Mammatus cloud at sunset on Wednesday. (Rex Block via Flickr)

The excessively hot weather is ready to take a break, maybe a long one.

There are compelling signs that the intensity and longevity of the heat wave that ended Thursday will not be exceeded again this year. We’ve likely reached an inflection point in which any forthcoming heat waves are no longer so punishing.

This last heat wave packed a punch. It strung together six straight days with high temperatures between 92 and 98 degrees, pushing 2019′s count of 90-degree days to 51, tied for fourth most on record year-to-date.

It has been a long, hot summer, indeed.

But, as it often does at this time of year, the jet stream — the high-altitude current of winds separating hot from cold — is more frequently nosing its way southward. Now it’s taking a plunge in the eastern United States, drawing in cooler and less humid air from Canada.


Blue-shaded zone indicates where jet stream dipped (through the Mid-Atlantic) Friday night. (WeatherBell.com)

After highs in the rain-cooled 70s on Friday, highs may hit only 80 this weekend, even when sunshine returns. Nighttime temperatures are set to dive into the 50s in our cooler locations and even 60 to 65 close to the city — low enough to give your air-conditioner a break. These will be the coolest daytime and overnight temperatures since June, between five and 10 degrees below average.

The drop in humidity may be the most noticeable feature in this pattern shift. Dew points, a measure of humidity, plunge from a sticky 70 degrees Friday to much more comfortable mid-50s by Saturday morning.


High-resolution NAM model forecast of dew points between Friday morning and Saturday morning.

While we have probably turned the corner in terms of extreme heat and humidity of long duration, it’s premature to declare summer over. On average, we expect to see another five days at or above 90 degrees and have witnessed 90-degree weather as late as the second week of October.

The jet stream is sure to retreat at times while high-pressure zones off the East Coast pump hot and humid air northward. But it’s unlikely that the intensity of heat waves as we head into September will match what we’ve just experienced. And cold fronts will probably sweep in before we can string together too many hot days in a row.

Through next week, the average computer model forecasts highs below 90 in Washington, although some upper 80s are predicted for the middle of next week and 90 can’t be ruled out, given possible model errors.


Seven forecast of high and low temperatures in Washington from blend of computer models. (WeatherBell.com)

The wild card, then, is what happens in September. The National Weather Service’s Climate Prediction Center September outlook suggests we certainly shouldn’t count summer out, favoring above-normal temperatures.

Matt Rogers, one of Capital Weather Gang’s long-range forecasting specialists, says the European model predicts another round of heat the week after Labor Day, around Sept. 5, with highs perhaps into the low-to-mid 90s.

“It shouldn’t be as hot as the last heat wave in terms of duration and intensity,” Rogers said. “Usually the heat in September is different. It’s not as humid typically. But it could still be pretty toasty.”

In September, the average high temperature starts off in the mid-80s and eases back into the mid-70s by month’s end. Normally, our last 90-degree day is around Sept. 11 and we see three days get that hot during the month, although there were a remarkable 14 such days in 1980, the most on record.

Rogers says a repeat of 1980 is unlikely. “I don’t see that durability [in the heat]. The pattern looks more variable than that,” Rogers said.

He agrees with the Weather Service that this September will end up hotter than normal, “but not as hot as last year.”

Temperatures were more than four degrees above normal last September, marking the fifth warmest on record in Washington. Not only it was abnormally warm, but it also was excessively humid and rainy. Rain fell on a record 16 days, totaling 9.73 inches, fifth most on record.