The rapid evolution of convective systems approaching the coastal metropolitan region of New York City (NYC) is a significant forecast problem. During the spring and early summer, many severe squall lines weaken rapidly or become distorted approaching this urban coastal environment. As a result, many severe thunderstorm warnings do not verify. This talk quantifies this problem using two case studies, a lightning climatology over the region, and a brief discussion of the failure of mesoscale models to forecast such events.

At 2200 UTC 18 May 2000, a severe squall line approached the NYC region and Long Island from the northwest. A few tens of kilometers upstream of the coast, the squall line had a distinct bow echo, leading convective line to 65 dBZ, and a 50-60 kt surface outflow. During the next hour as the convective line approached NYC and interacted with the cooler marine boundary layer, it slowed and became a band of heavy (35-45 dBZ) stratiform precipitation, which resulted in local flooding across NYC. The convective line weakened in a region where the CAPE rapidly decreased from 1500 J/kg to less than 250 J/kg at the coast. Another convective event on 19 April 2002 suggests that NYC itself can distort a convective line, since the convection weakened passing over the city. Both cases illustrate the transient nature of convection in this coastal urban environment. Four years (2000-2003) of cloud to ground lightning data were compiled for the months of June and August over the Northeast U.S. During June there is a clear reduction in the lightning activity approaching the coast around New York City and Long Island, which is consistent with the rapid weakening of squall lines. The cool marine boundary layer clearly impacts the evolution of convection during this early warm season month. In contrast, there is much more lightning activity along the coast in August, given the warmer sea surface temperatures, and thus larger CAPE values on average.

Mesoscale model simulations (MM5) down to 4-km grid spacing have had difficulty forecasting these convective lines approaching the coastal urban areas. For many cases the convection does not organize upstream under conditions of weak to moderate CAPE. Long-term verification of the MM5 shows that cool and dry biases near the urban areas may also hurt these convective forecasts.