Flood and Damaging Winds in Ohio, August 25, 1998

Synoptic and mesoscale conditions for August 23-25th...Impressive surface based instability developed late the night of the 23th ahead of a weak surface trough axis from southern lake Michigan back into western Ilinois where mid 70 dewpoints had pooled. Surface based CAPES were on the order of 3000 J/kg with lifted indicies to -10. At the same time a potent upper level trough was rotating through southern Ontario. Strong mid level capping inversion was in place at the time however increasing divergence underneath the exit region of a 100 knot jet streak across northern Michigan helped to intensify the low level jet across Illinois early on the morning of the 24th. Convection initated across lake Michigan and quickly became severe as it became surface based over land. Wetbulb zero and freezing level temperatures were favorable for large hail and mid level winds on the southern edge of trough axis increased to 50 knots which were favorable for damaging wind gusts.A stronger shortwave trough was located upstream across Montana the morning of the 24th. This system and associated frontal boundary across Minnesota into Michigan were forecast to combine across the area by late afternoon into the evening of the 25th and touch off another round of severe thunderstorms. The convection across southern Michigan into Ohio the morning of the 24th left behind an outflow boundary which helped pool upper 70 dewpoints across northern Indiana. By afternoon CAPES had increased to over 3000 J/kg. A surface wave developed along the cold front across Iowa by late morning where increasing surface convergence and mid level vorticity advection and cooling ahead of Montana shortwave helped to initiate a severe thunderstorm complex across northern Iowa. This system quickly turned into a derecho and raced into northern Indiana by mid afternoon. By late evening on the 24th... rapid destabilization occurred from northern Illinois into southern Wisconsin with the approach of Montana shortwave. Low level jet increased to 40 knots across central Illinois into lake Michigan which fired off another round of severe thunderstorms from the Illinois/Wisconsin border area eastward into southern lake Michigan. This convection quickly organized into a squall line by the morning of the 25th with several embedded supercells. One isolated supercell ahead of the line which failed to produce a tornado... did produce a swath of large hail and high winds across southern Michigan. Another supercell within the squall line interacted with the outflow of this isolated supercell and produced a long track tornado as it moved southeast from Niles to Bristol. One additional tornado which was likely a gustnado did develop along the leading edge of the bowing squall line between Dunlap and Goshen. As the bow matured, damaging winds became the primary threat.