A conceptual model for sustained low-level mesocyclones is tested as a tornado forecast tool with observations and forecasts from the operational Eta model. In the conceptual model, a balance between low-level storm inflow and outflow allows the development of a persistent low-level mesocyclone along the rear flank of a supercell thunderstorm, owing largely to the strength of the midlevel storm-relative winds. The present work draws on this conceptual model to identify preferred ranges of low (model surface level), middle (500 mb) and upper-level (250 mb) storm-relative wind speeds for 131 supercells, from gridded Eta model fields. The observations reveal that the 500 mb storm-relative wind speed has a distinct lower bound of approximately 8 ms-1 for the tornadic supercells, while differences between surface-level and 250 mb storm-relative wind speeds for tornadic and nontornadic supercells are much less pronounced. The storm-relative wind speeds are also compared to the bulk Richardson number shear for the purpose of discriminating between tornadic and nontornadic supercells. Test results of storm-relative wind speed at the Eta model surface level and at 500 mb, derived from gridded Eta forecast fields, demonstrate skill in distinguishing tornadic and nontornadic supercells in daily forecast operations at the Storm Prediction Center.