Summer arrives in Philly; 90-plus temperatures possible Sunday

A week after an unusually prolonged May cool streak, the region is experiencing a generous serving of summer.

Highs are forecast to approach 90 on Wednesday — as high as they get normally in late July — and might even exceed 90 on Sunday, forecasters are saying.

So, is this summer’s way of firing a warning shot?

Meteorologists caution that May isn’t the best time to be shopping for summer weather omens.

In fact, on Wednesday computer models were having a hard time making up their minds about the next two days, let alone the next three months.

» READ MORE: Summer of 2020 was third-hottest on record in Philly as warm-night trend continues

Threatening to ambush the warm spell was a so-called backdoor front, which would travel east to west and lure cool air from the ocean landward, the weather service said in its forecast discussion. As opposed to earlier guidance, models now were favoring it crossing the region as early as Thursday, as opposed to Friday.

That wouldn’t result in a cold wave, but the highs both Thursday and Friday are forecast to be several degrees lower than Wednesday’s; more June-like than July.

For the weekend, warm and well-stacked high pressure in the upper atmosphere was favored to let the sun bake the ground in much of the East and drive temperatures back upward.

Noticeably and perhaps disturbingly missing from the extended outlook are shots at significant rain.

Rainfall for the last 30 days has been about 40% of normal in Burlington, Camden, and Gloucester Counties, and about half of normal in Philadelphia and its neighboring Pennsylvania counties.

“Stream flow are running quite a bit lower,” said Robert Deal, meteorologist with the National Weather Service Office in Mount Holly. In Southeastern Pennsylvania, levels were in the 10th to 24th percentile range, he said, and “quite a few” are in the 10th percentile in South Jersey, he said.

Dryness can aid and abet heat waves. When the ground and foliage are moist, some of the sun’s energy must be diverted to evaporation. In the absence of moisture it can go to town.

However, Dave Dombek, a meteorologist with AccuWeather Inc., pointed out that can change faster than the weather on a late summer afternoon.

» READ MORE: What constitutes ‘normal’ weather in Philly and elsewhere is surprisingly complicated, and always changing

No overpowering signals have been evident to offer clues about what’s to come, says Todd Crawford, meteorologist with the Weather Company.

And what he said in issuing his seasonal outlook last week might sound familiar: “There is quite a bit of spread in our objective models for the summer.”

But do expect about a 100% chance of heat eventually, even if it’s not this weekend.