Even though summer is in its waning days, heat is expected to persist. Nineties are currently in the forecast into, and perhaps through the upcoming weekend in D.C.
This late-summer torch comes as part of an extensive heat dome of high pressure gripping the eastern two-thirds of the country.
Across the Lower 48, about 75 million people are under heat advisories and excessive heat warnings from the eastern Plains, to the Midwest and parts of the Southwest and Northeast. Temperatures in hot regions east of the Rockies were expected to rise to the mid-90s or around 100, with heat indexes as high as 105. Around Phoenix, temperatures may push 115 by Friday.
How 2021 stacks up so far
The number of hot days per year in D.C. has increased in recent decades. At the turn of the 20th century, the annual average of 90-degree days was about 25. In the mid-1970s, D.C. might have expected about 33 days so hot. From 2010 to 2020, the 90-degree average is about 49 days.
There are several reasons for the increase in hot days, most notably human-induced climate change and urbanization. Urbanization has been relatively limited near the record-keeping site in recent times, but it is probably still a growing factor given development across the area.
So far, 2021 appears to fit this increasing trend. With 42 days at or above 90 degrees, 2021 tied for the 17th most 90-plus degree days from the Jan. 1 to Aug. 25 period since 1871.
Despite all the 90-degree days, 2021 hasn’t necessarily been an exceptionally hot climatological summer (June through August) when comparing to recent years.
With an average summer temperature of 79.2 degrees through Tuesday, the city was sitting right in the middle of summertime standings from 2000 onward. Last summer was a full two degrees warmer at this point. It finished the fifth-warmest climatological summer. The top five hottest summers on record, again to the 1870s, are now all between 2010 and 2020.
Over the long term, Washington was running tied for 16th-warmest summer to date through Tuesday.
The upcoming outlook
The short term remains scorching as the sprawling heat dome controls the weather in our region for the rest of this week and into the weekend.
By next week, the city will most likely experience at least another five days at or above 90. This is in line with what we might typically expect through the rest of the season based on recent averages.
Temperatures are poised to reach the mid-90s for the rest of the workweek, before dipping slightly this weekend. The National Weather Service is forecasting 90 or above through Saturday, then temperatures close to that into early next week. Some recent weather modeling has trended hotter through Monday or Tuesday, so weekend and early-week temperature outlooks may rise.
If the city reaches 45 days at or above 90, it ranks within the top 20 percent of years since records began. If the tally reaches 50, that’s the top 13 percent and top 20 years overall. In 2020, the city recorded 46 such days, which could be surpassed this year.
Should the “heat wave” (commonly thought of as three days or more above 90) go through the weekend, it could reach or exceed the longest of the season. The previous longest streaks this summer, which took place in mid-July and mid-August, reached seven days each. On average, D.C. experiences five heat waves per year with a maximum length of typically nine or 10 days.
There are growing signs that the current heat dome will break down by the middle of next week, as a dip in the jet stream from Canada and potential tropical remnants from the south try to meet up over the area. This would likely allow temperatures to return closer to normal, which is about mid-80s.
One way or another, the D.C. region is running out the calendar, and temperatures will eventually drop. On average, the District typically sees its last day of 90-degree weather Sept. 12. Dulles typically experiences its last 90-degree day on Sept. 8; Baltimore’s is Sept. 9.
While there are some signs the final 90-degree date average is pushing deeper into September, it has been rather steady across the climate record of Washington, perhaps wandering up to five to seven days deeper into the shoulder season between summer and fall.
Rarely has the warm season extended deep into the fall months. In 2019, Washingtonians had to wait until October to see the final 90-degree day of the year. The latest on record came in 1919, when it reached 90 on Oct. 11.
This article was first published August 24, and it was updated August 25.
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Late-summer heat is turning it up in the DC area and the central US - The Washington Post
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