How Hot Can August Get?

Although average temperatures in August aren’t quite as hot as July in Washington, D.C., it can feature exceptionally hot weather.  Washingtonians have experienced triple digit heat during five Augusts over the last twenty years, compared to six different July’s – including last month.

Contributing to the warm summer weather in the eastern United States is the feature known as the “Bermuda High.”  A sprawling area of high pressure that sets up for most of the summer just off the East Coast – hence the name “Bermuda High” – this powerful area of high pressure acts as a heat pump ushering in warm and humid weather northward from the south.   One of the characteristics of an area of high pressure is sinking air, which also serves to suppress cloud formation.  That also allows the temperature to rise. 

The hottest temperature on record in Washington, D.C. is 106 degrees and has occurred twice (July 18, 1930 and August 6, 1918).  Meanwhile, the hottest temperature on record at National Airport (which opened in 1941) is 105 degrees and also occurred twice (August 17, 1997 and July 7, 2012).  However, there are rare instances where it can remain cloudy and cool during the summer months.  Such was the case on this date in 1996 when high temperatures reached only 75 degrees at both National and Dulles Airports.

Although Washington, D.C.’s average high temperatures are slightly warmer in July, there have been instances where August was hotter than July.  Such was the cause during the torrid summer of 1980 that saw the hottest August on record in the Nation’s Capital.  August’s average monthly temperature (combining daily highs and lows) is 78.1 degrees in Washington, D.C. compared to July’s average temperature of 79.8 degrees – based on NOAA’s 30-year average through 2010.  While August was warmer than July in Washington, D.C. seven times from 2000 through 2009, July has been warmer every year since.

A key difference in recent years has been the tendency for overnight temperatures to remain warmer in urban areas.  National Weather Service data indicates the Nation’s Capital just this week set a new record for longest stretch where temperatures remained above 70 degrees on 35 consecutive days from July 4 through August 8.  The previous record was 32 days from the record summer of 1980.  It’s interesting to note that three of the other top 10 streaks for highest number of consecutive days at or above 70 degrees in Washington, D.C. have occurred since 2010.  

The warmer overnights can be attributed to an increased amount of urban development and greenhouse gas emissions.  That creates a type of self-fulfilling prophecy since people will use more energy to keep cool and that will, in turn, lead to more greenhouse gas emissions.  Fortunately, average temperatures continue their gradual decline as Labor Day approaches.

Significant August Hurricanes

Category 5 Hurricane Allen (8-7-1980)  Source: NOAA

While Hurricane Andrew (1992) was a significant August hurricane, there are several other important storms that occurred in the 15 years prior to Andrew.

Hurricane Bob (1991) – Bob was a rare major hurricane to effect the Mid-Atlantic and northeastern United States.  Bob became a Category 3 hurricane with sustained winds of 115 mph while near the North Carolina Outer Banks and caused significant coastal erosion as it headed northward.  It weakened to Category 2 intensity before making landfall in Rhode Island on August 19.  Bob was the only hurricane to make landfall in the United States during the 1991 season and caused $1.5 billion in damages (unadjusted for inflation).  It remains the last hurricane to make landfall anywhere in New England.

Hurricane Alicia (1983) – Alicia became a major Category 3 hurricane on August 18 and made landfall in Texas with sustained winds of 115 mph.  A “major” hurricane is defined as a Category 3 or higher on the Saffir-Simpson Scale.  Alicia was the first hurricane to make landfall in the United States since Allen three years earlier.  It would remain the strongest hurricane to make landfall in Texas until Hurricane Bret in 1999 according to the National Hurricane Center.  Alicia was the costliest tropical system in the United States since Agnes in 1972 and caused $2.6 billion in damages (unadjusted for inflation).

Hurricane Allen (1980) – Allen was the first named storm of 1980 and set a few key records.  On August 7, Allen reached its peak intensity with 190 mph sustained winds and a minimum central air pressure of 899 millibars (standard sea level air pressure is approximately 1013 millibars).  Allen had the distinction of being the earliest Category 5 hurricane on record in the Atlantic Ocean, until Emily became a Category 5 on July 16, 2005.  Allen’s sustained winds remain the highest observed in any Atlantic hurricane and its minimum central air pressure makes it the Atlantic’s 5^th strongest hurricane on record.  Allen was also the first hurricane to intensify into a Category 5 on three separate occasions before it ultimately made landfall in south Texas as a Category 3 hurricane.

Hurricane David (1979) – David reached peak intensity on August 31 as a Category 5 storm with sustained winds of 175 mph and a minimum central air pressure of 924 millibars.  David remains a notorious storm in the Caribbean Sea since it not only caused extensive damage to a number of islands, but was one of the Dominican Republic’s deadliest hurricanes claiming over 2,000 lives.  Hurricanes bring copious amounts of rain and when an intense hurricane also brings high winds and storm surge, the results are often deadly.

There have been fewer hurricanes than average in the Atlantic Ocean during each season since 2013.  Just last week the Gulf of Mexico set a new record for longest period of time without a hurricane.  The last major hurricane to make landfall in the United States was Hurricane Wilma in October 2005.  Wilma was also the last hurricane to make landfall in the state of Florida.

August in Washington, D.C.

August is the third and final month of “meteorological summer” and remains quite warm in much of the United States.  However, Washingtonians eager for the cooler weather of fall, should like August’s downward progression of average temperatures.  Washington, D.C.’s average high and low temperatures on August 1 are 88 and 71 degrees, respectively, while they fall to 84 and 68 degrees by month’s end.  Official weather observations for Washington, D.C, are measured at National Airport.   

The highest annual average high temperature in Washington, D.C. is 89 degrees from July 7 through July 22.  After that, average temperatures start to decline.  At the same time, hours of daylight continue to contract during the month of August.

August can still be quite hot in Washington, D.C. with triple-digit heat occurring during five different years since 1997.  For example, the 105 degree temperature on August 17, 1997 is the hottest temperature on record at National Airport (it occurred again more recently on July 7, 2012) according to the National Weather Service.  However, the hottest temperature on record in the Nation’s Capital is 106 degrees which occurred twice before National Airport opened in 1941.  

By comparison, the coolest temperature on record during the month of August in Washington, D.C. is 49 degrees, which occurred most recently on August 29, 1986.  Thirteen of the last 20 Augusts have been warmer than average in Washington, D.C. The longest heat wave of 2016 continued yesterday with a high temperature of 91 degrees.  That was the 13^th consecutive day at or above 90 degrees and the 30^th of 2016.  Washington, D.C. averages 36 days at or above 90 degrees for the entire year.

August averages 3.44” of rain.  The wettest August on record in Washington, D.C. was in 1928 when 14.41” fell.  On the opposite end of the spectrum, Washington, D.C.’s driest August was in 1962 when just 0.55” of rain fell.  Decaying tropical systems sometimes impact the Mid-Atlantic Region in August, such as Hurricane Irene in 2011.  It brought the Nation’s Capital beneficial rain after initially making landfall in North Carolina’s Outer Banks.  NOAA’s outlook for August calls for an equal chance of above or below normal temperatures and precipitation in the Mid-Atlantic Region.  That means near average conditions can be expected in the Nation’s Capital this month.