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Did Someone Say “Recurrence Interval” – What Is That?

Contributed by James Bria, CompuWeather Sr. Forensic Meteorologist & Weather Expert

A recurrence interval is an estimate of the likelihood that an event will occur. The most common use of a recurrence interval as a meteorologist is to determine the magnitude of a rainfall during a specific duration. The duration can range from 5 minutes to as long as 60 days. CompuWeather  meteorologists are able to provide site specific rainfall amounts using Doppler radar and the duration of the rainfall. After calculating the rainfall total and the time period in which it occurred, CompuWeather meteorologists will determine the magnitude of such an event. The use of a meteorologist to determine the rainfall amount and duration is extremely important in order to calculate the recurrence interval. Often times, rainfall totals can vary greatly over small regions so it is not relevant to just use the closest rainfall data. It is important to note that not all rainfalls are equal and just knowing the rainfall total is not enough. Five inches of rain over 24 hours will most likely have less impact than five inches in 2 hour period. The 24 hour event may be considered a 5 year event while the 2 hour event may be considered 100 year event.

A 100 hundred year rainfall event can occur more than once every 100 years and if that happens it does not change the statistical probability of that of a 100 year event. A 100 year rainfall just means that there is 1 percent chance of that event occurring during any given year. There is also a difference between a 100 year rainfall and a 100 year flood. It is possible to have one and not the other. A flood event can often times occur when the sky is clear and no rain is falling. This can result from snow melting from the mountains and filtering into a river or even from an ice jam not allowing the river to flow as normal. CompuWeather meteorologists can be valuable to you in determining the true significance of a rainfall or flooding event that your case or claim involves.