Associating MRMS Hail and FLASH Parameters with the Occurrence of Severe Hail and Flash Flooding in the NWS Spokane County Warning Area

Recent studies have attempted to evaluate the accuracy and skill with which Multi-Radar Multi-Sensor (MRMS) radar and Flooded Locations and Simulated Hydrographs (FLASH) parameters can identify convection that produces severe hail and flash flooding. Although many of these studies assess regional and seasonal sensitivities of the parameters’ utility, incorporation of the findings when examining these parameters during the warning interrogation phase of hazardous convective events has been challenging at the National Weather Service in Spokane for several reasons, including (1) relative rarity of severe convective weather in the Inland Northwest compared to many other regions of the United States and resultant sample size uncertainties, (2) poor radar coverage in large portions of the Spokane county warning area, and (3) severe limitations in the utility of flash flood guidance in the northwestern U.S.

Nevertheless, with these parameters readily available for National Weather Service operational forecasters, the need to evaluate these MRMS hail and FLASH parameters to identify local instances of severe hail and flash flooding is apparent. After collection of storm reports of hail, flash flooding, and debris flows throughout the United States from January 2021 to July 2023, we compared the ability of several MRMS parameters to identify threshold exceedances of hail and instances of flash flooding nationally to those within a domain encompassing the Inland Northwest. Preliminary findings indicate that (1) the discriminating capability of MRMS hail parameters is generally similar when evaluating nationwide occurrences to those specific to the Inland Northwest, (2) the skill of the MRMS Maximum Expected Hail Size parameter is maximized around the threshold hail size of 2.5 cm (i.e., hail events designated as severe) with noticeably decreased skill at lower and higher thresholds, and (3) MRMS FLASH parameters provide meaningful information regarding the occurrence of flash flooding in the Inland Northwest, but primarily confined to areas with substantial urbanization.