Hottest Weather of the Summer?

Washingtonians are in the midst of the hottest weather since last August.  Today is poised to be the hottest day of the current heat wave with temperatures reaching the upper 90s in much of the DC-Baltimore corridor.  A “heat wave” is defined as any stretch of three or more consecutive days at or above 90°.  So far this year, Washingtonians have sweat through 20 days of at least 90°.  The Nation’s Capital didn’t experience day 20 of 90° last year until July 23.  

Sultry conditions in the DC Metro Area are common during the summer months.  Making the above average temperatures even worse is the high relative humidity that will help create a dangerous heat index of greater than 105° in many spots this afternoon.  That’s prompted the National Weather Service to issue a heat advisory for areas near and east of I-95 in the Mid-Atlantic Region today.

Heat advisories have been fairly recent occurrences in the last 10 years, with multiple hot summers and heat waves (particularly from 2010 – 2012 and in 2016).  Washington, D.C. averages 36 days of at least 90° in a given year, while the record is 67 such days.  For example, there were 58-90° days last year.  Days with temperatures of at least 95° are less common and 100° is rarer still in the Nation’s Capital.  That’s what makes the current heat wave so significant.  

Since weather measurements for the Nation’s Capital are taken at National Airport on the Potomac River, the wind direction sometimes plays an important role in temperatures.  If the wind blows out of the south at National, then that’s off the water and can keep temperatures a little cooler than they would be elsewhere.  A south wind would contribute to slightly higher dewpoints and relative humidity.  On the other hand, drier air heats up and cools off more than humid air.  So if the air is slightly drier, than the chances increase for 100° temperatures at National Airport.  While today’s record high at National Airport is 100° from 1954, the last time it was 100° there was August 15, 2016.

Area residents know that very humid conditions are common in this part of the country during the summer months.  That can also contribute to heavy rainfall.  Following the third driest June on record, DC had 2.27” of rain so far this month.  That’s more than half as much as the 3.13” of rain that occurred during all of last July.  More scattered showers and thunderstorms are likely tomorrow across the Mid-Atlantic Region.  Although this weekend will be less humid, 90° temperatures and the current heat wave can continue through Sunday.