A well-organized cluster of thunderstorms with a history of producing widespread damaging winds advanced into a hot, unstable airmass over the Mid Atlantic. This derecho produced widespread, significant wind damage from southern New Jersey southward into the Delmarva during the late evening and overnight of the 29th.
Strong thunderstorm development began across the Midwest and Lower Great Lakes regions during the afternoon of the 29th, with storms initiating across northern Illinois, near Chicago. Strong mid-level winds and an unstable airmass contributed to rapid development as well-organized storms progressed from northern Illinois through Indiana and Ohio. This robust convection continued to track southeastward into an airmass that was even more unstable due to abundant daytime surface heating. Late afternoon temperatures had peaked mainly in the mid to upper 90s with high humidity throughout the Mid Atlantic states, and as the potent thunderstorm cluster progressed from the Great Lakes to the central Appalachians and into the Mid Atlantic by late evening, thunderstorm activity was reinvigorated.
A robust gust front outpaced the main thunderstorm line, producing strong wind gusts as it traveled from the eastern shore of Maryland into Delaware and southern New Jersey. Shortly thereafter, the potent thunderstorm line tracked through the region, with additional damaging wind gusts. One cell in particular around Baltimore, MD experienced rapid intensification and expansion as it raced eastward into northern Delaware and southern New Jersey. This massive cell produced extreme damage throughout Salem, Cumberland, and Atlantic counties in New Jersey. Damage to trees, power lines, and buildings was also significant throughout Delaware and the eastern shore of Maryland. Thunderstorms also dropped hail in several locations, ranging from penny to golf-ball size. Destructive wind gusts, between 65 mph to nearly 90 mph, were measured as this derecho tracked from the Lower Great Lakes to the Mid Atlantic coast.

Thunderstorm Wind in QUEEN ANNE'S, MARYLAND
From 29-JUN-12 22:22:00 to 29-JUN-12 23:31:00 (EST-5)
Location: STEVENSVILLE
Magnitude: 56.00 EG
Property damage: 100.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A gust front outrunning a cluster of severe thunderstorms entered near Stevensville in western Queen Anne's County at approximately 11:22 pm EDT on the 29th. This gust front produced damaging wind gusts estimated at 65 mph as it traversed eastward across the county. Within approximately 20 minutes of the gust front passage, a potent line of severe thunderstorms tracked eastward through Queen Anne's County, producing another round of destructive wind gusts, estimated at 65 mph. Large tree limbs and electric wires were reported down across the county, with damage initially reported in the Stevensville area. Damage to a large chimney and the exterior walls of a home was also reported in Stevensville. Severe thunderstorms exited eastern Queen Anne's County, including the town of Sudlersville, at approximately 12:31 am EDT on the 30th.

Thunderstorm Wind in TALBOT, MARYLAND
From 29-JUN-12 22:22:00 to 29-JUN-12 23:31:00 (EST-5)
Location: TILGHMAN
Magnitude: 56.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: County Official

A gust front outrunning a cluster of severe thunderstorms entered the Tilghman Island area of western Talbot County at approximately 11:22 pm EDT on the 29th.  This gust front produced damaging wind gusts estimated at 65 mph as it traversed eastward across the county.  Within approximately 20 minutes of the gust front passage, a potent line of severe thunderstorms tracked eastward through Talbot County, producing another round of destructive wind gusts, estimated at 65 mph.  A significant number of trees and electric wires were reported down county-wide with damage first being noted on Tilghman Island.  Severe thunderstorms exited eastern Talbot County, including the town of Matthews, at approximately 12:31 am EDT on the 30th.

Thunderstorm Wind in KENT, MARYLAND
From 29-JUN-12 22:22:00 to 29-JUN-12 23:31:00 (EST-5)
Location: ROCK HALL
Magnitude: 57.00 MG
Property damage: 20.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A gust front outrunning a cluster of severe thunderstorms entered near Rock Hall in western Kent County at approximately 11:22 pm EDT on the 29th. This gust front produced damaging wind gusts estimated at 65 mph as it traversed eastward across the county. Within approximately 20 minutes of the gust front passage, a potent line of severe thunderstorms tracked eastward through Kent County, producing another round of destructive wind gusts, estimated at 65 mph. Many large tree limbs were reported down across the county, including in the Millington area. Severe thunderstorms exited eastern Kent County, near Massey, at approximately 12:31 am EDT on the 30th. A peak wind gust of 66 mph was measured at Tolchester Beach.

Thunderstorm Wind in CECIL, MARYLAND
From 29-JUN-12 22:27:00 to 29-JUN-12 23:40:00 (EST-5)
Location: PORT DEPOSIT
Magnitude: 61.00 EG
Property damage: 500.00K
Crop damage: 0.00K
Reported by: Emergency Manager

A gust front outrunning a cluster of severe thunderstorms entered near Port Deposit in western Cecil County at approximately 11:27 pm EDT on the 29th. This gust front produced damaging wind gusts estimated from 65 to 70 mph as it traversed eastward across the county. Within approximately 20 minutes of the gust front passage, a potent line of severe thunderstorms tracked eastward into Cecil County. One thunderstorm cell in particular experienced rapid intensification and expansion as it tracked from around Baltimore, MD across Cecil County and into northern Delaware. Destructive wind gusts, estimated at 70 mph, resulted in significant tree and power line damage county-wide. Approximately 35 roads were impassable due to numerous downed trees. In Chesapeake City, several large trees fell onto homes, resulting in one injury. A tree also fell onto a house in the town of North East. Damage to multiple buildings was reported in Warwick, and a large concentration of downed trees was reported in Cecilton. Severe thunderstorms exited eastern Cecil County, including the town of Elkton, at approximately 12:40 am EDT on the 30th.  A young man was critically injured on the morning of July 2 around 330 am EDT when a tree branch fell on his tent at Camp Rodney in Elk Neck State Park in Northeast, Maryland. It is believed that the tree was weakened by the derecho that went through the night of the 29th.

Thunderstorm Wind in CAROLINE, MARYLAND
From 29-JUN-12 22:40:00 to 29-JUN-12 23:49:00 (EST-5)
Location: HARMONY
Magnitude: 56.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Newspaper

A gust front outrunning a cluster of severe thunderstorms entered near Harmony in western Caroline County at approximately 11:40 pm EDT on the 29th. This gust front produced damaging wind gusts estimated at 65 mph as it traversed eastward across the county. Within approximately 20 minutes of the gust front passage, a potent line of severe thunderstorms tracked eastward through Caroline County, producing another round of destructive wind gusts, estimated at 65 mph. Trees and electric wires were reported down across the county. Severe thunderstorms exited eastern Caroline County, including the town of Henderson, at approximately 12:49 am EDT on the 30th.