Associating Monthly Hail Occurrence and Large Scale Environment for the 1 Continental United States

4 A new index relating large hail occurrence to environmental parameters has been developed. Hail 5 events are defined as the total number of three-hourly periods in a month that experience one or 6 more instance of hail larger than one inch. The index is derived using Poisson regression between 7 hail events and monthly mean variables describing the large scale environment from the North 8 American Regional Reanalysis (NARR) over the period 1979-2012. The index depends on four 9 parameters; convective precipitation, convective available potential energy, storm relative helicity 10 and the mean specific humidity between the surface and 90hPa. The new index differs in its 11 choice of parameters and their weighting from existing indices for the occurrence of either hail or 12 tornadoes. The index performs well simulating the monthly climatology of CONUS and regional 13 total occurrence of large hail. The spatial distribution of the index also favorably represents the hail 14 event climatology. However, in the index, the westward shift of maximum hail frequency during 15 the summer months is delayed relative to observations. Additionally, for the area just east of the 16 Rocky mountains, the index has a lower frequency of hail compared to observations. Despite these 17 limitations, annual index variations are realistic when compared to de-trended observations of hail 18 frequency. Little trend is seen in the index compared to the large increases in hail reports over 19 the three decades. The development of this hail index allows for future exploration into climate 20 scale linkages, the potential for seasonal forecasting and analysis of hail frequency under climatic 21 change. 22