Hatched: The struggle of communicating Intensity with the WPC Excessive Rainfall Outlook

The WPC Hydromet Testbed conducted the Flash Flood and Intense Rainfall experiment, with a portion of the experiment focusing on the Excessive Rainfall Outlook (ERO) modifications. In particular we added a Rainfall Intensity (referred to as “hatched”) component to the ERO, similar to what Trojniak et al (2022) has explored for an Annual Recurrence Intervals (ARI) based experimental ERO, in addition to adding a category between Slight and Moderate.

To define intensity we formally used the 10 year ARI for a 6hr period derived from NOAA Atlas 14. Grid point exceedances were calculated from the MRMS 6hr precipitation as verification. The MRMS data were interpolated to the 3km HRRR grid using a maximum filter, effectively preserving the most extreme rainfall. However, intense rainfall isn't restricted to 6hr periods and thus a looser definition was used as an alternative. Our experiences with participants using these definitions will be analyzed across 30 days of the experiment. Examples of different ways of analyzing intensity will be shown for a case study day. These include the formal 10yr 6hr ARI exceedances, including 1 or 3 hr, as well as simpler, hourly precipitation accumulations.

Preliminary results revealed that participants were uncomfortable with the “loose” definitions citing many concerns including: lack of a strict definition to communicate areas of intense rainfall, the precarious relationship with the areal coverage/intensity based definitions of the existing ERO categories, and the added value of the intensity component on existing categories. As the NWS moves toward Impacts based decision support, the intensity may or may not represent Impacts. Additionally, some participants felt that the significant severe component of the SPC outlook made it harder to add the visual of a “hatched” to the ERO and suggested it might cause confusion.