Quantitative precipitation estimate (QPE) and forecast (QPF) exceedance comparison with flash flood reports

Flash flooding remains a challenging prediction problem, which is
exacerbated by the lack of a universally accepted definition of the
phenomenon. In this article, we extend prior analysis to examine the
correspondence of various combinations of quantitative precipitation
estimates (QPE) and precipitation thresholds to observed occurrences of
flash floods, additionally considering short-term quantitative
precipitation forecasts from a convection-allowing model. Consistent with
previous studies, there is large variability between QPE datasets in the
frequency of “heavy” precipitation events. There is also large regional
variability in the best thresholds for correspondence with reported flash
floods. In general, Flash Flood Guidance (FFG) exceedances provide the
best correspondence with observed flash floods, except in the interior
western US where recurrence interval thresholds (for the southwestern US)
and static thresholds (for the northern and central Rockies) provide
better correspondence. Six-hour QPE provides better correspondence with
observed flash floods than 1-h QPE in all regions except the west coast
and southwestern US. Exceedances of precipitation thresholds in forecasts
from the operational High-Resolution Rapid Refresh (HRRR) generally do not
correspond with observed flash flood events as well as QPE datasets, but
they outperform QPE datasets in some regions of complex terrain and sparse
observational coverage such as the southwestern US. These results can
provide context for forecasters seeking to identify potential flash flood
events based on QPE or forecast-based exceedances of precipitation
thresholds.