When predicting hail, three factors need to be examined. They are the freezing level, CAPE, and the wet-bulb zero temperature. Lower freezing levels will allow hailstones less time to melt as they fall to the surface. Higher elevation areas (i.e. Colorado and Wyoming) tend to have a high frequency of hail since the freezing level is closer to the ground in these high elevation locations. The CAPE determines the potential size of hailstones before they fall. Higher CAPEs lead to hailstones being thrown higher vertical distances into the updraft and the potential for many "growth rings". The wet-bulb zero temperature is a function of how much dry air there is in the mid-levels of the atmosphere. Through evaporational cooling, the freezing level will drop closer to the earth's surface. On a Skew-T, if there is a large dewpoint depression in the mid-levels of the atmosphere, there will be evaporational cooling (leading to high winds and hail) associated with thunderstorms.

For maximum hail size you need the following: relatively high elevation area, low freezing level, dry air in mid-level of atmosphere, and a high value of CAPE. This combination is most often found in the Great Plains. The hail potential is minimized with this combination: low elevation area, high freezing level, low CAPE, and a moist atmosphere in the mid and upper levels. The southeast U.S. does not get as much hail and large hail as the Great Plains due to several of this minimizing factors (especially low elevation and moist mid-levels). Now, you are an expert hail forecaster!

Giant hail webpage: