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Rare derecho kills four, cuts power to more than half of a million people in mid-Atlantic

A derecho is a long-lived line of severe thunderstorms that travels a single direction for more than 250 miles.

At least four people are dead and more than half of a million electric customers were left in the dark when a rare derecho tore through eastern Pennsylvania and New Jersey on Wednesday.

The northern Plains, Upper Midwest and eastern Great Lakes were put on alert Tuesday night when the thunderstorms erupted. Then the threat shifted east to the mid-Atlantic on Wednesday.

The National Weather Service recorded more than 185 damaging wind reports while more than 525,000 customers were without power in Pennsylvania and New Jersey. By early Thursday morning, more than 350,000 customers were still in the dark.

“The storms raced across Pennsylvania and New Jersey, moving eastward at over 80 mph at times,” AccuWeather Meteorologist Brian Thompson said.

Roads were blocked and roofs were destroyed as the tropical storm-force winds were unleashed. A thunderstorm in Reading, Pennsylvania, produced a wind gust to 83 mph during the midday hours on Wednesday. A gust to 68 mph was recorded at Northeast Philadelphia Airport.

Philadelphia International Airport had two separate instances of damaging winds on Wednesday. After a thunderstorm produced a wind gust of 61 mph just after noon, a second storm was accompanied by a wind gust of 68 mph on Wednesday evening.

"That 83-mph gust might have been the fastest wind speed on record at an official site in Berks. Jeffrey R. Stoudt, a retired meteorologist and organizer of the Berks Area Rainfall Network, believes the gust beat an 82-mph wind recorded in 1954 in the passage of the remnants of Hurricane Hazel," AccuWeather Meteorologist Alan Reppert said.

Another gust of 73 mph was recorded in Philadelphia's Homlesburg area.

It appears the area of damaging wind gusts extended for about 250 miles from northwestern Pennsylvania to the New Jersey coast on Wednesday.

"Fast-moving lines of storms have an easier time producing widespread damaging winds, which this line did over southeastern Pennsylvania and portions of New Jersey," Thompson said.

A tree crashed through the roof of a building at the Philmont Country Club in Lower Moreland, killing a staff member. Two people died in separate incidents when trees fell onto their cars in Lower Merion, according to NBC Philadelphia. FOX 29 in Philadelphia reported that another man died after strong winds knocked power lines into a home and started a fire.

A tornado warning was triggered that encompassed Philadelphia Pennsylvania, and Camden, New Jersey, until 7:45 p.m. EDT on Wednesday night by the National Weather Service in Mount Holly.

"We are getting widespread reports of damage across southeast PA from these thunderstorms. This is a real situation and we urge people to take cover to protect themselves now," the NWS Mount Holly tweeted following the warning.

After the storms had passed, the NWS office in Mount Holly, New Jersey, confirmed that the intense storms were a derecho.

A derecho is a long-lived line of severe thunderstorms that travels a single direction for more than 250 miles, according to the Storm Prediction Center. The weather phenomena has wind gusts of 58 mph or higher, typically causing straight-line wind damage.

Derechos are sometimes referred to as inland hurricanes due to the hurricanelike conditions that occur over land with this weather phenomenon.

One of the most notable derechos in recent memory occurred in 2012, which started in Indiana and blasted eastward through the mid-Atlantic. Severe damage was reported all across the region, including in Washington, D.C.