Relationship between organized convective structures and severe weather type over the northeast U.S

The organizational structures of all warm season convection are determined by the ambient dynamic and thermodynamic conditions in which they develop. Over the Midwest, it has been shown that specific convective structures are more likely to produce certain types of severe weather (i.e. wind, hail tornadoes). Over the northeastern U.S, the relationship between convective organizational structure and severe weather type is unknown. This presentation will highlight this relationship over the complex surface conditions of the northeast U.S. as well as examine the environments that support the various severe structures.

For this study, organizational structures of convection over the Northeast were identified through the manual examination of 2-km NOWrad radar reflectivity imagery. Convection was classified into three types of cellular convection (individual cells, clusters of cells, and broken lines), five types of quasi-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. Both the Storm Prediction Center (SPC) and National Climatic Data Center (NCDC) storm reports archives were utilized to construct a dataset of severe reports over the Northeast. Severe wind and hail reports were identified for 2 warm seasons (May-Aug), including 2007 and a random warm season (2002-2006). To increase the sample size of tornado events, tornado reports were identified over an 11-year period from 1996-2007. Each storm report was matched to the convective structure that produced it.

Over the Northeast, the most common type of severe weather reported is severe wind, comprising 68% of all severe reports. This is dominance of severe wind differs from the central U.S, as previous studies have shown that hail is the most common severe weather type over the Midwest. Northeast linear systems are associated with the highest number of wind reports, closely followed by cellular convection. Cellular structures are the primary producers of hail over the Northeast, associated with two-thirds of all hail reports. Cellular and linear convection produce the majority of tornadoes, associate with 46% and 40% of all reports respectively. However, there have been destructive tornado events caused by nonlinear systems, including the 8 August 2007 event over NYC. Forecasting the development of tornadoes from nonlinear convective systems poses a difficult forecast challenge, given that it is unclear where to anticipate rotation within such a widespread storm. Therefore, the environments that support organized convective structures capable of producing severe weather will be highlighted, with particular emphasis on nonlinear convective events associated with tornadogenesis. Comparisons will be made between nonlinear tornadic and nontornadic events, especially for instability, low-lever shear, and the development of surface baroclinic zones.