Probabilistic Precipitation Forecasts for Flash Flooding in the 2018 NOAA/NSSL Hydrometeorology Testbed

In operations, National Weather Service (NWS) forecasters use deterministic forecast guidance based on quantitative precipitation estimates (QPEs), flash flood guidance, and Flood Locations and Simulated Hydrographs (FLASH) products to assess flash flooding risks and make warning decisions. The Hydrometeorology Testbed – Hydro (HMT-Hydro) experiment located in Norman, OK has provided a means to test and evaluate the latest advancements in this guidance within a quasi-operational environment. Most recently, the 2018 HMT-Hydro experiment introduced two experimental probabilistic precipitation forecasts from the FLASH system that are driven by either 1) QPE-forcing or 2) QPE-forcing coupled with QPF-forcing from the NSSL Experimental Warn-on-Forecast System for ensembles.

Differences between the usefulness and potential application of deterministic forecast guidance (condition 1), probabilistic forecast guidance with QPE-forcing (condition 2), and probabilistic forecast guidance with QPE-forcing and QPF forcing (condition 3) for flash flooding was demonstrated in the 2018 HMT-Hydro experiment. Three archived flash flood events were used during this demonstration: West Virginia on 23 June 2016, Oklahoma on 19 May 2017, and Texas on 25–26 August 2017. Over three weeks, a total of nine NWS forecasters analyzed each event using the three conditions in a case study mode. The first two conditions provided guidance only valid at the top of the hour, while the third condition provided guidance valid out to 3–6 hours (depending on the case). At the top of each forecast hour for a period ranging between 8–12 hours, participants independently cycled through viewing guidance for the three conditions and answered a set of questions designed to elicit what information they found useful, how they perceived the current and future flash flooding threat, and whether they felt actionable decisions were justified (e.g., issuing an advisory, warning, or update).

Comparisons between participants’ assessments of the flash flood threat for the three conditions will be qualitatively analyzed and compared, and attention will specifically be given to what utility probabilistic forecast guidance for flash flooding provides over the traditional deterministic forecast guidance. In addition to sharing findings from this analysis, topics that emerged from participants’ feedback during end-of-week group discussions will be included to further emphasize NWS forecasters’ initial impressions and uses of probabilistic forecast guidance for flash flooding during the 2018 HMT-Hydro experiment.