How unusual is triple-digit heat in the nation's capital?

 

DC's 7-day forecast (Source: The Weather Channel)

The nation’s capital is poised to experience one of its most intense heat waves in recent memory. High temperatures could reach or exceed 100° on four consecutive days starting this afternoon. Since triple-digit heat is so rare in the nation’s capital, having multiple consecutive 100-degree days would be an historic event. 

July is typically the hottest month of the year in the nation’s capital. As such, it isn’t surprising that triple-digit heat occurs most frequently in July. This century, Washington, D.C. has averaged one day of triple-digit July heat every other year, one such day every third August and one June day of triple-digit heat roughly every five to six years. This summer has already had one 100-degree day with a high of 100° on June 12. 

Triple-digit heat also tends to be feast or famine in the nation’s capital. For example, before the summer of 2024, that had six days of triple-digit heat, there hadn’t been any since 2016. The month with the highest overall number of 100-degree days in the nation’s capital remains July 2012 (7). However, the summer with the most 100-degree days remains 1930 with 11.

A heat wave is loosely defined as a minimum of three consecutive days that high temperatures reach at least 90-degrees. There have been multiple heat waves in the nation’s capital that didn’t have any triple-digit heat. The most notable such heat waves that occurred recently were a 12-day stretch in July 2019 and a 20-day stretch in 2020. However, while those heat waves were known primarily for their longevity, the upcoming heat wave will be known more for its intensity.

The typical summertime weather pattern in the United States is conducive for heat waves in the DMV (DC, Maryland and Virginia). That's when a semi-permanent area of high pressure sets up off the southeast coast of the United States. It's sometimes referred to as the "Bermuda High" since that's where they are often centered. The clockwise winds around this seasonal area of high pressure usher hot and humid air northward from the deep south into the Mid-Atlantic Region.

Tomorrow and Friday, July 2 and July 3, are setting up to be the hottest days in the DMV. That’s when temperatures are most likely to reach the triple-digit mark.

Longest streak of consecutive 100-degree days in Washington, D.C. (Source: NOAA)

Four Days

July 14 – 17, 2024
July 5 – 8, 2012
July 19 – 22, 1930

Three Days

August 13 – 15, 2016
July 8 – 10, 1993
July 15 – 17, 1988
August 3 – 5, 1930

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.