year’s total was only 42 such days.

DC residents could use a little rain as measurable rainfall has only occurred once this month prior to today.  That’s not to say there haven’t been scattered showers and storms in the DC suburbs so far this month, but the only time rainfall was officially observed in DC was last Wednesday, August 7 (0.9”).  Weather measurements for the Nation’s Capital have officially been made at National Airport since the early 1940s.  Despite having measurable rainfall on only one day so far this month, DC has a rainfall surplus of more than five inches this year.  

July 2019 was DC’s third consecutive wetter than average July and finished with nearly double the average amount of monthly rainfall.  Many area residents and tourists alike have enjoyed the relatively dry weather so far this August.  That’s following the month of July that featured rain on 15 days.  Area lawns and gardens are in need of a good watering in many parts of the DC Metro Area due to spotty rainfall the last few weeks.  

August 2019 has gotten off to a much different start than last August.  Washingtonians had measurable rainfall on six of the first seven days of August 2018.  That’s not to say this month won’t end up becoming DC’s third consecutive wetter than average August.  NOAA’s Climate Prediction Center indicates the Mid-Atlantic Region has a 33% chance of above average precipitation over the next 8-14 days.  However, the outlook for the entire month of August is for near average rainfall in the Nation’s Capital.

NOAA’s Hurricane Update

The Atlantic Ocean Basin also includes the Gulf of Mexico and Caribbean Sea

NOAA just issued an August update to its seasonal hurricane forecast.  When the season began on June 1, NOAA cited El Nino conditions as a primary reason for near to below average tropical activity in 2019 across the tropical Atlantic.  However, El Niño has dissipated and that opened the door to NOAA’s updated hurricane forecast.

In early June NOAA was expecting a near average Atlantic hurricane season (12 tropical storms, six hurricanes and two major hurricanes).  That was consistent with the El Nino conditions at the time.  An episode of El Nino often contributes to below average sea surface temperatures and above average levels of wind shear in the tropical Atlantic.  Both tend to preclude tropical development.

The season has gotten off to a quiet start with only two named storms, including a minimal hurricane.  However, June and July are normally quiet months in the tropical Atlantic.  NOAA’s updated hurricane forecast calls for a total of 10 – 17 tropical storms and 5 – 9 hurricanes, 2 – 4 of which become major hurricanes (Category 3 or greater with sustained winds of at least 111 mph).

Scientists at Colorado State University also tweaked their forecast and are expecting a seasonal total of 14 tropical storms, seven hurricanes and two major hurricanes.  Aside from El Nino conditions dissipating, there has been another key weather headline in the tropical Atlantic.  Atmospheric winds between 5,000 feet and 20,000 feet blew large plumes of dust into the tropical Atlantic from western Africa to west of the Cape Verde Islands.  That helped create some unusually dry and stable air in the same area that tropical storms develop during the height of the season.

There have been active hurricane seasons that got off to slow starts, as this season has.  The 2004 season comes to mind, which had 15 tropical storms, nine hurricanes and six major hurricanes after not seeing the first named storm of the season until August 1.  The overall number of tropical storms and hurricanes that develop is less important than where they develop.  For example, the 1992 Atlantic hurricane season featured only one major hurricane.  But that was Hurricane Andrew which remains one of the costliest and most intense hurricanes to ever impact the United States.  

The National Hurricane Center doesn’t expect any tropical development in the next five days.  The next named storm in the Atlantic Ocean will be “Chantal.”

Final Month of Summer

Strong to Severe Storms a "Slight" Risk in parts of the Mid-Atlantic  (Source: NOAA)

July 2019 was DC’s third consecutive wetter than average July.  While not as wet as 2017 or 2018, this July still finished with 6.49” (almost double the monthly average of 3.73”).  Although no record highs were set, Washingtonians endured 22 days of 90° heat in July, which finished with an average temperature of 82.1°.  That left last month tied with July 1955 for DC’s ninth warmest on record.

August has gotten off to a warm and relatively dry start, with no measurable rainfall yet at National Airport – DC’s official weather reporting site.  That’s not to say parts of the DC Metro haven’t had appreciable rainfall so far this month.  A thunderstorm last evening in Bethesda, Maryland, was its first significant rainfall in more than a week.  The Nation’s Capital averages 2.93” of rain in August.  Ten of the last 20 August’s have been wetter than average, including the last two.

The tropical Atlantic often gets busier during the month of August.  Such was the case in 2011 when a weakening Hurricane Irene brought a few inches of rain to the Nation’s Capital after its landfall in North Carolina.  August can also been a very dry month in Washington, D.C., such as in 1995 and 1998 when less than an inch of rain occurred during the entire month.  In fact, August 1998 ranked as DC’s second driest on record with only 0.59” of rain.

August is the third and final month of “meteorological summer” that began June 1.  Fourteen of the last 20 August’s have been warmer than average in the Nation’s Capital.  To add to that, August has been warmer than July in the Nation’s Capital in eight of the last 20 years.  That’s pretty remarkable considering DC’s average August temperature (combining daily high and low temperatures) is nearly 2° cooler than July (78.1° vs. 79.8°).  Its hottest August temperature on record is 106°, recorded on August 6, 1918 and also on July 20, 1930 – DC’s hottest overall temperature. 

DC’s coolest observed August temperature is 49° and has occurred three times, most recently on August 29, 1986.  Although triple-digit heat typically occurs only every few years in the Nation’s Capital, it happens more frequently than temperatures less than 60°.  DC residents experienced 100° August heat as recently as 2016.  Meanwhile, August temperatures haven’t fallen below 60° in Washington, D.C. since 2004.  NOAA expects this August to finish with near average temperatures and rainfall in the DC Metro Region.