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After 20 straight days with 90-degree temperatures or higher, D.C. fails to reach 90 on Thursday

Streak ends at second place, but the record for the most 90-degree days in any month is still at risk

People cool off at a pool in Arlington on Tuesday. (Salwan Georges/The Washington Post)
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Washington reached 20 days in a row with a high temperature of 90 degrees or higher Wednesday, moving the city to just one step away from the record of 21 straight 90-degree days. But on Thursday, cloudiness and a strong breeze off the Potomac River kept Reagan National Airport from reaching or exceeding the 90-degree mark, instead hitting 87.

Even though the streak won’t break an all-time record, it was still remarkable, with only two other periods since 1872 that have stretched to 20 straight 90-degree days.

Based on forecast guidance, temperatures are likely to climb further during the coming week or more, setting us up to break other heat records.

Going for gold

If the District had made it to at least 90 degrees Thursday, the 21 days in a row at or above 90 would have tied with 1980 and 1988 for the longest heat wave on record.

While both of those heat waves were more intense — which can be measured by their average highs — even with a one-day break, the ongoing heat wave is set to ramp up considerably.

Even though average highs during the ongoing event have been within a short distance of 90 degrees, 20 days at or above 90 was reached earlier in the year than either of the longer heat waves in the past. In fact, this one started earlier than any of the biggest historical heat waves in Washington.

The closest heat wave in terms of its date range occurred in 1872 and began a day later. Most of the other record heat streaks occurred from mid-July to mid-August.

Other Washington-area observing stations have not seen as long of a 90-degree streak, which may point to the influence of the urban heat island effect on temperatures at Reagan National Airport. But these other locations have still seen impressively long stretches of hot weather.

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At Dulles Airport and Baltimore, their 88-degree-plus streaks have continued to lengthen in a similar manner. The 20 days in a row at Baltimore is the fourth most on record, while the 17 days in a row at Dulles ties for sixth most. Baltimore has also recorded 16 days with high temperatures of 90 degrees or higher during the streak that D.C. has pulled 20.

So it hasn’t been super hot, but a prime overlap of peak daily heat and an unusually persistent weather pattern has kept the record endlessly playing.

Surging 90-degree annual total

After a slow start, the 90-degree story during the year changed rapidly with this streak. Tied with 2019 to date, the city has recorded 24 days with 90-degree or above high temperatures through Wednesday. The annual average is 36 such days, and this figure has been increasing over time due in part to human-caused climate change.

It seems the sky is the limit, since hotter weather is soon to come.

With 24 days at or above 90, the city is running about nine days ahead of the typical annual total. Remarkably, the count of 24 is more than the District saw during 52 individual years between 1872 and today.

As recently as 2004, there were only 11 days with high temperatures of 90 degrees or greater, with the record fewest being seven in 1905 and 1886. Typically, the District sees streaks of eight to 10 days of 90-degree heat in a given year.

Looking ahead, the July record of 25 days at or above 90 is also increasingly likely to fall.

If the pattern doesn’t relax soon, we’re also headed toward another high-end annual total after the past year’s third-most 90-degree days on record of 62. The record is 67, which was reached in 1980 and 2010.

Turning hotter in the days ahead

There’s not much light at the end of this hot tunnel.

A powerful and persistent heat dome is poised to continue to strengthen and expand over the coming days. As might be expected, this will turn up our thermostat.

Following Thursday’s struggle to get to or past 90, it should be somewhat easier to accomplish Friday. For the weekend, temperatures continue to rise. In the city, we’re looking at highs in the mid-90s on Saturday and the upper 90s for Sunday and Monday.

There is no major sign of a weather pattern change to come. The best forecast data into the future suggests above-average temperatures are going to persist through the rest of the month. The only possible cooling influence may be a somewhat better chance of storms than on recent days, which cool temperatures somewhat, at least temporarily.

A sign of things to come

Even though the core of the hottest air mass has set up outside of the Mid-Atlantic, across the Midwest, the region has been running 3 to 5 degrees above average for the first two weeks of July.

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While the recent trend is still relatively short, there has been an increase in the occurrence of 90-degree days in Washington, particularly since 2010. This fits with expectations based on scientific research that clearly show that in a warming climate, there is both an increased likelihood for heat waves as well as an intensification of such heat episodes when they do occur.

This is in fact one of the most robust conclusions of research into how global warming is already altering the occurrence and severity of extreme weather events.

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