The climatology and organization of convection over the Northeast United States

This presentation highlights the spatial distribution and evolution of deep convection over the Northeast U.S. during the warm season (April through September), as well as the favored convective organizations in this region. There is little knowledge of where convection is initiated over the Northeast U.S. and its relation to the terrain, coastal, and urban areas. A convective climatology was constructed for the Northeast U.S. using 2km by 2km resolution NOWrad radar data from 1996-2007 as well cloud-to-ground lightning from the National Lightning Data Network (NLDN) from 2001-2007. The different convective structures over the Northeast were classified during two warm seasons (2007 and randomly picked days from May to September of 2002-2006) using the same approach published for the central U.S. This involved perusing hourly NOWRAD (2-km grid-spacing) radar imagery and classifying the convection into five types of cellular convection (long-lived individual cells, short-lived individual cells, clusters of long-lived cells, clusters of short-lived pulse cells, and broken squall lines), five types of linear systems (bow echoes, squall lines with trailing stratiform rain, lines with leading stratiform rain, lines with parallel stratiform rain, and lines with no stratiform rain), and nonlinear systems.

There are preferred regions for convection within the Hudson Valley, western and southeast Pennsylvania, central New Jersey, and the Delmarva Peninsula. A favored initiation area includes the immediate lee of the Appalachians. There is a sharp gradient in convective frequency immediately west of the coast (around New York City) as a result of the cooler marine boundary layer. As the warm season progresses, the convective activity shifts more towards the coast with the warming sea surface temperatures. During the mid-day period (18-00 UTC), the maximum convection is clearly over inland areas, but by late at night (06-12 UTC) the convective maximum shifts more offshore from the southern New England coast southwestward to the Chesapeake Bay. Composites using the North American Regional Reanalysis (NARR) highlight some of the flow patterns attached to the favored areas of convective development. It was found that the Northeast U.S. has a similar distribution of different types of convective morphologies as the central U.S., but the production of severe weather varies significantly between the two regions. For example, Northeast hail events are evenly distributed among convective clusters, nonlinear, and non-stratiform linear systems. Central U.S. hail develops primarily with broken lines, bow echos, and parallel stratiform linear systems.