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.

Hail in NEW CASTLE, DELAWARE
From 29-JUN-12 23:15:00 to 29-JUN-12 23:15:00 (EST-5)
Location: NEWARK
Magnitude: 1.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A large and intense thunderstorm dropped golf-ball size hail in Newark.

Hail in NEW CASTLE, DELAWARE
From 29-JUN-12 23:20:00 to 29-JUN-12 23:20:00 (EST-5)
Location: WILMINGTON
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A thunderstorm dropped nickel-size hail in Wilmington.

Hail in NEW CASTLE, DELAWARE
From 29-JUN-12 23:21:00 to 29-JUN-12 23:21:00 (EST-5)
Location: NEW CASTLE
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A thunderstorm dropped penny-size hail in the town of New Castle.

Thunderstorm Wind in SUSSEX, DELAWARE
From 29-JUN-12 22:57:00 to 30-JUN-12 00:10:00 (EST-5)
Location: SEAFORD
Magnitude: 61.00 EG
Property damage: 100.00K
Crop damage: 0.00K
Reported by: Public

A gust front outrunning a cluster of severe thunderstorms entered near Seaford in western Sussex County at approximately 11:57 pm EDT on the 29th. This gust front produced damaging wind gusts estimated between 65 mph 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 through Sussex county, producing another round of destructive wind gusts, estimated from 65 mph to 70 mph. Large tree limbs were reported down across the county, including in Seaford, Lincoln, Lewes, and Millsboro. In addition, multiple downed trees were reported in Selbyville, and a wind gust to 70 mph was estimated in Bridgeville. Severe thunderstorms exited eastern Sussex County, including the town of Lewes, at approximately 1:10 am EDT on the 30th. Measured wind gusts included 69 mph in Dewey Beach and 63 mph in Lewes.

Thunderstorm Wind in NEW CASTLE, DELAWARE
From 29-JUN-12 22:57:00 to 29-JUN-12 23:50:00 (EST-5)
Location: NEWARK
Magnitude: 65.00 EG
Property damage: 100.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A gust front outrunning a cluster of severe thunderstorms entered near Newark in western New Castle County at approximately 11:57 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 New Castle County. One thunderstorm cell in particular experienced rapid intensification and expansion as it tracked into New Castle County. Destructive wind gusts, estimated near 75 mph, resulted in tree and power line damage across the county. Multiple downed trees were reported in Middletown and Odessa. In addition, power poles were snapped by wind gusts estimated between 70 and 75 mph in Middletown. Severe thunderstorms exited eastern New Castle County, including Delaware City, at approximately 12:50 am EDT on the 30th.

Hail in NEW CASTLE, DELAWARE
From 29-JUN-12 22:55:00 to 29-JUN-12 22:55:00 (EST-5)
Location: MIDDLETOWN
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Strong thunderstorms advancing into New Castle County dropped penny-size hail in Middletown.

Thunderstorm Wind in KENT, DELAWARE
From 29-JUN-12 22:57:00 to 30-JUN-12 00:10:00 (EST-5)
Location: SANDTOWN
Magnitude: 61.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A gust front outrunning a cluster of severe thunderstorms entered near Sandtown in western Kent County at approximately 11:57 pm EDT on the 29th. This gust front produced damaging wind gusts estimated between 65 mph 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 through Kent County, producing another round of destructive wind gusts, estimated from 65 mph to 70 mph. Large tree limbs were reported down across the county, including in Felton and Dover. In addition, several downed trees and an estimated wind gust to 70 mph were reported in Harrington. Severe thunderstorms exited eastern Kent County, including the town of Bowers, at approximately 1:10 am EDT on the 30th.