A deep low pressure system tracked across the Lower Great Lakes northeastward into the St. Lawrence Valley from the evening of the 20th into the daytime of the 21st. Its associated strong cold front swept eastward through the Mid Atlantic region and across Delaware during the early morning of the 21st. In addition, a secondary area of low pressure formed along the frontal boundary, west of Delaware Bay, and deepened rapidly. The approaching cold front and the deepening secondary low pressure produced a strong southeasterly flow over the region that resulted in peak southeast wind gusts ranging from 45 mph to 60 mph across the state during the early morning on the 21st.  The wind damaged trees, tree limbs and knocked down power lines.  Most of the reported downed trees were in New Castle County.  Delmarva Power reported about 650 of its customers lost power, 500 of them were in New Castle County. Peak wind gusts included 60 mph at the New Castle County Airport, 48 mph in Georgetown (Sussex County) and 45 mph in Lewes (Sussex County) and  Dover (Kent County).
In addition, the significant southeast flow allowed water to pile up along the Delaware Coast and into Delaware Bay, producing minor to major coastal flooding at the time of high tide on the morning of the 21st. The magnitude of the tidal flooding increased northwestward through Delaware Bay into tidal sections of the Delaware River. Moderate to heavy rain fell across the state, with storm totals ranging between 1 to 3 inches. This rain resulted in minor to moderate flooding of creeks and rivers in Northern Delaware.
The strong onshore flow contributed to higher high tides with minor tidal flooding occurring along the Delaware oceanfront. High tide at Breakwater Harbor (Sussex County) reached 6.67 feet above mean lower low water. Minor tidal flooding starts at 6.0 feet above mean lower low water. The southeasterly flow pushed water up the Delaware Bay and into the tidal portions of the Delaware River, causing major tidal flooding to occur at Reedy Point (New Castle County). High tide at Reedy Point reached 9.2 feet above mean lower low water. Major tidal flooding starts at 9.2 feet above mean lower low water. The high tide reached 9.47 feet above mean lower low water at Delaware City (New Castle County). There are no established flood categories for this site. Because of tidal effects from Chesapeake Bay which extended east through  the Nanticoke River, the Woodland Ferry (in Sussex County) canceled operations on the 21st.

Coastal Flood in NEW CASTLE, DELAWARE
From 21-DEC-12 04:00:00 to 21-DEC-12 08:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: WLON

High Wind in NEW CASTLE, DELAWARE
From 21-DEC-12 02:00:00 to 21-DEC-12 05:00:00 (EST-5)
Magnitude: 52.00 MG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: ASOS

Strong Wind in INLAND SUSSEX, DELAWARE
From 21-DEC-12 01:00:00 to 21-DEC-12 05:00:00 (EST-5)
Magnitude: 42.00 MG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: ASOS

Strong Wind in DELAWARE BEACHES, DELAWARE
From 21-DEC-12 01:00:00 to 21-DEC-12 05:00:00 (EST-5)
Magnitude: 40.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Mesonet

Strong Wind in KENT, DELAWARE
From 21-DEC-12 01:00:00 to 21-DEC-12 05:00:00 (EST-5)
Magnitude: 40.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Mesonet

Flood in NEW CASTLE, DELAWARE
From 21-DEC-12 05:30:00 to 21-DEC-12 12:20:00 (EST-5)
Location: GLASGOW
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

The heavy rain caused considerable flooding within Wilmington as well as in New Castle. In both locations, vehicles were stuck in high water and firefighters rescued stranded motorists. In Wilmington, the Southbridge area was impacted by the flooding the hardest. South Market Street (along the Christina River), South Herald Street, New Castle Avenue, South Walnut Street (along the Chrsitina River) were totally impassable. Elsewhere in the city, East 12th Street and Interstate 495 were also flooded.  Most of the flooding was gone by the middle of the morning on the 21st.
The Christina River at Coochs Bridge had moderate flooding and was above its 10.5 foot flood stage from 535 a.m. EST through 1136 a.m. EST on the 21st. It crested at 11.99 feet at 9 a.m. EST. The Red Clay Creek at Wooddale was above its 7 foot flood stage from 808 a.m. EST through 945 a.m. EST on the 21st. It crested at 7.11 feet at 845 a.m. EST. The Red Clay Creek at Stanton was above its 15 foot flood stage from 853 a.m. EST through 1217 p.m. EST on the 21st. It crested at 15.19 feet at 945 a.m. EST.
Event precipitation totals included 2.45 inches in Newark, 1.64 inches in Newport, 1.44 inches at the Wilmington Airport and 1.43 inches in Middletown.