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SUNY (Albany): "Terrain wind channeling and severe weather in the greater Capital District"

Final Report

This Partners Project was a case study on the effects of wind channeling by north-south oriented river valleys on the development of severe weather in the greater Capital District. The project was motivated by the destructive Great Barrington (GBR), Massachusetts tornado of 29 May 1995 that claimed 3 lives and injured more than a dozen people. The Great Barrington storm was noteworthy in two respects. First, the WSR-88D base velocity observations from the radar site in Berne, New York revealed the existence of a persistent, strong southerly low-level inflow into the storm region over the Hudson River valley. Second the NWS staff members on duty at Albany recognized the characteristic tornado mesocyclone signature on the 88D base velocity maps and were able to issue the first-ever tornado warning in advance of an actual touchdown for this area.

The research findings from the case study have demonstrated important terrain influences on the storm. Specifically, a mesocyclone moving eastward toward the Hudson Valley intensified in association with vortex tube stretching east of the Catskill escarpment with further vorticity spinup occurring in response to a thunderstorm-induced cool air surge down the Catskill Creek. The mesocyclone went tornadic just east of the Hudson River in Columbia County (NY). The tornado moved eastward into Massachusetts, temporarily weakened as it ascended the mountains west of the Housatonic Valley and reintensified just west of GBR as it encountered the favorable terrain-channeled southerly flow up the Housatonic Valley. These findings are convincing evidence for the importance of terrain effects on mesocyclone/tornado development from newly available WSR-88D observations.

WSR-88D training to date has concentrated on using rotational velocities (Vr) to assess mesocyclone strength. WSR-88D Build 8 software included procedures for easily calculating, not only Vr , but also the associated shear. Shear may be more useful in assessing mesocyclone intensity since it measures the change in Vr over distance. Participants used the WSR-88D shear algorithm extensively in their storm analysis. While this one event will not produce guidelines for its use in all situations, it will provide forecasters with a benchmark that will be useful in future WSR-88D operations.