A warm and very humid air mass was in place over the mid-Atlantic on July 17. A warm front which had sagged south into the region on the night of the 16th pushed back to the north on the morning of the 17th. Meanwhile, a cold front slowly approached by evening, with a pre-frontal surface trough ahead of it. The presence of multiple synoptic and mesoscale boundaries led to a highly complex convective evolution. Several areas and multiple rounds of convection affected the eastern mid-Atlantic over about a 12 hour period from early afternoon on the 17th to a little after midnight on the 18th. With shear falling in the weak to moderate range, convection mainly took the form of multi-cellular and linear clusters, though some semi-discrete supercellular structures were noted at times. A number of storms became severe with damaging wind. Areas of heavy rain and flash flooding also occurred due to the widespread and at times slow moving nature of the convection. Eventually, by later in the night, the approach of the cold front and significant overturning of the atmosphere from the day's convection caused remaining activity to weaken and shift offshore.

Thunderstorm Wind in BUCKS, PENNSYLVANIA
From 17-JUL-21 19:15:00 to 17-JUL-21 19:15:00 (EST-5)
Location: STANWOOD GARDENS
Magnitude: 50.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A tree was reported down.

Thunderstorm Wind in CHESTER, PENNSYLVANIA
From 17-JUL-21 20:32:00 to 17-JUL-21 20:32:00 (EST-5)
Location: FIVE PTS
Magnitude: 50.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

A downed tree blocked the road on Round Hill Rd at Kaoli Rd. Time estimated from radar and dispatch report.

Thunderstorm Wind in CHESTER, PENNSYLVANIA
From 17-JUL-21 20:29:00 to 17-JUL-21 20:29:00 (EST-5)
Location: KAOLIN
Magnitude: 50.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Downed tree limbs blocked Kaolin Rd at Limestone Rd. Time estimated from radar and dispatch report.

Thunderstorm Wind in LEHIGH, PENNSYLVANIA
From 17-JUL-21 14:10:00 to 17-JUL-21 14:10:00 (EST-5)
Location: CHAPMAN
Magnitude: 60.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Some siding was blown off a home. Time estimated from radar.

Thunderstorm Wind in PHILADELPHIA, PENNSYLVANIA
From 17-JUL-21 17:55:00 to 17-JUL-21 17:55:00 (EST-5)
Location: FOX CHASE
Magnitude: 55.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Utility Company

Several instances of power outages were reported in northeast Philadelphia. Time estimated from radar.

Thunderstorm Wind in MONTGOMERY, PENNSYLVANIA
From 17-JUL-21 18:00:00 to 17-JUL-21 18:00:00 (EST-5)
Location: GLENSIDE
Magnitude: 55.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Several reports of downed trees and power outages in the area. Time estimated from radar.

Thunderstorm Wind in NORTHAMPTON, PENNSYLVANIA
From 17-JUL-21 14:00:00 to 17-JUL-21 14:00:00 (EST-5)
Location: SHOENERSVILLE
Magnitude: 50.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Measured gust from the east with a microburst. A second gust of 57 mph occurred from the west a short time later. Trees and limbs were snapped nearby.

Thunderstorm Wind in BUCKS, PENNSYLVANIA
From 17-JUL-21 19:08:00 to 17-JUL-21 19:08:00 (EST-5)
Location: STANWOOD GARDENS
Magnitude: 55.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Several power outages were reported. Large branches were blown down and siding was also blown off a house. The spotter estimated winds above 60 mph. Time estimated from radar.

Thunderstorm Wind in BUCKS, PENNSYLVANIA
From 17-JUL-21 12:08:00 to 17-JUL-21 12:08:00 (EST-5)
Location: CARVERSVILLE
Magnitude: 55.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Several large trees and numerous branches were downed with power outages.

Thunderstorm Wind in LEHIGH, PENNSYLVANIA
From 17-JUL-21 14:10:00 to 17-JUL-21 14:10:00 (EST-5)
Location: FOGELSVILLE
Magnitude: 50.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A large tree was snapped. Time estimated from radar.

Thunderstorm Wind in NORTHAMPTON, PENNSYLVANIA
From 17-JUL-21 14:10:00 to 17-JUL-21 14:10:00 (EST-5)
Location: SHOENERSVILLE
Magnitude: 55.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Several trees were downed in North Catasauqua and Catasauqua. Time estimated from radar.

Thunderstorm Wind in PHILADELPHIA, PENNSYLVANIA
From 17-JUL-21 21:05:00 to 17-JUL-21 21:05:00 (EST-5)
Location: TACONY
Magnitude: 50.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

An mPing report of a downed tree.

Thunderstorm Wind in BUCKS, PENNSYLVANIA
From 17-JUL-21 18:05:00 to 17-JUL-21 18:05:00 (EST-5)
Location: LUMBERVILLE
Magnitude: 55.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Several trees were blown down near the intersection of Fleecy Dale Rd and River Rd.