How Hot Has August Been?

August is well positioned to finish as one of the warmest of all time in the Nation’s Capital – and that’s following the sixth warmest July on record.  The first half of August broke a slew of hot weather records across the Mid-Atlantic Region and northeastern United States.  Washington, D.C. has had four days of triple-digit heat this summer, including three consecutive days from August 13 – 15.

Since just one day of triple-digit heat is infrequent in the Nation’s Capital, having three consecutive days of 100 degree heat is quite remarkable.  In fact, this week’s streak of temperatures at/above the century mark ties it for the second longest streak on record.  The longest streak of consecutive days at/above 100 degrees in Washington, D.C. is four days and occurred most recently on July 5 – 8, 2012, according to the National Weather Service.  

This week’s stretch of three consecutive days above 100 degrees was the longest such streak to occur in August since 1930.  You have to go back to 1997 to just get consecutive August days at/above 100 degrees in Washington, D.C.  By coincidence, the second of two consecutive 100 degree days occurred 19 years ago today when Washingtonians sweated through a record high of 105 degrees.  The combination of record high temperatures and uncharacteristically high humidity creates particularly dangerous heat that necessitates heat advisories and excessive heat warnings. 

No records are kept of “heat index” temperatures – the “feels-like” temperature when you combine temperature and relative humidity – but the heat index in the Nation’s Capital in the last week has rivaled the 122 degree heat index that occurred on July 16, 1980.  More recently, on July 22, 2011, the heat index reached 121 degrees.  Those dates are two of the highest observed heat index temperatures in Washington, D.C.  

Since July 24, Washingtonians have endured seven days of low temperatures at/above 80 degrees, producing six record high low temperatures and tying the 2011 record.  Such warm overnight temperatures have contributed to an average monthly temperature (combining daily highs and lows) of 83.8 degrees in Washington, D.C. through August 16.  This is significantly warmer than the 78.1 degree average at National Airport. 

Records kept by the National Weather Service indicate another unprecedented event happened this summer.  There have been two streaks of at least seven consecutive days with temperatures at/above 95 degrees – from July 22 – 28 and the current streak that began August 10.  Yesterday was also the sixth consecutive day of at least 97 degrees in Washington, D.C. which ties for second longest streak since 1953.  Fortunately, starting tomorrow there is going to be a change in the weather pattern with an end to the extreme heat in the Mid-Atlantic Region.

Washington, D.C.’s Warmest Augusts (Degrees Fahrenheit - Source: NWS)

1.  82.8 (1980)

2.  81.3 (1995), (1978)

4.  81.1 (2002)

5.  81.0 (2012, 1983)

7.  80.7 (1988)

8.  80.6 (2006)

9.  80.3 (1959)

10.  80.2 (2010)

Historical August Heat

The Beach is a Wonderful Place to Cool Off

Although Washingtonians experienced their first triple-digit heat in four years last month, the current heat wave has been even more extreme with three consecutive days of temperatures at record levels in the Nation’s Capital.  A number of other cities across the eastern United States including Hartford, New York City and Atlantic City also set record highs over the weekend.  Even if temperatures don't reach the century mark for a third consecutive day, the combination of hot temperatures and oppressive humidity will make for heat index temperatures well over 100 degrees for Mid-Atlantic residents today and tomorrow.

Washington, D.C. has had an extreme heat wave with temperatures at or above 95 degrees every day since August 10.  A record high was tied Saturday at National Airport when it reached 99 degrees.  Also, high temperatures reached 101 degrees on Saturday, August 13, tying the daily record and just yesterday the record high of 100 degrees was also tied.  That was the first time since August 16 -17, 1997, that Washingtonians have experienced triple-digit heat on consecutive days in August.

The unusually high heat index temperatures have made the current heat wave especially dangerous.  As I wrote about in Friday’s column, the “heat index” is the feels-like temperature when you factor in relative humidity and air temperature.  Given the very tropical-like dewpoints in the mid-70s for much of the last few days, heat indices of 110 to 120 degrees have been common across the DC Metro Region.  That caused the National Weather Service to issue heat advisories from Virginia to New England, illustrating the danger of the record-setting temperatures.  An excessive heat warning was also issued for Washington, D.C. over the weekend for the first time since 2013. 

Through August 14, Washingtonians have had 10 days at or above 90 degrees this month including 5 days above 95 degrees and 2 days of triple-digit heat.  The Nation’s Capital hasn’t had two days of triple-digit heat in August since 2002 (though not on consecutive days).  Since the hot summer of 1993, Washington, D.C. has averaged approximately 10 days of 90-degree heat and 3 days of 95-degree heat in August with triple digit heat roughly every three years.  We’ve already exceeded August’s average number of 95 and 100 degree days and today will be the eleventh day above 90 degrees.  

Making this summer incredible is the fact that it’s been so hot since July 5.  Despite having only 5 days at or above 90 degrees through Independence Day, Washingtonians still had their sixth warmest July on record with temperatures at or above 90 degrees on 23 of the final 27 days of the month.  In a given year, Washington, D.C. averages 36 days of at least 90 degrees with 38 such days so far this year.  

Weather Quiz

Heat Advisories from Virginia to New England today (Source: NOAA)

 

 

 

 

Has triple digit heat occurred more frequently in June, July or August during the last 25 years in Washington, D.C.?

A.  June

B.  July

C.  August

D.  A tie between two of the months.

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Answer to Weather Quiz question from July 25.

B.  Although all of these are hot scenarios, the National Weather Service defines a “heat wave” as any stretch of three or more consecutive days at or above 90 degrees.  Washingtonians are currently experiencing their sixth heat wave of 2016.

Dry Heat vs. Oppressive Heat

"Heat Index" will Be Tough This Weekend (Source: WUSA9)

As Washingtonians endure their sixth heat wave of 2016, some may say the current heat wave is the worst.  Even if temperatures don’t reach the century mark during the next several days, the high “heat index” temperatures are what’s making the current heat wave especially dangerous.  The “heat index” is the apparent or feels-like temperature when you combine the air temperature with relative humidity.

The higher the relative humidity is, the worse high temperatures feel.  Humidity is relative to a given temperature and meteorologists use the dewpoint temperature to calculate RH.  Dewpoint temperatures above 70 degrees are indicative of an oppressive air mass.  A natural cooling mechanism is evaporation of one’s sweat.  However, whenever it’s more humid out, the body can’t effectively cool itself and this makes the heat especially dangerous.    

Such is the case during the current heat wave because heat indices the next several days are going to be above the century mark across the majority of the Mid-Atlantic Region.  Although records aren’t kept for high heat index temperatures, triple-digit heat indices are somewhat rare in the Mid-Atlantic Region.  One of the highest heat index temperatures observed in the Nation’s Capital according to the National Weather Service occurred on July 16, 1980 when an air temperature of 103 degrees and a dewpoint temperature of 78 produced an astonishing heat index of 122 degrees.

The hottest days in the Nation’s Capital are when the wind blows primarily from the west.  That’s a slightly drier wind as opposed to southerly winds that usher in more humid air.  The official weather reporting site for Washington, D.C. is National Airport and a southerly wind there tends to keep temperatures slightly cooler as the wind blows off the Potomac River.  Southerly winds are poised to keep the oppressive humidity levels in place until early next week.  The more humid conditions so common in the eastern United States during the summer months serve a purpose, too, in making triple-digit heat relatively rare.  More humid air may feel worse, but higher relative humidity also prevents the air temperature from rising as high as a drier air mass.

Unlike the arid desert Southwest where triple-digit heat is a daily occurrence during the summer, triple-digit heat occurs only every few years in the Nation’s Capital.  Prior to July 25, it hasn’t been 100 degrees in Washington, D.C. since 2012.  Minimizing time spent outdoors and drinking extra water is especially important during such extreme heat waves.  My colleagues and I on the WUSA9 weather team will keep you apprised of the latest weather conditions both on-air and online.