Hourly Precipitation Potential Index - a Pragmatic Approach to Communicate Precipitation Timing

The National Weather Service (NWS) provides gridded forecasts of weather elements to partners via a National Digital Forecast Database (NDFD).  The database is available for users to feed decision support systems and to create text, graphic, gridded, and image products of their own.   Precipitation forecasts are made available as Probability of Precipitation (PoP) for 12-h periods out through 7 days and as deterministic precipitation amounts for 6-h periods out through 3 days.

In August 2015, the NWS introduced an hourly Precipitation Potential Index (PPI) as a new experimental element in the Conterminous United States (CONUS) for the first 36 hours from NDFD issuance time, and at 3-h temporal resolution after the first 36 hours through forecast day 3.   The purpose of PPI is to provide greater detail regarding precipitation timing than is currently available from 12-h Probability of Precipitation forecasts.

Although PPI has been shown as an element on NWS point and click web pages for many years, its introduction into NDFD has been controversial.  What is the scientific merit for such an index?   Do users understand it?   Can it be verified?   What are the alternatives?   Should the new National Blend of Models provide PPI guidance?   This presentation will describe PPI, and discuss many of the issues associated with it.