In fact, average high temperatures will begin to decline starting Sunday, and in 10 days, low temperatures will also begin to drop off.
So far, this summer hasn’t been that bad. We’ve notched 19 days at or above 90 degrees through Wednesday. Yes, that’s a little above the average of 16 days, but the summer’s highest temperature so far — 95 degrees — is on the cool side of what we typically expect.
Before we celebrate our good fortune, it’s important to understand how much summer we still have to endure, as well as some lessons from the past.
We’re at the peak of the peak. Here’s what that means.
Earlier this year, we explained that our average highest temperature now peaks at 90 degrees, from July 6 to July 27. It wasn’t always this way, but climate change has bumped up our average temperatures (using data from 1991 to 2020), and we now spend three weeks of summer with a “normal” high of 90, rounding to the nearest whole number.
But we can provide narrower windows for when temperatures “peak” if we don’t round these temperatures and instead allow for one decimal place. Doing so yields the following peaks:
- High temperatures peak from July 13-17 at 89.9 degrees.
- Low temperatures peak from July 15-25 at 72.7 degrees.
- Average temperatures peak from July 14-21 at 81.3 degrees.
In other words, peak heat is here or about to be here.
Corresponding with these peak temperatures, the graph below shows that July 16 and 17 are the most common dates to see highs of 90 degrees or higher. On these dates, we hit at least 90 degrees 73 percent of the time between 1991 and 2020.
But, by early next week, the chance of hitting 90 degrees on any given day begins to fall.
That said, the chart also shows that the chance of reaching 90-degree heat stays reasonably high well into August and that there are secondary peaks. Whereas the chance of eclipsing 90 degrees rises steadily in June and early July, the descent into fall is bumpier, and periods of high heat can come in waves.
Where do we go from here?
Returning to where we stand this summer, we’re on our fourth day of a heat wave and probably have about three days left. We’re defining a heat wave as a stretch of at least three straight days with 90-degree (or hotter) weather. Over the past three decades, our average longest heat wave has been nine or 10 days. It’s possible we won’t see one that long this summer, but with two weeks left in July and all of August ahead of us, that’s far from certain.
The three heat waves we’ve seen this summer are about par for the course season-to-date, because we average about six per year and have about half the summer left.
Since the mid-1940s, the city has seen the incidence of heat waves rise from less than four to around six per summer, which is attributable to climate change and urbanization.
Although we’ve managed a typical number of heat waves this summer, temperatures overall have tended to “underperform,” falling short of forecasts in recent weeks. That may be because a good deal of rain has fallen, making it somewhat harder to heat the air.
The high temperature of the year so far of 95 degrees is four below our average maximum summer temperature of 99. If that holds, it would be the coolest maximum since 2004, when the highest temperature was only 92. But over the past 30 summers, the highest temperature of the year came after this date 60 percent of the time. Indeed, every year from 2015 to 2020 saw its highest temperature post the second week of July.
Signals from long-term weather models suggest that heat may come in bouts in the weeks ahead, as it has much of the summer thus far. Following the present heat wave, a several-day cool-down seems likely, but for now, it looks as if the end of July should be toasty again.
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July 15, 2021 at 03:03AM
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D.C.’s summer heat is near its peak. Up next? The descent toward winter. - The Washington Post
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