RainyDay

RainyDay, from Daniel Wright's Hydroclimate Extremes Group (University of Wisconsin-Madison), implements stochastic storm transposition: it resamples and spatially transposes observed storms from gridded rainfall archives to synthesise thousands of plausible extreme-rainfall realisations over your watershed — rainfall frequency analysis built from real storm structures rather than point statistics and area-reduction factors.

The approach shines exactly where design-storm convention is weakest: large watersheds, rare recurrence intervals and physically realistic space-time storm structure for 2D models. It underpins a growing body of probabilistic flood work, including FEMA-adjacent probabilistic frameworks, and remains the reference open implementation of SST.