Helping Business Weather the Storm

New Perspective of Rainfall Data Gives Users a Sense of Its Rarity

by Daphne Thompson, on Sep 3, 2010 11:13:21 AM

WINDSOR, Colo., /PRNewswire/ -- As our society and aging infrastructure become increasingly more vulnerable and impacted by extreme rainfall, the requirement for timely, accurate and informative rainfall information presented in a meaningful context is paramount to saving lives, protecting property and efficiently managing water. An innovative new way of looking at rainfall data is providing a wide variety of users a powerful perspective of how unusual, or common, recent rainstorms have been. Metstat, Inc., which provides accurate, innovative and detailed precipitation information to help private and public clients make cost-effective decisions, has launched iRainStat, a service that translates rainfall maps into average recurrence interval (ARI) maps in near real-time. iRainStat provides information analogous to the 100-year flood, but for rainfall instead.

The ARI is the average period of time between rainfall events of the same magnitude. For example, the ARI of six inches of rain in 24 hours in Washington, D.C. is 25 years. In other words, Washington, D.C. can expect six or more inches of rain in 24 hours to occur, on average, every 25 years. An ARI can also be expressed in terms of probability. For example, a 25-year event has 1-in-25 or 4% (1/25) chance of occurring in any given year.

iRainStat maps are created by translating rainfall into its equivalent ARI. The rainfall-to-ARI conversion is based on official precipitation frequency data published by the National Oceanic and Atmospheric Administration (NOAA) together with high-resolution gauge-adjusted radar rainfall from Weather Decision Technologies and the National Weather Service. To see a free, live example of today's ARI map visit

The concept of ARI rainfall mapping has been used in recent government documents pertaining to extreme rainstorms. For instance, according to a report posted on the Nashville, Tenn. National Weather Service website, the catastrophic flooding that occurred early last May across western Tennessee was the result of two-day rainfall exceeding the 1,000-year ARI. However, iRainStat is the first and only service that provides ARI rainfall maps in near real-time for the United States. Metstat's six-year archive of ARI rainfall maps is providing a unique perspective of extreme rain storms across the United States. In fact, surprisingly a 50- or 100-year rainstorm occurs somewhere in America nearly every day.

Although the ARI of rainfall does not necessarily equate to a flood of the same ARI, iRainStat provides an excellent indicator of flooding potential. Floods are caused by heavy rain, spring snowmelt, dam/levee failure and/or limited soil absorption. The degree of flooding from heavy rainfall depends on the rainfall intensity, duration, topography, soil conditions, ground cover, basin size and infrastructure design. For instance, rain associated with a one to five-year ARI can cause significant urban flooding since most urban storm water systems are typically designed for one to ten-year ARI.

iRainStat is a powerful and unique tool that benefits a wide variety of users, including dam owners/operators, media, storm water managers, county/city flood control districts, insurance companies, sewer districts, irrigation districts, emergency managers, departments of transportation, and private weather forecast companies.

Media contact:

Tye Parzybok, 970.686.1253,

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SOURCE Metstat, Inc.

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