Choosing the “Right” Isentropic Surface(s)
Compute a cross section of isentropes and isohumes ( mixing ratios) normal to a jet streak or baroclinic zone in the area of interest.
Choose the low-level isentropic surface that is in the moist layer, displays the greatest slope, and stays 50-100 hPa above the surface.
A rule of thumb is to choose an isentropic surface that is located at ~700-750 hPa above your area (as with the “Garcia” method for forecasting snowfall)