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‘Derecho,’ calm, and supercell thunderstorms a 'rare’ and deadly combination for Philly region

Four deaths in the region were attributed to the derecho; the highest number of derecho-related fatalities around here in the period of record dating to 1950. In all Wednesday, 563,000 lost power.

A commercial trash truck opted not to go under the canopy of branches from a fallen tree along School House Lane in East Falls Wednesday.
A commercial trash truck opted not to go under the canopy of branches from a fallen tree along School House Lane in East Falls Wednesday.Read moreALEJANDRO A. ALVAREZ / Staff Photographer

For all the mayhem that unfolded Wednesday, it was a prolonged period of tranquility that was an essential ingredient for making June 3, 2020, one of the wildest days for weather in the region’s history.

Meteorologists themselves said they were in awe of what transpired around here from about 11 a.m. to 8 p.m. No one could recall a day in which two extreme wind events were generated by two very different systems.

“It was definitely a rare day,” said Jonathan O’Brien, a meteorologist with the National Weather Service in Mount Holly.

In all, more than 850,000 utility customers on both sides of the river lost power as a result of Wednesday’s storms.

The “derecho," which crossed the region around lunchtime, set off hurricane-force gusts — 93 mph — at the Shore. At Philadelphia International Airport coinciding with a round of “supercell” thunderstorms, a gust of 68 mph was recorded at 7:32 p.m. By around noon, 335,000 PECO customers had lost power, said spokesman Greg Smore said.

Four deaths in the Philadelphia region were attributed to the derecho, all related to tree damage, the most derecho-related fatalities around here in the period of reliable records dating to 1950, according to the weather service.

“That derecho event got a lot of headlines, but that late-afternoon, evening event was impressive," O’Brien said.

It was characterized by strobe-like lightning, and prolonged thunder that evoked an oceanic roar, an acoustic effect attending to thunderstorm clusters, said Bill Bunting, branch chief at the government’s Storm Prediction Center, in Norman, Okla.

» READ MORE: Thursday has a chance of damaging storms; Wednesday’s brought deaths, but no tornado in Philly region

Torrential rains were significantly heavier than they were during the passage of the rapidly moving derecho, which had to do with radically different natures of the two storms.

What is a derecho?

A derecho is essentially a squall line moving at turnpike speeds over a path of at least 240 miles long and having gusts of 58 mph or better.

The one on Wednesday qualified by traveling 254 miles — from Northwestern Pennsylvania to the Shore, said Jack Boston, a senior meteorologist with AccuWeather Inc., and it more than cleared the bar with winds. Also, it cut a 70-mile-wide path, not unlike that one would have expected with an oversize EF-0 tornado.

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

According to the storm center, the term was minted by Gustavus Hinrichs, a physics professor at the University of Iowa, in 1888. Derecho is Spanish for straight ahead. The term had a long career in obscurity before it came into popular use in the mid-1980s.

Conditions on Wednesday were dangerously derecho-friendly, said Boston. The air was laden with tropical moisture from the Gulf of Mexico — a key element.

Meanwhile, to the north was the perfect conspirator: dry high pressure in the upper atmosphere over the central United States. The clash of the contrasting air masses created extreme updrafts that set off storms. Specifically, because air condenses as it rises, producing the convection that causes lightning and thunder.

Then, winds from the northwest aloft propelled the derecho toward the Atlantic.

Boston said that on average, the Philadelphia region can expect about one of these events annually, usually in June or July. Let’s hope we’ve met our quota.

What is a supercell?

Unlike “pop-up” thunderstorms, a supercell is a well-organized cluster of storms that can take on a rotation and cover areas 10 miles in diameter.

They are fed by rising currents of air that set up convection and sometimes can spawn tornadoes, although none were reported around here Wednesday.

They usually occur late in the day and often are accompanied by “anvil” and wall clouds, powerful wind gusts, and hail.

Derechos, on the other hand, could care less about the time of day. In June 2012, one struck in the dead of night, killing a 2-year-old and a 7-year-old in Pittsgrove, Salem County.

The timing of Wednesday’s derecho turned out to be a huge problem.

What went wrong

The derecho’s destructive journey began around 9 a.m. in Northwestern Pennsylvania, said AccuWeather’s Boston, moving 70 mph in a southeasterly direction.

It reached Philadelphia just before noon, and had slowed to 60 mph. Temperatures dropped from 88 to 69 in just an hour. The downpours were as furious as they were short-lived, and that eliminated any flooding threat.

It got to the Shore in the early afternoon, and reached a peak as it was collapsing, which would have been typical, said Boston; they tend to save their best for last. A gust of 93 mph was measured at Beach Haven, on Long Beach Island, and 92 at Surf City.

Unfortunately, the sun made reappearances throughout the region on a positively steamy day, which provided copious fuel for the supercell outbreak.

That period of calm, said Boston, “restored” the atmosphere and primed it for the encore.

Restored probably isn’t the word most people would use.