The four primary forecasting keys for hail (Hales 1996) are:
Some operational considerations include the size and distribution of CAPE, using a reasonable lifted parcel, and the environmental lapse rates. Another important factor is the melting of the hailstone as it falls to the surface from the freezing level (Sturtevant 1995). Forecasters should look for a cold pool at 500 mb (usually associated with a closed low), as it moves into an area of moderate low level moisture.
Figure 2 has Miller's two methods of predicting hail size. These charts are over 25 years old but still work quite well. Additional information can be found in Polston (1996), D. Smith (1996) and Shanklin (1989).