Northeast severe weather distribution as a function of flow regime

This talk will report on the results of a COMET-funded research project between the University at Albany and the National Weather Service in Albany, New York. The focus of the research is a study of how the occurrence and location of severe weather (tornadoes, hail, high winds) in eastern New York and western New England is modulated by the underlying terrain as a function of the prevailing large-scale flow patterns. It is hoped that the results of this study will help forecasters isolate preferred regions for severe weather (more specifically, tornadoes) based on the large scale flow pattern. The study has been divided up into two main areas: 1) a climatology of severe weather events (focusing on tornado events) in eastern New York and western New England using Storm Data reports from 1955 to 1998, and 2) a synoptic analysis of tornado events in this region from the period 1980 to 1998 using NCEP Reanalysis Data.

All available severe weather reports were stratified based on the 700mb flow direction as determined from the Albany radiosonde reports. The 700mb level was chosen in hopes that the winds would be high enough to reflect some of the large scale flow, yet low enough to be affected by the topography. Preliminary observations indicate that there is a tendency for severe weather to occur in preferred regions based on the mid-level flow direction. This suggests that funneling of wind by the river valleys may be an important factor in creating preferred regions for severe weather. Currently, work is being done to quantify these suggested results. The region has been divided into 0.5 degree latitude/longitude boxes, and in each box the average number of events per severe weather day with northwest flow is being compared to the average number of events per severe weather day with southwest flow. In addition, an attempt is being made to account for some of the known biases in the Storm Data reports, such as population.

The goal of the second area of this project is to determine the evolution and structure of synoptic-scale flow features during and preceding tornadoes in eastern New York and western New England. We examined 47 reported tornado events from 1980 to 1998. For each tornado event, we are documenting the large scale flow pattern (from the surface to the tropopause), as well as parameters important to severe weather such as vertical motion, location of jet streaks, moisture, and instability (CAPE). Once this study of the various large scale parameters is complete, composites will be made of events with similar large scale features to illustrate representative signatures. In addition, detailed case studies of selected events will be done. It is hoped that results of this part of the project will assist forecasters in identifying particular flow patterns that might indicate a preference for severe weather to occur in a particular area of the forecast region.