This study differs from prior HRRR and RAP investigations in that it focuses on the spatial (1km) and temporal (hourly) scales at which the NWM operates. The results show that the precipitation forecasts from HRRRx appear to be more skillful than those from the RAPx in depicting the precipitation occurrence and amount compared to the Stage IV analysis across the Contiguous United States (CONUS) during the warm season of May-October 2015. Both QPF sources exhibit the highest skill in the northeast and the lowest quality over the southwest United States, and QPF skill decreases with increasing lead times and accumulation thresholds. Additionally, the RAPx and HRRRx models using all 24 forecast issuances perform better than using 4 issuances at 00 UTC, 06 UTC, 12 UTC and 18 UTC. When averaging QPFs over different hydrologic unit codes (HUCs), overall the analysis suggests that precipitation skill decreases with the size of HUC, although more details are seen at the finer HUC scale. The study also evaluates the ability of each QPF source to capture the historic heavy rainfall that resulted in catastrophic flooding over South Carolina during 1-5 October 2015 and finds that the HRRRx accurately forecasted the timing, location and amount of extreme rainfall during the flooding event.
This evaluation is not only helpful for identifying the strengths and weaknesses of the precipitation forecasts—which is important for atmospheric model development and improvement—but is also valuable for forecasters and other end users to better understand the reliability of NWM hydrologic forecasts in aiding their decision-making process.