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MetStat’s homepage, with AEP calculator tab at top.

We are pleased to announce a new, powerful utility on our website called the AEP Calculator which provides a means of translating precipitation depths into an equivalent Annual Exceedance Probability (AEP) or Average Recurrence Interval (ARI) and vice versa. MetStat’s AEP Calculator can be reached via this link or our home page. The translation of precipitation to AEP or ARI is based on the most current government-issued precipitation frequency information.  Most of the continental United States is covered by precipitation frequency estimates from NOAA Atlas 14 published between 2013 and present; only the Pacific Northwest (OR, WA, ID, MT and WY) is not available at this time.

Although at MetStat we routinely compute precipitation with rarer AEP/ARIs, MetStat’s AEP Calculator is constrained to resolving AEPs out to 1 in 1000, which is the limit of the underlying NOAA Atlas 14 data. The AEP calculator provides the “best estimate” along with upper and lower bounds of the 90% confidence interval. The probability that a precipitation frequency estimate will be greater than the upper bound (or less than the lower bound) is 5%. We provide the upper/lower bounds to convey the uncertainty in the statistical analysis of the precipitation data. Although these uncertainty bounds do not explicitly account for climate change, we believe any short-term climate variations would still result in estimates within the 90% confidence limits. Naturally there is greater uncertainty at rarer frequencies because storms of this magnitude are less frequent for establishing more certain precipitation frequency estimates.

The utility requires a user to input the latitude and longitude of a location, a precipitation amount and a duration. There are a number of websites that provide latitude and longitude information, our favorite is After submitting these variables, a graph will appear that shows the how rare that precipitation amount is in terms of both an AEP and ARI. Below is an example of the output given 11.8 inches of precipitation in Dallas, TX over a 24 hour period.

ARIplot precip

             Example output of the AEP calculator, in precipitation mode.

This graph shows that 11.8 inches of precipitation over 24 hours in Dallas yields an AEP of 0.003, which is equivalent to an ARI value of 337 years. In other words, a 24-hour precipitation amount of 11.8 inches in Dallas has a 1 in 337 or 0.3% chance of occuring in any given year. Alternatively stated, this magnitude of precipitation happens in 24 hours, on average, every 337-years, but that’s not to say it won’t rain that hard more than once in a year or in consecutive years. The term “return period” is equivalent to the ARI, but not widely used since it gives the false sense of an events only occurring every x number of years, when really ARI is the average number of years between exceedances of a given precipitation amount; implicit in this definition is the years between exceedances are generally random.

The utility can also work in the opposite direction. To use the above example, say you were interested in what amount of precipitation in 24 hours in Dallas, TX has a 0.3% chance of occuring in any given year, or an AEP of 0.003. You can simply enter a desired AEP or ARI over the specified duration to obtain the equivalent precipitation needed to meet that AEP or ARI. Below is a graphic showing the equivalent precipitation for an AEP of 0.003 in Dallas, TX.

ARIplot aep 1

Example output of the AEP calculator, on AEP mode.

It is important to understand the frequency of precipitation, it does not equate to the same frequency of the resulting flood. This utility does not tell you the “100-year flood.” The degree of flooding is largely the result of the precipitation, but other factors such as soil type, antecedent conditions, slope, vegetation, season, snowpack, regulations on water flow and other environmental factors,  influence the degree of flooding. In fact, under the right conditions, such as an burn scar where vegetation has been removed and the soil backed into a crust, even a 10-year precipitation event can cause a 100-year flood.

Effective communication of extreme precipitation, in a probabilistic perspective, is a challenge we face almost daily at MetStat. The common misconception is that “100-year storms” appear are happening far more frequently than what the term “100-year” ARI or 1 in 100 would imply. The fact is there are hundreds of 100-year storms in the United States each year, but rarely at the same exact location. For more information on interpretation of the utility’s results, please see the “Terminology, Semantics & Definitions” portion of our Frequency Analysis page.

We hope this tool will be helpful for those wishing to ascertain the rarity of precipitation, including weather observers, hydrologic professionals, or anyone who has an interest in meteorology and would like to explore the climatology in their area. We have several future improvements to make to this utility, including additional durations, point-and-click features and more, so watch for those later.