Four days after the derecho, and a supercell-thunderstorm encore, a few thousand customers still were waiting Sunday night for their power to be restored.

Some PECO customers along the Main Line complained about a wait that extended over three of the sultriest days of the year and that the utility kept moving back the scheduled restoration times.

Utility officials said the biggest obstacle to crews confronted was the obvious: quantity.

More than 850,000 Peco, PSE&G, and Atlantic Electric customers lost power on Wednesday, the majority of those resulting from the derecho squall line that roared through the region from late morning into the early afternoon. About 3,000 still waiting for power late Sunday night.

However, additional outages occurred after supercell thunderstorms in the evening added to the outage totals and delayed efforts to restore power after the derecho-related outages, Peco spokeswoman Alexandra Coppadge said Sunday night.

The four derecho-related deaths in the Philadelphia region were the most ever in reliable records dating to 1950, the National Weather Service said.

Of Peco’s 563,000 outages, 335,000 were reported around noon Wednesday, the company said. The derecho’s passage felt like “the worst 10 minutes ever,” said Ben Armstrong, spokesman for a consortium that includes PSE&G, which reported 200,000 outages. Atlantic City Electric added about 80,000.

The derecho’s passage was followed by several hours of tranquil weather that allowed the atmosphere to ramp up for an outbreak of supercell thunderstorms, meteorologists said. The derecho wind gusts reached 93 mph at Beach Haven, and a 68-mph gust was recorded Wednesday evening at Philadelphia International Airport.

Unfortunately the prolonged outages that followed coincided with three steamy days in which the air was soaked with water vapor, which finally began to relent late Saturday.

Coppadge said “occasionally” circumstances can result in changes to restoration estimates, but customers don’t have power after their neighbors get it back, they should report the outage again, at 1-800-841-4141.

Sunday, provided spectacular cleanup weather for those confronting the avalanches of tree debris from Wednesday’s mayhem.

And given that a derecho, a Spanish term connoting “straight ahead,” visits only about once every two years around here — a deadly one also occurred in June 2012 — odds of a reoccurrence this summer would be small.

Conditions have to be just right (or wrong). A derecho is a rapidly moving squall line that traverses a path of 240 miles or more and generates damaging winds in at least three places about 40 miles apart. All the damage must occur within three hours.

This one made the grade with honors, traveling 254 miles from near Erie to the Jersey Shore, with speeds as high as 70 mph at the outset.

On Wednesday, the atmosphere was ripe for one to form. The air over the East was swollen with moisture-laden air, said Jack Boston, a senior meteorologist with AccuWeather Inc. As it surged north it interacted with dry high pressure in the upper atmosphere over the central United States. The collision generated extreme updrafts that set off storms in northwestern Pennsylvania

The cluster rode upper-air winds from the northwest to plow its destructive path. The supercell thunderstorms that formed later might have caused even more damage had the derecho not already executed some of the dirty work.

Dave Shea walks near a fallen tree in his Radnor Township neighborhood on Wednesday.
STEVEN M. FALK / Staff Photographer
Dave Shea walks near a fallen tree in his Radnor Township neighborhood on Wednesday.

What followed was a massive restoration project for utilities. Peco summoned help from as far away as Oklahoma.

On Saturday, an armada of utility trucks and crews from Eldorado, Ill., 830 miles away, were in Upper Merion Township, Montgomery County, working to restore power to those who hadn’t had it since Wednesday. To get here, trucks had driven through six hours of torrential rain from Columbus to Harrisburg, said crew member Dennis Collins.

An armada of utility vehicles is parked on an Upper Merion Township street as utility crews from Illinois work to restore power on Saturday.
Anthony R. Wood/Staff photo
An armada of utility vehicles is parked on an Upper Merion Township street as utility crews from Illinois work to restore power on Saturday.

No doubt customers are grateful to have the lights back, and it might not hurt to have air-conditioning later in the week when temperatures are due to head back to the mid- and upper-80s after cresting around 80 on Monday.

Looking ahead, the Climate Prediction Center is saying odds favor below-normal temperatures in the East in its 8- to 14-day outlook.