A tornado outbreak killed 64 people in Pennsylvania 35 years ago. We still know terrifyingly little about the storms.

The horror was unmistakable even on the primitive 1970s radar screen.

A “clear echo” was evident, the calling card of a powerful tornado. An embedded "ball image” indicated that it had ingested massive quantities of debris, recalled Greg Forbes, at the time a Penn State professor who later would become one of the country’s most visible severe-storm specialists.

A colleague who was watching the radar with him recalled hearing Forbes say: “People are dying right now.”

They were. On this date 35 years ago, the deadliest tornado outbreak in the state’s history cut through the heartland of Pennsylvania, killed 64 people, and spawned the state’s only F5 tornado on record, with peak winds better than 200 mph. On storm surveys the next day, Forbes saw stacks of ripped-down trees piled 10 feet high.

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Killer twisters also rampaged through northern Ohio, southwestern New York, and Ontario, in regions unaccustomed to encounters with the most ferocious storms on earth. The odds against that happening in those areas are “incalculable," said Harold Brooks, senior scientist at that National Severe Storms Laboratory.

In its post-storm report, the National Oceanic and Atmospheric Administration stated soberly: “Perhaps the lesson to be learned from the 1985 outbreak is that under the proper atmospheric conditions, major tornadoes can occur irrespective of the location or terrain."

Thirty-five years later, despite remarkable advances in radar and other technologies, tornadoes and their inner lives remain enigmas to atmospheric scientists.

Like about everything else these days, tornadoes are behaving somewhat peculiarly, said Brooks, and that might be related to climate change, but they are directing some of that toward guarding their secrets.

With vast landmasses beneath subtropical, moisture-laden air surging from the Gulf of Mexico and its polar opposite sagging from Canada, the United States is a world capital of tornadoes.

The most-favored areas are in “Tornado Alley,” between the Rockies and the Mississippi River, from central Texas through South Dakota, where contrasting dry air that often is part of the tornado recipe, is plentiful.

The diverse air masses clearly have a whole lot to do with igniting the “supercell” thunderstorms that are the source of some of the most dangerous tornadoes, but indicting those ingredients alone would be a “gross oversimplification,” according to the Storm Prediction Center.

Tornadoes get their spin when winds blow in different directions in a column of air. But what causes that to happen?

Only about 20% of those supercells spawn tornadoes, and scientists aren’t sure why some do and some don’t. Some tornadoes form independently from supercells.

“The truth is that we don’t fully understand,” says the storm center.

The peak period in the central United States is early spring, with the crest of the wave gradually rippling eastward later in the spring and in June. However, in the Mid-Atlantic and Northeast, conditions usually aren’t as favorable for tornadoes.

Around here, the ideal air masses are present in summer, said Brooks, but the ideal winds tend to occur in winter. In the period of record, few have occurred in Pennsylvania before June.

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That’s just one reason why the 1985 outbreak was so shocking.

On average only 5%, or 20 to 25, of the 1,000 to 1,200 tornadoes that touch down annually reach “severe” criteria — EF3s on the Enhanced Fujita scale with winds of 136 mph or greater — with only one making it all the way to EF5, winds stronger than 200 mph.

Half of the 20 tornadoes that hit Pennsylvania that day would have qualified, and one of those was an EF5.

Tragically, the tornadoes hit where people live. Most don’t. Much of the nation’s land is uninhabited or un-built-upon. Some Midwestern counties have populations as low as one person per mile. (That likely would facilitate social distancing.)

“For casualties,” Brooks said, “you have to have tornadoes, you have to have people.”

Just before dawn on May 31, 1985, a warm front lifted across the Ohio-Pennsylvania border and temperatures soared into the 80s in Cleveland and Erie, according to the National Weather Service account. The air felt like water-vapor soup. Meanwhile, a cold front was tracking across the Midwest. What followed was atmospheric fireworks.

More than 10 tornadoes touched down in Ontario. Just after 4 p.m., the Cleveland weather service office posted a thunderstorm warning, and “it quickly became clear that a tornado outbreak of an unprecedented magnitude was taking place.”

Greg Forbes recalled that an outbreak had been expected, but no one expected this.

What he saw in storm surveys was almost unimaginable. One of them had traveled 65 miles through forestland, “up and down deep slopes" just north of I-80, somehow dodging two structures. “It mowed down just about every tree in the forests, in a path more than a mile wide at times, and stacking fallen trees atop each other piled 10 feet high.”

Nothing like it has occurred so far removed from the prime tornado zones since, and the Philadelphia region in all likelihood would not see anything similar, Brooks said.

“Getting something as big as far east as you is really hard,” he said. “The atmosphere has to work harder.”

That said, a powerful tornado that packed winds up to 200 mph took away a house and killed three people in Limerick, Montgomery County, in July 1994, and a killer tornado struck the Philadelphia region, heavily damaging parts of Bucks and Montgomery Counties and leaving four people dead, on May 28, 1896.

As for whether a changing climate means a changing tornado climatology, that would be impossible to know, at least for now, said Brooks.

He said researchers have seen a change in tornado variability. In the 1970s, 150 days per year saw at least one confirmed tornado, with 30 or more occurring on a single day once annually. Recently, the average is 90 days with at least one tornado, and two or three days with 30 or more.

But tornadoes have a long history of peculiar behavior. Totals vary wildly from year to year.

Forbes said he would not rule out a recurrence of 1985 in the next 50 years or so.

“It could happen again.”