Summer-like Heat on Tap

Heat Index  (Source: NOAA)

Summer officially arrives in the northern Hemisphere during the month of June.  Although DC’s highest average temperatures don’t occur until July, some of the hottest weather on record in the Nation’s Capital has occurred in June.  No correlation exists between a hotter or cooler than average month of June in Washington, D.C., and what the rest of the summer will be like. 

Since 1993, Washingtonians have averaged more than 7 June days of 90° heat, nearly 2 days of 95° heat with 100° heat in June roughly every five years.  June has been hotter than average in Washington, D.C. in nine of the last ten years, but seven of the previous ten Junes, dating back to 1997, were cooler than average.  That helps illustrate that above or below average monthly temperatures tend to be cyclical.  For example, from 2010 through 2012, June averaged 13 days at/above 90° while from 1997 through 2000 Washingtonians saw an average of only 6 days at/+ 90°.

June 1994 ranks as the second hottest on record in the Nation’s Capital with 17 days of at least 90°, including a stretch of 14 consecutive 90° days.  However, that was the hottest stretch of the summer and included a record high temperature of 101° on June 15.  That would be the last 100° in Washington, D.C. for more than three years.  Washingtonians didn’t experience their first 90° heat until June 11 last year, with only five 90° through June 30, 2016.  However, from July 1 through September 30, 2016, the Nation’s Capital sweat through 53 days at/above 90° including 4 days of 100° heat.

The hottest June temperature on record in Washington, D.C. is 104° (that occurred on June 29, 2012).  However, that’s narrowly below DC’s overall record high temperature of 106° that occurred once each in 1918 and 1930.  More recently, June has seen bouts of extreme heat such as in 2010, which surpassed 1994 as being DC’s warmest June on record.  While there was only one more June day of 90° heat (18) than occurred in June 1994, what made the difference were the warmer overnight low temperatures.  That essentially means overnight low temperatures didn’t cool off as much in June 2010, as they did 16 years earlier.

Washingtonians are poised to experience their first heat wave (3 or more consecutive days at/above 90°) starting on Sunday.  Some record high temperatures will likely be approached as well but no 100° is expected at this point.  However, the combination of temperatures in the 90s and higher relatively humidity will make the heat index temperature meet or exceed the century mark.

                                                        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)

Watch out for Severe Weather in June

June has the second highest average annual number of tornadoes (243).  Severe weather is defined as a thunderstorm that has any of the following: hail 1” in diameter or greater, wind gusts 58 mph/+ or a tornado.  The Mid-Atlantic Region has seen many severe weather outbreaks during the month of June.  Below are some of the more significant ones that have occurred locally.

2016: On June 21, there were several dozen severe weather reports that included large hail, high wind and a tornado.  An EF-0 tornado with estimated wind speeds of 80 mph traveled more than 12 miles in Howard County, Maryland, where minor damage occurred.  Hail nearly 2” in diameter fell at Dulles Airport and several other locations in the DC Metro Area.  Also, last June 16 there were several cases of hail at least 2” in diameter in north central Virginia, including the town of Bluemont in Loudoun County.

2013: A total of six EF-0 tornadoes developed on June 13 in the DC Metro Area.  Although they were all relatively weak, some occurred in heavily traveled parts of Montgomery and Charles Counties in Maryland and caused minor damage.  This was one of the wettest Junes on record in the Nation’s Capital, with a monthly total of 9.97” of rain at National Airport.

2012:  The June 29 Derecho was one of the most destructive events on record from parts of Illinois to the Mid-Atlantic Coast.  The DC Metro Area saw peak wind gusts of 71 mph at Dulles Airport, 70 mph at National Airport and 66 mph at BWI Airport.  This caused widespread damage and power outages.  The month began with a series of tornadoes on June 1.  However, none of the tornadoes were as destructive as the Derecho was at month’s end.

2008:  One of the more active severe weather days on record in the Nation’s Capital occurred on June 4.  Unlike Derecho day on June 29, 2012, that featured a singular destructive event, there were multiple rounds of severe weather on this date.  The first round was a powerful squall line that passed through the DC Metro Area in mid-afternoon.  Later that evening, several supercell thunderstorms developed and some produced tornadoes.

The Arrival of Meteorological Summer

What can DC area residents expect now that meteorological summer has arrived?  Some of the hottest weather in the Mid-Atlantic Region has occurred in June.  In Washington, D.C., average high and low temperatures rise from 80°/62° on June 1 to 88°/70° on June 30. 

June’s hottest recorded temperature in the Nation’s Capital is 104° which occurred relatively recently on June 28, 2012.  That was the last time Washingtonians experienced 100° heat in June.  The earliest in the season it’s been 100° in the Nation’s Capital was on June 5, 1925.  Meanwhile, DC’s coolest June temperature remains the 43° which occurred on June 2, 1897.  While weather records in Washington, D.C. date back to 1871, measurements have only been made at National Airport since it opened in 1941. So, the 104° and 43° record June temperatures were measured in different locations.

The Nation’s Capital averages 3.78” of rain in June.  Twelve of the last 20 Junes have been wetter than average.  The remnants of a tropical system occasionally bring the Mid-Atlantic Region significant rainfall in June, such as Agnes in 1972.  Severe weather can also occur during the month of June in the Mid-Atlantic Region.  One of the strongest tornadoes on record in the state of Maryland occurred in Frostburg on June 2, 1998.  Also, the infamous “derecho” occurred on June 28, 2012.

Washingtonians have had a rainfall deficit since August 2015.  Fortunately, the wetter than average month of May 2017 made up some of that deficit and eliminated the local drought conditions.  It was the first wetter and cooler than average month in Washington, D.C. since May 2016.  That’s despite DC experiencing its hottest May day in nearly six years on May 19 when it was 93°.  Last month was also the fifth wettest May on record at Dulles Airport (8.49”).

There is no correlation between a cool and wet May and what June’s weather will be like.  Washingtonians had a cooler and wetter than average May last year, but last summer was one of the hottest on record.  Also, a wetter than average June doesn’t foreshadow what the entire summer will be like.  DC’s wettest June on record (14.02”) occurred in 2006, but the rest of the summer was drier than average.  NOAA’s outlook for June is for near average temperatures in the Mid-Atlantic Region with the potential for above average precipitation.