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. This lead to flooding across portions of the area. Meanwhile, it was cold enough in the higher elevations of the Southern Poconos of Northeast Pennsylvania for freezing rain on Sunday. In particular, the Pocono Mountains Airport ASOS recorded 0.19 inches of ice accretion, and there was tree damage and power outages reported in Tobyhanna Pennsylvania.
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. By Monday morning and afternoon, tidal flooding made its way up the Delaware River to Philadelphia as a result of the onshore flow.

Flash Flood in DELAWARE, PENNSYLVANIA
From 16-APR-18 06:35:00 to 16-APR-18 09:45:00 (EST-5)
Location: EDGEMONT
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Water rescues took place where Mount Alverno Road crosses Chester Creek between Middletown Township and Aston Township.  A number of creeks left their banks, including Crum Creek and Chester Creek.  The Crum Creek gauge near Newtown Square reached 8.05 feet, exceeding its flood stage of 6 feet.  The Chester Creek gauge at Chester reached 10.78 feet, exceeding its flood stage of 8 feet.

Flood in BUCKS, PENNSYLVANIA
From 16-APR-18 10:30:00 to 16-APR-18 20:00:00 (EST-5)
Location: LANGHORNE
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

The Neshaminy Creek At Langhorne rose above flood stage of 9.0 feet. The maximum height was 10.29 feet.

Coastal Flood in LOWER BUCKS, PENNSYLVANIA
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 in the Croydon section of Bristol Township with water covering Bank Street, Minot Avenue and Sixth Avenue.  Peak tide was 10.61 feet MLLW at Burlington (Burlington County, New Jersey) and 11.37 feet MLLW at Newbold Island.

Flash Flood in PHILADELPHIA, PENNSYLVANIA
From 16-APR-18 06:30:00 to 16-APR-18 08:00:00 (EST-5)
Location: WEST PHILADELPHIA
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Water rescues took place in the vicinity of Cobbs Creek.  A number of small streams and creeks left their banks, including Cobbs Creek and Frankford Creek.  The gauge on Cobbs Creek reached 7.09 feet, exceeding its flood stage of 7 feet.  The gauge on Frankford Creek reached 7.89 feet, exceeding its flood stage of 7 feet.

Winter Weather in MONROE, PENNSYLVANIA
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: ASOS

The Pocono Mountains Airport recorded 0.29 inches of radial ice accretion due to freezing rain. Radial conversion (0.90 planar accretion). In Tobyhanna, tree damage and multiple power outages were reported due to the ice accretion.

Flash Flood in PHILADELPHIA, PENNSYLVANIA
From 16-APR-18 08:20:00 to 16-APR-18 09:55:00 (EST-5)
Location: MANAYUNK
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

A number of small streams and creeks left their banks, including the Wissahickon Creek.  The gauge on the Wissahickon Creek peaked at 6.07 feet, exceeding its flood stage of 5 feet.

Coastal Flood in PHILADELPHIA, PENNSYLVANIA
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: Emergency Manager

Moderate coastal flooding along the tidal Delaware River and its tidal tributaries.  Flooding occurred along Christopher Columbus Boulevard from Spring Garden Street to below the Ben Franklin Bridge.  Peak tide was 9.47 feet MLLW at Philadelphia.

Flash Flood in MONTGOMERY, PENNSYLVANIA
From 16-APR-18 08:45:00 to 16-APR-18 09:55:00 (EST-5)
Location: FLOURTOWN
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

A number of small streams and creeks left their banks, including Wissahickon Creek.  The gauge on the Wissahickon Creek at Fort Washington peaked at 10.93 feet, exceeding its flood stage of 10 feet.

Coastal Flood in DELAWARE, PENNSYLVANIA
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.