The storms over southwestern Oklahoma have recently increased in intensity and have warranted NWS and EWP severe thunderstorm warnings, with multiple reports of quarter to golf ball sized hail. The ProbSevere values for these storms with golf ball sized hail are well in excess of 90%–even some at 100% for times. The evolution of these storms and the generation of severe thunderstorm warnings is demonstrated in Figure 1.
A question was posed to the forecasters on the KOUN desk within EWP today, “There have been a number of storms with ProbSevere values in excess of 50% today, some of which seem to be false alarms, is this hampering the use of the product?” The forecasters replied,
“No I like that the storms with the highest ProbSevere probabilities are in excess of 90% and these are the ones producing golf ball sized hail–these are not marginal hail stones. The ProbSevere is drawing me to which storms are likely to need severe warnings in the near future. The false alarm storms, while they have high probabilities today, are in the 70-85% range–which makes them different than these with 90+% probabilities. I’ve easily been able to adjust to the probabilities and use them appropriately. Also I know the ProbSevere model has not been validated as a tool to reissue severe warnings, but I like how the probabilities diminish a bit when the radar intensity decreases and then come back up if the storm re-intensifies. If you look at this display (Figure 2) it has high probabilities on the right storms, but the ones further west are only (correctly )10-20%”
Figure 2. NOAA/CIMSS ProbSevere and MRMS composite reflectivity valid 2204 UTC 7 May 2014. This was the display the NWS forecaster in the EWP was referring to when he was discussing the ProbSevere continuing to identify the severe storms, but had much lower probabilities for the storms further north and west.
-Sieglaff, Week 1 ProbWarn PI