1. Introduction
Over the course of five days from 29 May to 2 June 1998, Pennsylvania was struck by three major outbreaks of severe weather. During the third event on 2 June, two tornadoes crossed paths in Somerset County near a farm that had sustained damage from a tornado just four days earlier during the 29 May outbreak. These two tornadoes, which reached F2 and F4 intensity, respectively, were produced by supercell storms with rotating mesocyclones. A supercell storm differs from an airmass thunderstorm in that it develops separate inflow and outflow structures which allow it to persist for several hours. The mesocyclone is a rotating column of air in the updraft region of the storm usually associated with severe thunderstorms and thought to play a role in tornadogenesis.
A supercell thunderstorm entered Somerset County, PA around 2300 UTC 2 June 1998. By 2316 UTC, the cell was located just southwest of New Centerville, PA. This cell and the cell immediately to its north had developed distinctive hook echoes in the base reflectivity imagery for 2316 UTC from the Pittsburgh, PA WSR-88D Doppler radar.
The RHI cross-section in the base reflectivity imagery for 2321 UTC 2 June 1998 from Pittsburgh, PA shows a pronounced weak echo region in the location of the tornado southwest of New Centerville, PA.
By 2322 UTC 2 June 1998, the tornado was just southwest of Casselman, PA. Distinctive hook echoes for this tornado and a possible tornado farther north are evident in the base reflectivity imagery for this time.
The rotation associated with the confirmed tornado southwest of Casselman, PA is marked with an inverted red triangle in the storm relative velocity scan for 2322 UTC 2 June 1998 from Pittsburgh, PA. This triangle was generated by the tornado detection alogorithm in the WSR-88D software. The rotation marked by the second tornado marker in this figure did not produce a confirmed tornado.
