An Examination of Conditional Significant Tornado Probabilities on the Outlook Scale as a Function of Convective Mode and Environment

The NOAA/NWS/Storm Prediction Center (SPC) has developed a database of hourly mesoanalyses of convective storm environments dating back to 2003. In addition, a database of convective mode has also been developed at SPC for a filtered subset of tornado reports, also dating back to 2003. Several studies have utilized these databases to examine conditional probabilities (by EF-Scale) of tornadoes as a function of environment and/or convective mode. However, these studies have generally focused on the conditional probabilities for individual events on the warning time scale (<= 1 hour into the future). In this study, we apply this type of analysis to the time scale of SPC convective outlooks. We examine the conditional probability of “significant” (EF2+) tornadoes, given the occurrence of a tornado, in terms of the spatiotemporal constraint of SPC’s probabilistic Day 1 tornado outlook, which is defined as the probability of tornadoes within 25 miles of a point over a 24-hour period. Relationships between conditional strong tornado probability on the outlook scale and environmental parameters known to be related to severe convection are examined. In addition, we also incorporate convective mode into the analysis, examining whether the conditional probability increases for events that are largely dominated by supercells versus QLCSs or other modes.

The ultimate goal of this study is to provide improved estimates of strong tornado probability, given a forecast of overall tornado probability, which SPC produces operationally, and some categorization of the expected storm environment and convective mode, which are already implicit in the SPC forecast process. Such estimates could be used to enhance SPC’s current significant tornado forecast, which presently consists only of a single 10% probability line, as well as to provide input to hazard models that estimate the potential impact given significant tornado probabilities and other predictors of societal risk.