Some Perspective on the Summer of 2015

A Beautiful July Sunset!

Washington, D.C.’s longest heat wave in several years concluded this past week when National Airport’s 13 day streak of 90 degree heat ended on August 5.  Not since 2011 had there been a streak of more than 11 consecutive days with high temperatures of at least 90 degrees.

The hottest temperature measured during this nearly two week stretch was 96 degrees on August 4.  Although this summer’s total of 38 days of 90 degree temperatures is the highest since 2012, it’s near the seasonal average of 36 days in the Nation’s Capital.  While 2015 has featured appreciably more 90 degree heat than last year (when there were only 24), no record highs have been set this summer.

On the other hand, Baltimore has seen only 17 days of at least 90 degrees and Dulles Airport has seen only 11 such days.  While National Airport’s longest “heat wave” of 2015 was the recent 13 day stretch with temperatures greater than 90 degrees, Dulles and BWI Airports haven’t seen more than three consecutive days of at least 90 degrees this year.  This summer’s heat in the Nation’s Capital can at least partially be attributed to the “urban heat island effect."

July’s average temperature at National Airport was almost two degrees warmer than average at 81.5 degrees, while at Dulles Airport it was only 76 degrees (almost a degree cooler than average).  Baltimore’s average temperature in July was exactly average at 77 degrees – combining daily highs and lows.  Although being away from major urban areas helps keep those locations cooler than downtown Washington, D.C., more study is needed as to why the temperature is consistently so much warmer at National Airport (the official weather reporting site for the Nation’s Capital).  

August often features an appreciable amount of hot weather, but average high temperatures gradually fall from 88 degrees to 84 degrees by the end of the month.  So Washingtonians looking forward to fall should be able to notice a difference to their liking as the month wears on.  That’s not to say that conditions won’t remain ideal for a trip to the beach until well into September.  My colleagues and I on the WUSA9 weather team will keep you apprised of the latest weather forecasts – including when any 90 degree temperatures may return this summer. 

Weather Quiz

Hurricane Rita as a Category 5   (Source: NASA)

What was the last Category 5 (as strong as hurricanes get with sustained winds of at least 156 mph) to make landfall in the United States?

A.  Katrina

B.  Rita

C.  Andrew

D.  Isabel

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Answer to Weather Quiz question from July 24.

True.  Through May 31 (the latest date for which accurate data is available) there have been a confirmed 578 tornadoes nationally.  Over the last few years, data from NOAA’s Storm Prediction Center indicates the United States has averaged 476 tornadoes from January 1 through May 31.  So, although tornadoes have occurred at an above average pace during the first few months of 2015, we’ll have to see what happens during the rest of the year.

What is August like in the Nation's Capital?

A Great Place to Cool Off!  (Great Falls, MD) 

As some may know, August is the third and final month of “meteorological summer.”  Although average high temperatures fall during the month, August is still quite warm across the Mid-Atlantic Region.  While July is the hottest month on average in the Nation’s Capital, every so often the month of August is hotter and that happened as recently as 2009.

The average high at National Airport on August 1 is 88 degrees, but that falls to 84 degrees by August 31.  Official weather observations for Washington, D.C have been measured at Reagan National Airport since it opened in 1941.  Washington, D.C.’s hottest August temperature on record is 106 degrees on August 6, 1918 and on July 20, 1930.  The last time there was triple digit heat during the month of August in the Nation’s Capital was in 2011.  August’s coolest temperature at National Airport is 49 degrees.  That’s happened three times – as recently as August 29, 1986. 

According to the National Weather Service, 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, the driest August on record was in 1962 when only 0.55” of rain fell.  The tropics typically get more active during the month of August and the Mid-Atlantic Region sometimes sees enhanced rainfall as a result.  Many Washingtonians will recall the heavy rain, high winds and coastal flooding caused by Hurricane Irene in late August 2011.

Today will be the 11^th consecutive day with temperatures of at least 90 degrees in the Nation’s Capital – the longest such stretch since 2012.  Fortunately, this summer has also had above average rainfall in addition to above average temperatures, making the heat less taxing on area reservoirs and gardens.  Since June 1, Washington, D.C. has a rainfall surplus of 9.33”.  As this week progresses, temperatures will cool to more seasonal levels in the 80s as the prospect of showers and thunderstorms return.

Just how Warm was July 2015 in the Nation's Capital?

Cooling off at the beach, Pasadena, MD

July’s average monthly temperature of 81.5 degrees (combining daily highs and lows) has made it the hottest month in the Nation’s Capital since July 2012.  That also happens to be the last time any triple digit heat occurred in Washington, D.C.

July 2015 featured 14 days with temperatures of 90 degrees and concluded with Washington, D.C. in the midst of its longest heat wave of the summer.  Daily high temperatures have been at least 90 degrees since July 24 and are expected to continue to be at least that warm well into next week. 

Meteorologists define a “heat wave” as any three-day period with temperatures of 90 degrees or hotter.  So, with a running total of 8 such days we have exceeded the minimum definition by a considerable margin.  The previous longest heat wave of the summer was a six-day stretch in June.  If the forecast comes to fruition the next few days, Washingtonians will see their longest heat wave since the summer of 2011.

The hottest temperature to occur in the Nation’s Capital this year was 98 degrees on July 19, while July’s coolest temperature measured at National Airport was 66 degrees.  Longtime Washingtonians may remember the summer of 2000 as being significantly cooler than average.  That was also the coolest July on record at National Airport with a monthly average temperature of 74.7 degrees.  According to the National Weather Service, July 2015 was also wetter than average with a monthly total of 5.01” of rain at National Airport compared to an average of 3.73”.

As we turn the calendar to August, will the warmer and wetter than average weather continue in the Mid-Atlantic Region?  The Climate Prediction Center is calling for near average temperatures during the month of August in this part of the country.  However, triple digit heat has occurred as recently as 2011 during the month of August, so we’ll have to see what happens.  Fortunately, my colleagues and I on the WUSA9 weather team don’t have any triple digit heat in the forecast during the next seven days.