Were you startled by a loud boom and a bright flash this morning?
You aren’t alone -- the loud booms shook people and left them asking what happened.
We asked the NBC10 First Alert Weather Team to explain the phenomenon.
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Loud thunder claps shook buildings, including our own NBC10 studio in Bala Cynwyd, around 10:20 a.m. Wednesday. From Delaware to Chester County to South Jersey people reported feeling buildings rattle and seeing bright flashes in the sky.
So many people locally tweeting about thunder, I'm starting to think that when the building shook a few minutes ago, it wasn't a truck. — Rachel (@raewing) Feb. 19, 2014
The NBC10 weather team says the loud boom was caused by lightning that amplified thanks to the cold air. Since the air is colder this time of year it has a higher density than air we have during thunderstorms that we get in spring and summer.
NBC10 First Alert Weather meteorologist Glenn "Hurricane" Schwartz said that numerous lightning strikes caused the loud bangs that felt almost like earthquakes.
Hurricane says that it's unlikely the rattling was anything more than lightning and the U.S. Geological Survey didn't report any earthquakes anywhere on the East Coast.
"Sound is affected by the density of the air," said Hurricane.
"The National Weather Service believes density is the main factor."
NBC10 First Alert Weather meteorologist Bill Henley said that the lack of leaves and foliage also causes sound to travel more clearly. It's the same reason why you are more likely to hear a far off train in winter rather than summer.
The fact that lightning bolts can be dozens of miles long also could have led to rolling thunder heard around the region.
The loud boom felt in Bala Cynwyd could also be felt in Roxborough, Northeast Philadelphia and the Main Line. Other loud booms were also felt in Bucks, Chester, Mercer, Montgomery and Camden Counties. There were also reports of rumbling thunder and lightning as far south as Newark, Del.
The weather team said the string of lightning strikes lasted less than 20 minutes but the damage was done.
The storm broke windows, knocked down power lines and turned the lights out on hundreds of homes and businesses in Philadelphia's Roxborough neighborhood.
Sharon DiFranco McMurray felt the boom and saw the lightning along Roxborough Avenue.
"Felt like a military jet flying overhead & then a loud Boom! The lightning lit up my house. Very scary and the thunder lasted 30 seconds."
The storm began to move out of the area by mid-afternoon.