Two Major Heavy Rain/Flood Events in the Mid-Atlantic: June 2006 and September 2011

The fourth and sixth largest flood events in the recorded history of the northern Mid-Atlantic region are compared in an effort to assist forecasters in recognizing favorable synoptic and mesoscale patterns capable of producing historic flooding. In June 2006, significant flooding and flash flooding impacted much of the Mid-Atlantic region as a continuous supply of deep tropical moisture moved north from the subtropical Atlantic ahead of a slow moving cold front. A three-day period of heavy rain resulted in nearly 38.1 cm (15 in) of rain across portions of the northern Mid-Atlantic with record flooding along the mainstem Susquehanna and Delaware Rivers. In September of 2011, tropical moisture associated with the remnants of Tropical Storm Lee resulted in a 24-hour period of heavy rain over which rainfall totals approached 30.3 cm (12 in) across portions of central New York and northern Pennsylvania. Numerous river stage records set in the June 2006 event were easily shattered along the mainstem Susquehanna River. Combined in both events, damage estimates resulting from flash flooding and major river flooding exceeded $2 billion. Multiple counties across the northern Mid-Atlantic were declared disaster areas and 22 lives were lost as a direct result of the flooding.

Both flood events were investigated to identify meteorological features and patterns responsible for extreme rainfall. Several crucial similarities were identified, which likely combined to allow both events to be regarded as historic in terms of socioeconomic and environmental impacts. One of the similarities was that each event had a well-established atmospheric river in place, which allowed for the uninterrupted supply of deep tropical moisture. Additionally, it is shown that while these events displayed many of the large-scale characteristics identified in previous flash flood classification schemes, their location near the east coast of the United States resulted in the presence of coastal fronts, which appeared to make these cases different than many otherwise similar, and previously documented flood cases.