What is a “Heat Wave?”

Keeping Cool on the Water

A “heat wave” is loosely defined by most people as a few hot days.  That’s a rather subjective definition as no two people are likely to have the same definition of what constitutes “hot” weather.  So it’s important to use the official definition of a heat wave from the National Weather Service. They define a “heat wave” in the Nation’s Capital as any stretch of 3 or more consecutive days at or above 90°. 

Today is the second day out of what will become the first heat wave of 2017 in the DC Metro Area.  In fact, record heat is possible both today and tomorrow as high temperature approach the mid to upper 90s.  Official weather measurements for Washington, D.C. are made at National Airport (DCA).  Located on the Potomac River, temperatures there can be influenced by the water temperature. 

That’s why wind direction is an important factor to consider regarding National Airport temperatures.  For example, a southerly wind tends to keep temperatures in the spring and summer a few degrees cooler as the wind blows off the relatively cool waters of the Potomac River.  However, a westerly wind largely negates any influence of the Potomac River on DCA temperatures.  While the Potomac has a cooling influence on temperatures during the summer months, it has the opposite effect in the winter.  Consequently, there is often a difference of several degrees between temperatures at National Airport and areas downtown when the wind is blowing off the Potomac River.

Area residents will be lucky to have the cooling influence of being on or near the water today as temperatures approach the record high of 95° this afternoon.  It’s true that today’s record is the lowest record high temperature of the entire summer (June 1 – August 31) in the Nation’s Capital so that makes it an easier record to eclipse.  The Nation’s Capital has twelve June record highs of 100° or hotter.  Temperatures aren’t forecast to reach the century mark this week, but the combination of temperatures in the 90s with higher dewpoints and relative humidity will make the heat index feel at or above the century mark on Tuesday.

That means drinking extra fluids and staying indoors in the air conditioning as much as possible is extra important for people and their pets.  This week’s heat wave is only expected to last a few days which while significant for being 2017’s first such stretch of hot weather in the Nation’s Capital, it isn’t an historic heat wave.  June’s longest heat wave remains a 14-day stretch of 90°/+ heat in 1994.  Overall, the last heat wave to occur in Washington, D.C. was on September 6 – 10, 2016.

                                                  Record High Temperatures

June 12

DCA: 95° (2002)
IAD: 96° (1986)
BWI: 96° (1986)

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June 13

DCA: 96° (1954)
IAD: 94° (1994)
BWI: 97° (1956)