I'd like to get the dT/dz and the potential temperature. Could you explain me how to get those values?
Both of these can be obtained by looking at the forecast vertical profile on READY. Go to the Current Meteorology page at after entering your location, choose the SOUNDING program and the GFS meteorological data. Then for Graphics, select All to get the regular vertical temperature profile (where you can calculate the DT/DZ) and the potential temperature profile.