The siege began just after 3 p.m., the atmosphere primed with water vapor. Hailstones the size of quarters, falling as fast as 40 mph, were pounding a patch of Montgomery County. Winds were gusting perhaps 80 mph, air conditioners suddenly went silent, trees came down.

Ken Hellendall, Cheltenham Township’s emergency director, placed an urgent call to his deputy, Joseph O’Neill. O’Neill was nonplussed.

“He literally didn’t know why I was calling him,” Hellendall said. It turns out that O’Neill was returning from the Willow Grove Shopping Center, a mere five miles away, where it was just another steamy Wednesday in July.

As so many storms recently on both sides of the river, those that struck the region Wednesday had randomness that Gus the lottery-pitching groundhog might envy. Many of them have had one thing in common, however: They have focused their ferocity on limited geographic areas.

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Peco reported more than 125,000 power outages from the Wednesday storm, with 100,000 of them confined to a zone from Germantown to southern Montgomery County, said spokesperson Greg Smore, a tiny percentage of its service territory.

Hellendall said it was the worst storm he had experienced in his 40-plus years in the township. “I’ve never seen anything like this,” he said.

And it came a few days after Cheltenham had experienced the benign side of nature’s caprice.

On Saturday night, a horrific storm struck areas around Bensalem Township, about 12 miles from Cheltenham. Motorists on the Pennsylvania Turnpike encountered rains so hard that it was not unlike trying to drive through a car wash with debris ramming against the car doors.

“We saw the massive damage to Bensalem,” said Hellendall, “and we got nothing.”

Local, local

With the summer thunderstorm business, it’s usually more retail than wholesale, and in some cases it might even seem door-to-door.

Thunderstorms tend to pop up spontaneously. Save for the ones that are part of larger systems like squall lines, on average they are no more than 15 miles in diameter and are history in about 20 minutes. The core of the heaviest rain might be less than 5 miles wide, said Matthew Elliott, warning coordination meteorologist at the government’s Storm Prediction Center.

With solar energy at its annual peak in the Northern Hemisphere, the temperature contrasts that spawn well-organized cool-season storms weaken. Meanwhile, the upper-air currents that steer the weather systems become a lot like the rest of us — lazy and disorganized.

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Randomness rules. Thunderstorms are fueled by ground heating that simmers the air, forcing it upward where it condenses in the cooler air layers higher up.

Meanwhile, areas on the perimeter of intense storms actually experience a collateral benefit: The descending currents away from the rising air have a drying effect, which would explain how Cheltenham could remain rain-free while Bensalem gets creamed.

A misnomer?

Thunderstorms might more properly be called “lightning storms.”

It is the lightning, explosively heating the air to about 50,000 degrees Fahrenheit, that sets off the thunder. Lightning packs a million times more voltage than household currents, says John Jensenius, a meteorologist who is head of the National Lightning Safety Council.

The lightning aspect is particularly deadly, dangerous, and insidious, and it has been a huge factor in power outages and those Western wildfires whose smoke visited us last week.

Why us?

The Philadelphia region, situated near major bodies of humidity-producing water and with more summer heat than most people would want, happens to be in a prime pop-up storm zone, said Elliott, who used to work in the Baltimore-Washington National Weather Service Office.

We are prone to the less organized, scattershot “multicellular” thunderstorms, he said, such as the ones that attacked several times this month, including on Wednesday.

In almost guerrilla style, he said, storms can intensify ever so briefly, and “you get these pockets of damage, quite intense, certainly.”

As to where they will appear, “You might know a general area,” said Elliott, but before they materialize, pinpointing precisely where they will hit would be virtually impossible.

The forecast

For now the limits of the science dictate that the public will have to settle for the so-called point probability forecasts, as in “30% chance of showers.” That means literally there’s a 30% chance that 0.01 inches of rain or more will fall where you are sometime during the forecast period.

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A common reaction, said Jensenius, is, “Well, you’re not telling me anything.”

He begs to differ: Such a forecast should be viewed as a prompt to keep an eye on the skies and radar.

Are ‘micro’ storms the future?

Yes, and they have been in the past, said Elliott. “These types of events are quite common,” he said, especially this time of year. “This is prime time.”

Cheltenham’s Hellendall said he could not recall a storm “more micro” than the ones that occurred Wednesday and on July 13. The hail was a clear indicator that Hellendall’s township had weathered an especially powerful storm, said Jensenius.

But it is unclear whether these storms are more focused and/or are happening more frequently, said Elliott.

Climate change clearly has had impacts, but one thing that has changed dramatically in the universe of severe storms is the quantity of observations and reporting, he said. With social media and the internet, almost nothing can happen in peace.

“Ten years ago, lots of these reports you would never have found,” he said. That compounds making comparisons with past events.

But he said the new floods of data hold great promise for tracking whatever climate change is doing to the behavior of thunderstorms.

In the meantime, expect a 30% chance of showers.