A strong backdoor cold front moved from northeast to southwest across the area late Saturday afternoon April 15th into Sunday morning April 16th. Not only was there a significant drop in temperature coinciding with the passage of the front, but also a stark difference between high temperatures on Saturday and Sunday. On Saturday, high temperatures ranged from the mid-70s to mid-80s, but only made it to the mid-30s to mid-40s on Sunday. In general, there was a 40 degree temperature drop from Saturday to Sunday!
The backdoor front became stationary south of the region then moved northward as a warm front Sunday night into Monday morning. Meanwhile, a strong cold front approached from the west and swept across the area Monday afternoon and evening, along with an attendant area of low pressure. This robust storm system tapped large amounts of moisture, which was lifted along the warm front as overrunning precipitation, and along the cold front as a line of showers and thunderstorms.
As a result, heavy rainfall occurred across the region. A widespread 1 to 3 inches of rainfall occurred, with localized amounts over 4 inches, especially across the northern half of New Jersey. As a result, flooding occurred across portions of the area. Meanwhile, it was cold enough in the higher elevations of Sussex County New Jersey for freezing rain on Sunday.
Along the coast, an extended period of onshore flow occurred, beginning with east-northeast winds Saturday Night, which then shifted to the southeast on Sunday Night into Monday Morning April 16th. Widespread winds gusts of 40 to 50 mph occurred in the east-northeast flow Saturday Night and Sunday, with a lull Sunday Night, then occasionally again in the south-southeast flow along and just in advance of a heavy line of showers and thunderstorms on Monday Morning. The onshore flow, and to a lesser extent the heavy rainfall, lead to multiple rounds of coastal flooding along the oceanfront and back bays with the high tide cycles on Sunday evening and again on Monday Morning.

Coastal Flood in MIDDLESEX, NEW JERSEY
From 15-APR-18 20:00:00 to 15-APR-18 23:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Moderate coastal flooding with a number of road closures in the communities along Raritan Bay.  Peak tide was 8.13 feet MLLW at Perth Amboy.

Coastal Flood in MERCER, NEW JERSEY
From 16-APR-18 14:00:00 to 16-APR-18 18:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Moderate coastal flooding along the tidal Delaware River.  Peak tide was 11.37 feet MLLW at Newbold Island (Bucks County, Pennsylvania).

Coastal Flood in NORTHWESTERN BURLINGTON, NEW JERSEY
From 16-APR-18 13:00:00 to 16-APR-18 18:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Moderate coastal flooding along the tidal Delaware River and its tidal tributaries.  Flooding occurred along the riverfront in Burlington City.  Peak tide was 10.61 feet MLLW at Burlington.

Coastal Flood in CAMDEN, NEW JERSEY
From 16-APR-18 13:00:00 to 16-APR-18 17:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Moderate coastal flooding along the tidal Delaware River and its tidal tributaries.  Peak tide was 9.47 feet MLLW at Philadelphia.

Coastal Flood in GLOUCESTER, NEW JERSEY
From 16-APR-18 13:00:00 to 16-APR-18 17:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Moderate coastal flooding along the tidal Delaware River and its tidal tributaries.  There were a number of road closures including New Jersey Route 47 in Westville and US Route 130 in Logan Township.  Peak tide was 9.47 feet MLLW at Philadelphia.

Coastal Flood in SALEM, NEW JERSEY
From 16-APR-18 10:00:00 to 16-APR-18 16:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Moderate coastal flooding along the tidal Delaware River and its tidal tributaries.  Peak tide was 8.94 feet MLLW at Wilmington (New Castle County, Delaware).

Flood in SOMERSET, NEW JERSEY
From 16-APR-18 17:00:00 to 17-APR-18 18:45:00 (EST-5)
Location: MILLSTONE
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

The Millstone River at Blackwells Mills rose above the flood stage of 9.0 feet and reached a  maximum of 10.63 feet.

Flood in MORRIS, NEW JERSEY
From 16-APR-18 23:30:00 to 17-APR-18 13:45:00 (EST-5)
Location: BOONTON
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

The Rockaway River at Boonton Below the Reservoir rose above flood stage of 5.0 feet and reached a maximum of 5.28 feet.

Flood in SOMERSET, NEW JERSEY
From 16-APR-18 21:00:00 to 16-APR-18 23:15:00 (EST-5)
Location: CAMP KILMER
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

The Raritan River at Manville rose above flood stage of 14.0 feet and reached a maximum of 14.11 feet.

Flood in MORRIS, NEW JERSEY
From 17-APR-18 18:15:00 to 19-APR-18 19:15:00 (EST-5)
Location: HANOVER ARPT
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

The Passaic River at Pine Brook rose above flood stage of 19.0 feet to a maximum of 19.24 feet.

Winter Weather in SUSSEX, NEW JERSEY
From 16-APR-18 00:30:00 to 16-APR-18 09:00:00 (EST-5)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trace of ice accretion was observed on elevated surfaces due to freezing rain.

Flood in MIDDLESEX, NEW JERSEY
From 17-APR-18 20:00:00 to 17-APR-18 23:45:00 (EST-5)
Location: OUTCALT
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

The Manalapan Brook at Spotswood rose to flood stage at 19 feet, reaching a maximum of  19.02 feet.

Flood in SOMERSET, NEW JERSEY
From 16-APR-18 22:15:00 to 17-APR-18 00:45:00 (EST-5)
Location: BOUND BROOK JCT
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

The Raritan River at Bound Brook rose above flood stage of 28.0 feet and reached a maximum of 28.10 feet.