The National Weather Service Explains the ‘100-Year Flood’ Phenomenon

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flood, Montana, Missouri,100 year flood
Does it make sense now? Credit: NWS

Been hearing a lot about “100 Year” flood lately? Are you confused as to what that means? The NWS Missouri Montana have made a graphic in hopes of clearing it up.

An example to put it into perspective: on the evening of Friday, May 11th, the Clark Fork above Missoula reached 13.82 feet (32,500 cubic feet per second). That level does not exceed the 50-year or 100-year flows. It was, however, the second highest recorded level since 1908 (100 years)… but this does not technically classify it as a “100 Year Flood”.

Such records actually use Cubic Feet per Second (CFS) measurements to distinguish records. The Clark Fork above Missoula’s 100 Year measurement is 42,500 CFS. Thus, the level it reached on May 11th does not exceed that flow.

Now, if the Clark Fork above Missoula were to reach 14.0 feet (33,200 CFS) it would also not exceed the 100 Year flood requirement, or even the 50 Year flood requirement.

We know it’s a lot of numbers, and it’s confusing to put it all together, but hopefully, this sheds some light on the matter!

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