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Thursday was the final day of widespread record highs in the Washington-Baltimore region, and the late-season heat wave solidified its place in the record books. No heat wave this late into September has been this hot.
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Since Sunday, the region’s three airport weather-observing locations — Reagan National (the District’s official site), Dulles International and Baltimore-Washington International Marshall (Baltimore’s official site) — have endured their hottest five-day September periods on record.
Amid the heat wave, Dulles reached 100 for the first time in September.
Records fell for a fifth straight day Thursday. Since the District’s highest September temperature of 104 came on the same date in 1881, it was not challenged. But heat indexes, a measure of how hot it feels factoring in humidity, were that high.
Days and days of high temperatures in the upper 90s to around 100 are more typical of the hottest portion of midsummer, and some years it never gets so hot. The heat is wildly out of place for September.
Key records
The five days of high temperatures reaching at least 97 degrees in Washington marks the hottest such stretch this late in the year in records going back to 1872, when observations began. Only four years have seen longer streaks in any month, according to data from the Southeast Regional Climate Center.
In Baltimore, its five-day streak reaching at least 98 degrees tied its second longest on record, regardless of the month. The same was true at Dulles for its five-day streak at or above 97.
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Between Sunday and Wednesday, calendar day record highs were set every day at Dulles and in Baltimore, as well as three times in the District. Dulles picked up another Thursday.
Here’s a quick breakdown:
The District set calendar day record highs of 98 and 99 on Monday and Tuesday. Its high of 98 on Wednesday tied the calendar day record. The 99-degree high on Tuesday marked its fifth-highest temperature recorded this late in the year. Dulles set a September record high when it hit 100 on Wednesday. Before that, it tied the previous September record of 99 on three straight days. Thursday’s high of 97 marked a calendar day record. Baltimore hit a calendar day record on Wednesday, reaching 100 for only the fourth time in September since 1872. Before that, it set calendar day record highs of 98, 99 and 99, Sunday through Tuesday.
#Baltimore is experiencing its most severe September heatwave on record. The ongoing heatwave includes Baltimore's first September extreme heat event (Clarke et al., 2014 methodology) on record. Baltimore's most notable September heatwaves are below. #historic #heatwave #mdwx pic.twitter.com/TVezmnxXfw
— Don Sutherland (@DonSuth89069583) September 7, 2023
July or September?
Hot weather can last into September with ease in the Mid-Atlantic, but it’s usually less intense than during the core summer months. Not so in 2023.
Before this heat wave, the District’s previous highest temperature of the year was 97 on July 29. That has been topped four times in recent days. It’s the first time September has featured the highest temperature of the year since 1970, and one of only a few instances overall.
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High temperatures have run about 15 degrees above normal during this stretch. The average high in Washington is around 83 or 84 degrees.
The first week of the month was about as warm as it gets, despite two cooler-than-normal days.
A hot end to a mild summer
Part of the pain of this late-season heat wave is that much of the region had a relatively mild summer overall. While intense heat waves persistently scorched the Southern United States, frequent cold fronts and intrusions of smoky Canadian air kept extreme heat in check locally.
Washington has had only 31 days at or above 90 degrees through Thursday, compared with an annual average of around 40 such days to date.
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Oddly, the District has seen fewer 90-degree days than Dulles and Baltimore, which are ordinarily cooler. They have seen 44 and 34 such days, respectively, compared with an average of 33 and 35 annually.
All three locations have seen a below-average number of days reaching at least 95 degrees this summer.
Throughout the summer, temperatures north and west of Washington have tended to be warmer than those south and east. This may be partly attributable to increasing drought focused northwest of the Beltway since drier land surfaces are easier to heat up.
The end is near?
While cooler than previous days this week, Friday will remain unseasonably hot for this late in the summer season. But once the 90-degree heat departs, that may be the end of it until next summer.
In an average year, the final 90-degree day comes during the second week of September in the District. Last year, it occurred Sept. 4. That said, the area didn’t get its final taste of 90s until the first week of October in 2018 and 2019.
Toward mid-September, the weather pattern favors hotter weather in the West and average to somewhat cooler-than-normal weather in the East. There is no real sign that we’ll see a resurgence of heat.
If you want to mark your calendar, the latest 90-degree day on record in the District is Oct. 11.
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