At approximately 22:30Z on July 25, 2002, a tornado formed about 15 miles west of Granite Falls at the southern end of a squall line. It moved along a 20 mile path through that city where it damaged over 300 homes and buildings, injured 10, and caused one death. The pre-storm 22Z environment ETA sounding showed a CAPE of over 3,600 J/kg but a weak helicity of 93 J/kg and thus did not seem to favor any isolated supercell development. WSR-88D radar from MPX 100 miles away indicated no mesocyclone activity until the time tornado struck Granite Falls.

We investigate the origin of the rotation by modeling the interaction of a mesoconvective system that formed west/southwest of Granite Falls and the parent storm of the tornado, both of which merged in this area. We use ARPS to model the development of the interaction of the two storm to determine the role of the interaction in the intensity of the low-level rotation in the mesocyclone supercell.