Strong low pressure with ample moisture led to a significant rain and flooding event across eastern Pennsylvania into northern and central New Jersey from March 10, 2011 through March 13, 2011. This low pressure system organized over the lower Mississippi Valley and progressed northeastward into the Ohio Valley, drawing in Gulf of Mexico moisture. From Thursday March 10 through Friday March 11, low pressure then tracked from the eastern Ohio Valley into southeastern Canada. Its associated cold front swept through the Middle Atlantic region during Thursday night into Friday. This large storm system also pulled in plenty of Atlantic moisture. Heavy rain fell throughout the region, with the highest totals found at points north and west of the I-95 corridor. Streams and creeks quickly spilled over their banks, and moderate to major river flooding resulted in northern New Jersey and eastern Pennsylvania.

 

With a previous heavy rain event having just occurred from March 6 into March 7, 2011 across the Middle Atlantic region, saturated ground and already high stages on area creeks and rivers raised concerns for a significant flood event. A Flood Watch was first issued at 418am Tuesday March 8 for the following areas: New Castle, Cecil, Sussex, Warren, Morris, Hunterdon, Somerset, Middlesex, Western Monmouth, Mercer, Gloucester, Camden, Northwestern Burlington, Carbon, Monroe, Berks, Lehigh, Northampton, Chester, Montgomery, Bucks, Delaware, and Philadelphia. The Flood Watch was then extended at 1100am Wednesday March 9 to inlcude the remainder of New Jersey, Delaware, and the eastern shore of Maryland. During the course of Thursday March 10 and Friday March 11, dozens of river flood warnings were issued as water levels rose rapidly in response to the copious amount of rainfall. Flood warnings were raised for locations along the Passaic, Delaware, and Lehigh Rivers. Flood warnings were also issued for many points along smaller tributaries and creeks. In addition, Areal Flood Advisories and Warnings were also issued to account for widespread urban and small stream flooding.

 

Southeasterly surface winds provided a strong fetch of moisture off the Atlantic Ocean as low pressure and its associated cold front approached the Mid Atlantic region. The most significant rainfall came in two waves across the Mount Holly county warning area. During Thursday into Thursday evening, the first heavy band of rain tracked northward from Chesapeake Bay into southeastern Pennsylvania and the Lehigh Valley. Heavy rain then spread northward into the Poconos and northern New Jersey where upslope flow over the higher terrain enhanced precipitation efficiency and rainfall rates. By early evening Thursday March 10, locations in Carbon, Monroe, and Northampton counties were already reporting over 2 inches of rainfall. A second extensive band of heavy rain then developed off the Delmarva Peninsula during the late evening hours of Thursday March 10. This large area of rain spread northward through early Friday providing all of the county warning area with another round of heavy precipitation.

Storm total rainfall from this event was quite impressive. Rainfall ranged from 2.5 to 5 inches along and north of interstate 78 to between 1 and 2.5 inches elsewhere. Rainfall from this event and the one previous on Sunday March 6 into Monday March 7 totaled between 7 and 8 inches in some upslope regions of the Poconos and far northern New Jersey. These amounts pushed many rivers, streams, and creeks out of their banks, and widespread flooding was the result. Several locations along the Passaic River basin in northern New Jersey experienced major flooding as a result of this event. Along the Passaic, the 5th and 6th highest crests on record were recorded at Little Falls and Pine Brook, respectively. These are preliminary determinations with final analysis of crest data being performed by the USGS. Moderate flooding was experienced along the Lehigh River at Walnutport, with the 3rd highest crest on record occurring Friday morning March 11. Several locations from Tocks Island to Trenton along the Delaware River reported minor to moderate flooding, but crests did not approach record levels during this event.

The widespread flooding that occurred across the region as a result of heavy rainfall forced the closure of many major and secondary area roadways. Flooding of homes and businesses was also common near the swollen rivers and creeks. Locations along the Passaic River remained above flood stage for more than a week following this event. Saturated ground and brief gusty winds also resulted in a few downed trees across the region.

 

Information contained in this summary is preliminary. More complete and/or detailed information may be contained in subsequent monthly NOAA storm data publications.