D.C.’s Infamous 2012 Derecho

 

June 2012 Derecho (Source: NOAA)

Today marks 11 years since one of DC’s most destructive severe weather events.   It’s safe to say that prior to the evening of June 29, 2012, most Washingtonians had never heard the meteorological term “derecho.”  That date will forever live in weather infamy in the nation’s capital because a “derecho” knocked out power to more than a million people in the DC Metro Area. 

D.C.’s hottest June day on record was June 29, 2012 with a high temperature of 104°.  There was also a high degree of atmospheric instability, which helped create the ideal environment for severe thunderstorms.  Consequently, area meteorologists grew increasingly apprehensive as a cluster of severe thunderstorms developed over eastern Iowa and northern Illinois on the morning on June 29.  Thunderstorms within the cluster quickly reached severe limits and tracked eastward as the day wore on. 

“Derechos” are a type of squall line.  A “squall line” is a line of thunderstorms that can become severe, with the primary threat often being wind gusts of at least 58 mph.  “Derechos” are longer lasting squall lines that travel at least 240 miles, according to NOAA’s Storm Prediction Center.  Wind gusts in a derecho are typically higher than 58 mph but less than 100 mph.  Such was the case when the June 2012 derecho passed through the Mid-Atlantic Region.  The highest wind gusts in the D.C. Area were 71 mph at Dulles Airport, 70 mph at National Airport, and 66 mph at BWI Airport. 

Overall, the highest wind gust in the 2012 derecho was 91 mph in Fort Wayne, Indiana.  That is equivalent to the wind in an EF-1 tornado.  Unlike other types of thunderstorms, the primary type of damage that occurs in derechos are from straight-line winds.  That’s generally easy for experts to determine as the majority of damage occurs in linear fashion. 

The June 29, 2012 derecho came through the D.C. Metro Area between 9:00 – 11:30 at night.  It knocked down countless trees and power lines, which created one of the largest power outages on record.  Many roads were impassable and treacherous.  Normally, it takes me less than 20 minutes to drive home after work, but it took more than an hour to navigate the roads that night.  According to NOAA, there were 13 fatalities during the event combined with an estimated total of 4 million customers without power for a week, from the Midwest to the East Coast.

Not until the next morning, a Saturday, did Washingtonians fully appreciate how damaging the derecho was.  The extensive tree damage even caused Saturday’s round of the AT&T National golf tournament at Congressional Country Club in Bethesda, Maryland, to be closed to the public.  Adding to the hardship of the widespread power outages was an extended heat wave with highs of at least 95° at National Airport every day through July 8.

How cool has June been ?

 

A June day at the Avenel Pool in Potomac, Maryland

D.C. residents who don’t like extreme heat should be pleased that another June will pass without any triple-digit heat.  Not only that, but June 2023 has been an unusually cool month in the nation’s capital.  Only four of the first 24 days this month have been warmer than average.  That’s helped produce a monthly temperature 2.6° cooler than average. 

There have been more days with highs in the 70s (four) than days in the 90s (two) since June 1.  The nation’s capital hasn’t finished with fewer than five 90° June days since 2014.  That’s significant since the nation’s capital has averaged between seven and eight 90° June days over the last 30 years, according to NOAA data.  It has also averaged one day of triple-digit June heat roughly once every six years over the same time period.  However, D.C. residents haven’t experienced 100°/+ heat in June since 2012.

Triple-digit heat is relatively rare during any month in the nation’s capital, typically occurring only every three to four years with none since August 2016.  It’s important to distinguish between the actual air temperature and feels-like temperature.  The “heat index” is what it feels like when you combine the air temperature with the relative humidity. 

On very humid days, it feels hotter and more oppressive compared to less humid days.  The heat index exceeds the century mark several times over the course of a typical D.C. summer, but the actual air temperature rarely touches 100°.  D.C.’s hottest temperature on record is 106° and has occurred twice, most recently on July 20, 1930.  D.C.’s second hottest all-time temperature is 105° and has also occurred only twice, on July 7, 2012 and August 17, 1997. 

Although weather records in the nation’s capital date back to 1871, the more significant date to remember is 1941.  That’s when National Airport opened and where weather records have been kept ever since.  Prior to that, weather measurements were made downtown.  Conditions are quite different between the two locations because one has the moderating influence of the Potomac River and the other has more of an urban heat island effect.

There were some very hot summers in the 1930s prior to when National Airport opened.  However, a key difference between then and now is that urbanization has increased dramatically.  Consequently, overnight low temperatures remain much warmer than they used to.  That’s why the nation’s capital has had many of its warmest months on record over the last 10 to 20 years.  For example, June 2010 was D.C.’s warmest on record with the highest number of 90° days (18). 

Weather Quiz

 

Washington, D.C. (June 2016)

True or False.

Washington, D.C. has felt significant impacts from tropical systems during the month of June.