The Washington PostDemocracy Dies in Darkness

PM Update: With 20 days in a row of 90 degrees or higher, D.C. sits on the edge of history

A beautiful summer scene along the C&O Canal in Georgetown. (C JRCook/Flickr) (JR COOK/Flickr)
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As Washington closes in on three weeks of 90-degree-or-higher temperatures, the city is now only one day from tying the record for the longest such streak, set in 1980 and tied in 1988. Despite the extended run of hot readings, the relatively tame nature of the heat and humidity combination has left many saying it’s been a passable summer so far. If the humidity is a heavy factor in that play, it’s time to prepare for things to go downhill.

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Through tonight: Bubbly clouds of the day will diminish with sunset. Additional clouds may build through the night as a more humid air mass becomes entrenched. Lows will range from the upper 60s to the mid-70s, from suburbs to city center. Winds will be light from the south.

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Tomorrow (Thursday): Some people will say this is a wimpy if historically long heat wave, because there have been so many near-90 days. Others (like me) will say we’ve really had to earn it with a number of nail-biters. This one could be no different. Highs will again be within a few degrees of 90, with clouds or lack of clouds offering much of the story. It will turn more humid, as well, with winds out of the south around 10 to 15 mph. There’s a small chance of a late-day shower or storm.

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Pollen update: Tree and grass pollen are low/moderate, as are mold spores. Weed pollen is low.

Can we do it? We’re all on the edge of our seats as the forecast for Thursday is fine-tuned. You may have noticed that I chose not to make a specific declaration on whether we can get Day 21 of 90-plus Thursday. If I had to make a call, I’d lean yes. That said, it’s going to be complicated.

The local National Weather Service office did a great job explaining the setup in a technical discussion earlier today:

“While the background air mass will be warming, it will also become increasingly humid. The more humid it is, the longer it takes for the air to heat up (humid air is denser and requires more energy to raise its temperature).

“But with the increasing humidity will come a deck of clouds. These clouds should lift and scatter by late morning and may actually help to trap heat tonight leading to a warmer start. But if the clouds linger too long, the air might not have enough time to heat up to 90 F, especially at the airport with an onshore wind right off the Potomac River (the water temperature at nearby Washington Channel is 85 F).”

As we pointed out Tuesday, this heat wave has been lengthy but not extreme in high heat.

Coming into today, of the 19 days in a row at or above 90, five were 90, four were 91, and 10 were 92 or above. The highest temperature of the heat wave so far of 97 is actually somewhat lower than we’d expect the mercury to rise in a D.C. summer. Of course, we’ve still got plenty of time for that. Potentially in this heat wave, should it not break briefly on Thursday.

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