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The EPI is a color-shaded map of the average number of years between the recurrence of a similar precipitation event, otherwise known as the Average Recurrence Interval (ARI) or “return period.” The EPI allows users to quickly ascertain areas with the most unusual precipitation and potential for flooding rather than using simple precipitation amounts, since what is deemed heavy rain in one part of the U.S. may be typical in another. EPI maps provide an objective, timely and accurate depiction of the magnitude and extent of high-impact precipitation and allow users can make appropriate decisions. The conversion of precipitation to a EPI removes the distraction of heavy, but not abnormal precipitation thereby only highlighting the high-impact, most unusual precipitation. These EPI maps are updated every 6-hours (0000 UTC, 0600 UTC, 1200 UTC & 1800 UTC), which coincides with updates of the gauge-adjusted radar-estimated precipitation from the National Weather Service (NWS).
These maps are neither official nor certified and only represent ARIs up to 100-years.