Remembering DC’s infamous derecho

 

The infamous June 2012 derecho (Source: NOAA)

Today marks 14 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 DMV (DC, Maryland and Virginia) 

June 29, 2012 was already a memorable day in the nation’s capital having featured DC’s hottest June temperature on record of 104°. However, there was also a high degree of atmospheric instability and that 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 DMV. Severe wind gusts were observed at all three D.C. Area airports during the derecho, including: 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 across the DMV and created one of the largest power outages on record. Many roads were impassable and treacherous. Normally, it took 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 up a week, from the Midwest to the U.S. 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.

Could a warm and dry June predict the rest of DC’s summer weather?

 

A June day in the nation's capital

June is the third hottest month of the year on average in the nation’s capital. Washingtonians know there have been some significant June heat waves in recent decades, as well as some exceptionally wet weather. However, June’s weather isn’t always a good harbinger of what the entire summer’s weather will be like.

Longtime Washingtonians may recall the summer of 1994. That June featured a 14-day heat wave with temperatures of at least 90° from June 13 – June 26. The hottest day was June 15 when a record high of 101° occurred. This was DC’s hottest stretch of weather during the entire 1994 summer. June was also warmer than August in 1994. June being warmer than August is a relatively rare occurrence in the nation's capital having happened only four times since 1994. 

The summer of 2016 was another standout season for DMV (DC, Maryland and Virginia) residents since the spring and first third of June were rather cool. The nation’s capital didn’t experience its first 90-degree day of 2016 until June 11. That’s several weeks after DC’s average first 90-degree day of the year occurs around May 18.

However, despite the cool start to the 2016 summer in the nation’s capital, a hard pivot to hotter weather occurred and DC went on to have its fourth hottest summer on record. Multiple days of record, triple-digit heat occurred in July and August 2016.

A wet or dry June also isn’t a strong indicator of what might occur in July and August. For example, Washington, D.C. had 14.02” of rain in its wettest June on record in 2006, but was followed by below average July and August rainfall. Meanwhile, June 2017 was DC’s third driest, but was followed by wetter than average conditions in July and August. 

This June is well-positioned to finish as a warmer and drier than average month in the nation’s capital. NOAA’s Climate Prediction Center expects July to also be a warmer than average month in the nation’s capital, but with near average rainfall. 

DC’s extreme June weather

 

A summer's day in Washington, D.C.

The nation’s capital has seen no shortage of extreme June weather over the last two decades. Some have been very rainy while others have been quite hot and dry. If June were to end today, then it would rank among DC’s warmest and driest on record.

Five of DC’s 10 warmest June’s have occurred since 2008. Official weather records in the nation’s capital date back to 1871 and are kept by NOAA. Having five of the 10 warmest June's so recently is a remarkable feat. What has contributed to make so many of DC’s warmest June’s occur relatively recently, in addition to a changing climate, is a more pronounced urban heat island effect.

It wasn’t until recent decades that the DMV (DC, Maryland and Virginia) has become as urbanized as it has with so much ground covered by buildings and roads with automobile traffic. Warmer overnights are largely what has led to many of our warmest months in recent decades. That’s because urban areas don’t cool off as quickly or as much as rural areas that have more green space with fewer buildings and roads. Not surprisingly, the most recent of DC’s Top 10 coolest June’s occurred more than 50 years ago in 1972.

Three of DC’s five wettest June’s have also occurred in the last 20 years. While it might make sense that a rainier June could also be cooler than average, that’s not always the case. For example, June 2015 finished as DC’s second wettest, but was also DC’s seventh warmest with 12 days of 90-degree heat. The nation’s capital has averaged between seven and eight 90-degree June days over the last 30 years, according to NOAA data.

Higher temperatures can hold more water vapor which means warmer weather can also feature rainier days. June 2015 had three days that more than 2” of rain occurred. As such, roughly 64% of DC’s rainfall in June 2015 occurred on those three days. On the opposite end of the spectrum were June 2024 and June 1976. Each of those months rank among DC’s Top 10 warmest and Top 10 driest June’s.

DC’s wettest June’s (Source: National Weather Service)

1. 14.02"  (2006)
2. 11.94"  (2015)
3. 11.53"  (1972)
4. 10.94"  (1900)
5. 9.97"   (2013)
6. 8.55”   (1883)
7. 7.87”   (2003)
8. 7.53”   (1916)
9. 7.41”   (1929)
10. 7.40”   (1968)

 

DC’s coolest June’s (Source: National Weather Service)

1. 65.9°  (1907)
2. 67.0°  (1903)
3. 68.6°  (1927)
4. 69.1°  (1926, 1878)
5. 69.6°  (1897)
6. 69.7°  (1916, 1910)
7. 69.9°  (1886)
8. 70.2°  (1972, 1955)
9. 70.4°  (1912)
10. 70.6°  (1915)