Real-time Average Recurrence Interval Rainfall Maps for the U.S

Describing floods in terms of an average recurrence internal (ARI) or “return period” (100-year) has been used for decades to convey the rareness of major flooding events. However, describing the intensity of heavy rainstorms in a similar manner has not been as routine, but provides an equally good perspective of extreme rainfall events. Although the ARI of rainfall does not necessarily equate to a flood of the same ARI, ARI maps of rainfall provide an excellent indicator of flooding potential. Plus, real-time ARI maps of rainfall provide forecasters and media outlets a critical alternative to stream gauge information during rain-induced flooding events, especially considering flooding can often destroy or cause stream gauges to malfunction.

The concept of ARI rainfall mapping has been used in recent government documents pertaining to extreme rainstorms. However, this paper/presentation will introduce the concept of real-time ARI rainfall maps, a new tool for conveying the magnitude of heavy rainfall events to the public. This is especially important as society and infrastructure become increasingly more vulnerable and impacted by extreme rainfall events, making timely, accurate and informative rainfall information paramount to protecting property, saving lives and efficiently managing water. Expressing the rarity (or commonness) of rainfall in terms of an ARI provides an objective and useful perspective of the rainfall that most people understand, even for those not familiar with the rainfall in a particular area. Real-time ARI rainfall maps provide a new and powerful means for the media to communicate where the rainfall is most unusual and most apt to produce flooding.

The rainfall-to-ARI conversion is based on official rainfall frequency data published by the National Atmospheric and Oceanic Administration (NOAA) together with high-resolution (1 km x 1 km) gauge-adjusted radar rainfall from Weather Decision Technologies, Inc. (WDT) and the National Weather Service (NWS). The rainfall-to-ARI process is highly computational given the millions of grid cells that comprise the contiguous U.S. domain, but software has been optimized in to operate in a highly reliable, fast and redundant data center. Depending on the extent of rainfall across the country, a contiguous U.S. ARI map is processed in 4-8 minutes on a multiple-processor Linux computer. Operational ARI maps/grids are available within 25 minutes of their valid time and converted into a number of common formats. Once ARI maps for discrete periods (e.g. last 24-hours) are created, maps of the maximum ARI over a given period of time (e.g. 72-hours) can easily be created. These so-called maximum ARI maps/grids have the unique ability of portraying the ARI of a storm event, regardless of when the heaviest rainfall occurred across an area.

An overview of rainfall frequency estimates as well as 6- and 24-hour ARI maps, including several case studies, animations and real-time examples will be presented.