The spatial and temporal variability of convective storms over the Northeast United States during the warm season

Aspatial and temporal climatology of convective storms over the Northeast United States during the warm season (April-September) is presented using composite National Operational Weather radar (NOWrad) data at 2-km grid spacing from 1996 to 2007 as well as cloud-to-ground lightning from the National Lightning Data Network (NLDN) on a 10-km grid from 2001 to 2007. There are preferred regions for convective storms within New York's Hudson River Valley, western and southeastern Pennsylvania, central New Jersey, and across the Delmarva Peninsula. 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 midsummer 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 in 2002-05, which was partially the result of more frequent and amplified trough activity in 1998-2001, which helped to trigger convective storms. 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 westsouthwesterly flow at low levels and an approaching shortwave trough at midlevels.