![]() In order to compute the “return period” of a flood, pre-existing flood frequency statistics and a measure of the flood (i.e. 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. 100-year) has been used for decades to convey the rareness of major flooding events. Although the ARI of rainfall does not necessarily equate to a flood of the same ARI, the ARI of rainfall is an excellent indicator of flooding potential.ĭescribing floods in terms of a “return period” (e.g. (WDT) together with official rainfall frequency grids/maps published by HDSC. In a highly computational GIS environment, real-time ARIs are computed based on gauge-corrected radar-estimated rainfall maps/grids from the National Weather Service (NWS)> and Weather Decision Technologies, Inc. New methodologies address each of these issues. Historically, calculating the ARI of a given rainfall event has been inefficient, because it is generalized for an area based on a single observation and/or lacking reliable underlying data. The ARI is the average number of years between exceedances of a given rainfall depth for a given duration at a specific point location. ![]() For those who prefer a probability, an ARI can be easily converted into a probability. To clarify the meaning and to be consistent with NOAA’s National Weather Service Hydrometeorological Design Studies Center (HDSC), the term “average recurrence interval” (ARI) is used instead. The use of the term “return period” has been criticized for leading to confusion in the minds of decision makers and the public. Average recurrence interval (ARI) maps of rainfall provide information that is analogous to the 100-year flood, but for rainfall instead. Knowing how much rainfall fell at a particular location during a certain amount of time is useful, but expressing the rarity of rainfall in terms of a “return period” provides an objective and useful perspective of the rainfall. To make rainfall data more meaningful, an innovative technique for translating near real-time rainfall maps into “return period” maps has been created so users have an objective, timely, and accurate depiction of recent rainfall. Accurate rainfall data is becoming increasingly available, but translating it into meaningful information to support decisions is lacking. ![]() Boren Blvd, Ste 270, Norman, OK 73072 PH (405) 579.7675 FAX (405) 579.7780 email: Inc, 4764 Shavano Drive, Windsor, CO 80550 PH (970) 686-1253 email: society and infrastructure become increasingly more vulnerable and impacted by extreme rainfall events, the requirement for timely, accurate and informative rainfall information is critical to protecting property, saving lives and efficiently managing water. Hultstrand 3ġMETSTAT, Inc, 4764 Shavano Drive, Windsor, CO 80550 PH (970) 686-1253 email: Decision Technologies, Inc., 201 David L.
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