From Thomas Grisa, director of public works at the City of Brookfield, Wisconsin - - Relabeling Extreme Rainfall Events to Improve Public Understanding.
From the article:
"The 100-year rain storm is understood poorly by the public — this can lead to confusion or worse. Unlike rainfall events, natural disasters, including earthquakes, tornadoes, and hurricanes, are not rated using recurrence intervals. Instead, they use straightforward scales, such as the Richter, Fujita, and Saffir-Simpson scales that rate the severity of the event, not the rarity of the event. The public understands that, in general, the higher the number, the worse the event.
The current rating for rain events uses recurrence intervals based on the probability of a storm occurring. The system defines how rare the storm is, not its severity. While it is generally true that severe storms happen rarely, defining storms by their frequency misleads the public into thinking that once the storm happens, it will be a long time until it happens again. Instead, defining storms by their severity impresses on the public that these can be significant — but not necessarily rare — events, and that they should consider protecting themselves to minimize the storm’s consequences.
One solution is to develop an alternative system similar to the rating of other natural disasters. One example, called the Rain Storm Severity Index, rates rain events on a scale of 1 to 10 based on the storm’s severity. This rating system builds on existing and accepted hydrologic science, so there is no change in background data or analysis. It does, however, change the designation from a recurrence interval to a simple scale for presentation to the public in much the same way that a graphic user interface makes computer programs easier to use."