Five days of showers and thunderstorms with heavy rain culminated with flash flooding of smaller streams in northeastern Morris County during the late afternoon and early evening of the 18th and then larger stream and river flooding in eastern Morris County that lasted into the 22nd. Minor flooding occurred along the Rockaway and Passaic Rivers and moderate flooding occurred along the lower stretches of the Pompton River. Five day event precipitation totals averaged 3.5 to 6.0 inches. The heaviest rain fell during the afternoon and early evening on the 18th with precipitation totals of around two inches.
The periods of heavy rain were caused by a persistent and blocky weather pattern which forced a low pressure system over the southeastern United States to only slowly move to the northeast. The surface and its upper level low pressure system did not reach New York State until the morning of the 20th. The persistent southeast flow coupled with some of the highest astronomical tides of the year also produced around five days of widespread minor tidal flooding during the evening and overnight high tide cycles from the 15th through the 19th. The highest tides occurred during the evening and overnight high tide cycle on the 17th. The highest tides included 7.66 feet above mean lower low water at Sandy Hook (Monmouth County), 6.52 feet above mean lower low water in Atlantic City (Atlantic County), 7.67 feet above mean lower low water in Cape May (Cape May County), 7.86 feet above mean lower low water at Reedy Island, Delaware and 8.73 feet above mean lower low water in Philadelphia, Pennsylvania.

Flood in MORRIS, NEW JERSEY
From 18-MAY-11 15:45:00 to 22-MAY-11 20:45:00 (EST-5)
Location: POWERVILLE
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: River/Stream Gage

Five consecutive days of periods of heavy rain led to flooding of brooks and rivers in the eastern part of Morris County. The Passaic River at Pine Brook was above its 19 foot flood stage from 700 a.m. EDT on the 19th through 945 p.m. EDT on the 22nd. It crested at 19.68 feet at 830 a.m. EDT on the 20th. Farther downstream, the Passaic River at Circle Burners or Lincoln Park crested at 6.42 feet at 1005 p.m. EDT on the 20th. Flood stage is 5.5 feet.  The lower areas of the Pompton River had the worst flooding with this event as flood stages reached moderate levels. Nevertheless, this was less flooding than what occurred in mid March and mid April of this year. The Pompton River at Ryerson Road was above its 12 foot flood stage from 900 a.m. EDT on the 19th through 952 a.m. EDT on the 21st. It crested at 13.73 feet at 300 a.m. EDT on the 20th. Farther downstream, the Pompton River at Midwood was above its 7.5 foot flood stage from 759 a.m. EDT on the 19th through 845 a.m. EDT on the 22nd. It crested at 9.44 feet at 530 a.m. EDT on the 20th.
The East Ditch at Beaver Brook Road was above its 2.5 foot flood stage from 753 p.m. EDT on the 18th through 1138 a.m. EDT on the 19th. It crested at 3.06 feet at 315 a.m. EDT on the 19th. The Rockaway River above the Boonton Reservoir was above its 5 foot flood stage from 1003 p.m. EDT on the 18th through 855 p.m. EDT on the 19th. It crested at 5.53 feet at 7 a.m. EDT on the 19th. The Rockaway River below the Boonton Reservoir was above its 5 foot flood stage from 445 p.m. EDT on the 18th through 730 a.m. EDT on the 21st. It crested at 6.20 feet at 1145 a.m. EDT on the 19th.
Event precipitation totals included 5.85 inches in Denville, 5.81 inches in Boonton, 5.71 inches in Rockaway, 5.58 inches in Morris, 4.98 inches in Jefferson, 4.90 inches in Parsippany, 3.75 inches in Mine Hill, 3.72 inches in Mount Olive, 3.71 inches in Long Hill, 3.70 inches in Chatham and 3.54 inches in Randolph.

Flash Flood in MORRIS, NEW JERSEY
From 18-MAY-11 15:45:00 to 18-MAY-11 18:00:00 (EST-5)
Location: LAKE TELEMARK
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Heavy rain during the late afternoon on the 18th led to flash flooding of smaller streams in northeastern Morris County. Larger stream and river flooding followed. This was the fifth consecutive day of heavier rain within the county. Storm total amounts on the 18th within the axis of heavier rain averaged 1.5 to 2.0 inches. The axis of heavier rain extended from East Hanover and Montville Townships northwest through Parsippany, Kinnelon, Boonton and Rockaway Townships.