Convective storm evolution and frequency around coastal southern New England

A difficult forecast problem along coastal southern New England is determining how convection will initiate and evolve given the complex land-sea-urban boundaries in this region. The spatial and temporal variations of convective storms around southern New England was explored during the warm season (April through September) using composite radar (NOWrad) data at 2-km grid spacing from 1996-2007 as well as cloud-to-ground lightning from the National Lightning Data Network (NLDN) on a 10-km grid from 2001-2007. A favored initiation area during the early afternoon is the immediate lee of the Appalachians, with a tendency for these convective systems to move eastward to the coast by late evening. There is a sharp gradient in convective frequency within 20 km of the coast on average as a result of the relatively stable marine boundary layer, but as the sea surface warms by mid-summer this convective activity increases near the coast, with a nocturnal (0600-1200 UTC) convective maximum over the coastal waters. Convective frequency can vary by more than 40% interannually across subregions of the Northeast. There was 40-50% more convection across southern New England and Long Island during 1998-2001 than 2002-2005, which was partially the result of more frequent and amplified trough activity in 1998-2001.

Spatial composites using the North American Regional Reanalysis (NARR) highlight some of the synoptic flow patterns associated with the enhanced convective frequencies. Convective storms tend to weaken rapidly from west to east across southern New England when there is low-level southerly flow from the relatively cool ocean. Severe convection is favored over the populated New York City and Long Island coastal regions when there is warm, moist, and unstable air extending northward along the mid-Atlantic coastal plain, with west-southwesterly flow at low-levels and an approaching short-wave trough at mid-levels. A Weather Research and Forecasting (WRF) simulation highlights the importance of the marine boundary layer for a representative case by completing experiments with and with cool SSTs near the coast.