The Influence of Terrain on the Severe Weather Distribution across Interior Eastern New York and Western New England

Abstract

Forecasters have surmised that prominent mountain ranges and river valleys in eastern New York and western New England (e.g., Hudson and Mohawk River valleys; Adirondack, Catskill, Green, and Berkshire Mountains) affect convective initiation and subsequent severe weather distribution. The purpose of this research is to document the climatology of severe weather in this region with respect to the terrain and the synoptic-scale flow direction. The area of study was subdivided into overlapping 0.5° grid boxes, and the number of severe weather reports from the database (1950?98) was tabulated for each box. These severe weather reports were then normalized and contoured over a terrain map. A logarithmic correction factor was applied to the data in order to minimize potential population bias effects. The results of this correction were compared with cloud-to-ground (CG) lightning strikes (independent of population bias) from 1989 to 1998 (1990 missing) for severe weather days in the same region. The severe weather and CG lightning database also was stratified by 700-hPa flow direction into northwest and southwest flow regimes to see if subtle terrain influences on the severe weather distribution could be detected. Regions where the CG lightning and severe weather stratifications agree well include the southern Adirondacks, Berkshires, and the Litchfield Hills of northwest Connecticut. Regions where discrepancies exist between the two stratifications include the Catskills and the mid?Hudson valley. The results of both severe weather and lightning stratifications show that there are preferred regions of upstate New York and western New England for both CG lightning and severe weather to occur depending on the 700-hPa flow direction.