Multiple-Doppler data and a numerical hail model are used to identify factors in supercell storms that am important to hail growth. Most hailstone mass is acquired in one pass across the updraft at a nearly constant level at temperatures warmer than -25°C. The embryos, however, appear to undergo recycling and likely originate from a variety of locations.The maximum updrafts in the storms studied are sufficiently strong (50 m s1) to carry embryos aloft before much growth can occur. The critical flow factor is a broad region of moderate updraft allowing hailstones to remain balanced in the prime growth regions. It is shown that even if embryos are abundant storms will not produce significant hail without this growth area. Since measurements of storm kinematics are well within the capability of existing technology, this suggests a powerful method for real-time identification of severe hailstorms.