The flash flood of 12 July 2004 in Burlington County, New Jersey: A case study

Flash flooding is rare in southern New Jersey, owing mainly to the flat topography and the sandy, porous soils found there. However, on 12 July 2004, up to 13 inches of rain fell over portions of Burlington County, with most of the heavy rain occurring in just a 4-hour period during the late afternoon and evening. The excessive rainfall caused record-breaking flash flooding along nearly every stream in the Rancocas Creek Basin, leading to the failure or damage of 44 dams in Burlington County and subsequent national media attention. Based on precipitation frequency estimates from NOAA's National Weather Service Office of Hydrologic Development, this event represented a one-in-one-thousand year flood.

On 12 July 2004, a stationary front extended west to east approximately along the Maryland-Pennsylvania border, and then curved southeast through the Delaware Bay. The airmass south of the front became very moist and unstable by afternoon, with precipitable water of 2.0 to 2.5 inches and surface based lifted index (LI) of –6 to –8, while north of the front the airmass actually became more stable (surface LI of +2 to +4). Thus, the boundary intensified throughout the day as a surface low-pressure center tracked slowly eastward along it. Southwest winds at 850mb to 700mb advected the very moist and unstable air up and over the front during most of the afternoon, while by late afternoon a strong low-level (925mb to 850mb) southeast flow developed north of the front over southern New Jersey. As thunderstorms developed over southern New Jersey in the early evening, they produced intense rainfall rates of up to 3.5 inches per hour. Local rainfall totals over parts of Burlington County, New Jersey were greatly increased as multiple thunderstorm cells “trained” across the same area for several hours.

While several other locations in the nearby mid-Atlantic region received heavy rainfall earlier on this day, the rainfall over Burlington County was a particularly extreme event. A nearly total lack of cloud-to-ground lightning over the Burlington County area suggests that highly efficient warm-rain processes were more active there than for other heavy rain events that day. Another possible factor is the low-level southeasterly flow that brought in maritime air from the Atlantic Ocean. This case study will document the overall synoptic and mesoscale conditions that favored heavy rainfall this day, with a further emphasis on the unique hydro-meteorological factors that produced extreme rainfall and resulting extensive flood damage in Burlington County.