PHILADELPHIA — Warning of the potential of triple-digit temperatures and conditions that could make it feel like it’s 108, the National Weather Service has issued an “extreme heat” warning for Sunday through Wednesday.

The City of Philadelphia also has declared a heat emergency, as has Montgomery County.

Meanwhile, more than 40,000 Peco customers remained without power Saturday afternoon as restoration efforts continued from Thursday’s quick-hitting storm that took down, trees, branches and wires.

The atmosphere is notorious for unpredictable behavior, but the coming extreme heat is all but a certainty, Alex Staarmann, meteorologist with the National Weather Service in Mount Holly, said Saturday.

And in this case, it isn’t a “localized” event, he said, but is going to impact a significant portion of the East, from South Carolina to New England.

Given how generally temperate the weather has been the last several weeks around here, this level of heat could be especially hazardous, he said.

“It hasn’t been anywhere near this hot yet,” he said. While June heat waves aren’t all that unusual, they typically don’t last as long as hot spells later in the summer.

But forecasters have been warning that this one may have some staying power and could persist into next weekend.

The heat was getting off to a relatively gentle start Saturday with highs getting up to 90. Readings will reach the mid- and upper 90s on Sunday and perhaps break 100 Monday and Tuesday.

By the time the worst of the heat bakes the region, the Peco customers who lost power in Thursday’s storm should have their lights and air-conditioning systems operating.

At one point more than 325,000 customers had lost power, and Peco spokesperson Tom Brubaker said that the outages were spread throughout the service territory.

He said a major problem was the uprooting of trees, ripped out of ground made soggy from recent rains. Wind gusts past 55 mph were reported as the fast-moving storm ripped through the region between 5 and 6 p.m.

Plus, trees are not full leafed out, adding weight to the branches. Winds can sail through bare branches, but not leaf-bearing deciduous trees. Brubaker said power should be restored to the “vast majority” of customers by Sunday night.

Thunderstorms are not in the picture for the next several days, just heat.

It has been 13 years since the city recorded an official reading of 100, the longest 100-less stretch since the early 1900s.

Staarmann said the atmospheric conditions are ripe to end that streak, with dry winds, and almost no chance of rain. “It’s probably one the most favorable patterns we’ve had in recent years to get to 100 degrees,” he said.

Winds from west and northwest will be light, he added, but if they are too gentle, it’s possible a renegade breeze from the Delaware River could keep the thermometer from reaching the century mark.

Even if it doesn’t quite reach 100, it’s still going to be plenty hot, he said, and to the human body, 99 likely feels about the same as 100.

Mayor Cherelle L. Parker said the city’s heat response system is primed and ready.

It will open cooling centers and remind people to look in on elderly neighbors and acquaintances.

The Philadelphia Corporation for Aging’s (PCA) Heatline, 215-765-9040, will be operating from noon to 8:30 p.m. on Sunday, and 8:30 a.m. to 8:30 p.m. on Monday, Tuesday and Wednesday.

The line would stay in operation if the heat were to continue.

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