EF0 tornado in Butler County, Pennsylvania

A closed caption security video captured a tornado touching down northwest of Northvue, causing the roof to lift off of the Center Township salt dome and depositing it in the parking lot behind the facility. The first sign of damage was north of Unionville Road, where less than five trees had large branches down and one was uprooted. The worst damage happened to the salt dome as described above, where the maximum wind of 85 MPH was estimated. The only other damage nearby to the salt dome was a few hardwood trees that had large limbs broken off. The twister lifted in an open field just northeast of the salt dome facility.

An occluding surface low slowly drifted across Ohio on May 21st. Bands of showers with embedded thunderstorms moved across southwest Pennsylvania. During the mid to late afternoon, one convective band, associated with ample 0-3km CAPE and surface vorticity, was able to generate four tornadoes across southwest PA. Three were relatively brief and weak, but one longer-track EF1 was noted in Washington County. Multiple rounds of slow-moving heavier rain also produced a number of flash flooding incidents.