Suddenly it's summer; Philly sets record with high of 91. But spring is coming, honest

After a cruelly endless spell of winter that elbowed its way through March and most of April, the atmosphere this week evidently decided to skip over May and try a new season.

The official temperature reached 91 at Philadelphia International Airport on Thursday, breaking the record for the date, set in 2001.

And for the second consecutive day, the National Weather Service and the Air Quality Partnership hoisted an air-quality alert for higher levels of that unwelcome summer staple, ground ozone.

"It's a little early," said Sean Greene, manager of air quality programs for the Delaware Valley Regional Planning Council. Make that very early. The partnership doesn't begin issuing forecasts until May 1.

"Welcome to summer," he said.

Temperatures on Friday again are expected to challenge a record, 91, also set in 2001, but a potent front is due to approach in the afternoon, accompanied by showers and winds possibly gusting to 40 mph.

Meteorologists were assuring everyone that spring indeed would return by the weekend, with temperatures in the 70s for the next several days.

They also advised that even though the early seasonal outlooks have warm looks, the early pulse of heat wasn't necessarily a warm-up for a steamy or potentially deadly summer.

"You're not going to remember it as a particularly hot or cold summer," said Max Vido, a meteorologist with AccuWeather, which is calling for slightly above normal temperatures that would be a shade higher than last summer's, which generally were pleasant. The WeatherBell Analytics' forecast is similar.

The Climate Prediction Center's outlooks through August favor above-normal temperatures, but that is based largely on robust recent trends.

In the 20^th century, the June-through-August temperatures in Philadelphia averaged 74.7; in the 21^st century, the average has been 77.2.

Other than the trends, the clues aren't terribly illuminating. A weak El Niño – an anomalous warming of surface waters in the tropical Pacific – could develop, said Vido, but El Niño's effects on summer temperatures around here are unclear.

Some of the recent atmospheric patterns and sea-surface temperature profiles in the Gulf of Mexico and the Atlantic match up well with those of recent years in which summer temperatures were slightly above normal, Vido said.

Seasonal forecasts remain works in progress, and the warm seasons have been particularly vexing.

In the short term, strong upper-air jet-stream winds, the boundaries between warm and cold air, have been "screaming" across Canada, he said.

That has shunted the chill to the north – temperatures in Toronto and Montreal were in the low 60s on Thursday  – while much of the United States has basked in the warmth.

Said National Weather Service meteorologist Dean Iovino, "We went from winter to summer rather quickly."