What is a '1 in 100 year flood'?

WEATHER: Like It or Not!
CHICAGO (WLS) -- You may have seen they are calling the flooding in New York City a "1 in 100" or a "1 in 500 year flood." That can be a confusing term and really has nothing to do with years.

The saying "1 in a 100 year flood" is another way to express a very low percent chance of something happening. For example, a "1 in a 100 year flood" is actually a 1% chance of that event happening in any given year. It is based off a calculation using past climate reports.

Another example: a "1 in 500 year flood" is a 0.2% chance of that flood happening in any given year.

For Chicago, the amount of rain needed to be considered a 100 year or 500 year flood are below.



A "1 in 100 year flood" (or a 1% chance in a given year) would amount to 8.57 inches of rain in a 24-hour period.

A "1 in 500 year flood" (or a 0.2% chance in a given year) would amount to 11.24 inches of rain in a 24-hour period.



If it feels like you are back in statistics class, let me add another fun statistic about snow for Chicago from the National Weather Service office.

There's around a 5% chance every winter of seeing 12 inches of snow or more in a calendar day. Or to put that in year terms, a 1 every 20-year event.

In Chicago, there is also:
- a 100% chance for a given year to have a 2-inch or greater snowfall in one calendar day
- about a 50% chance for a given year to have a 6-inch or greater snowfall in one calendar day
- about a 5% chance for a given year to have a 12-inch or greater snowfall in one calendar day
- and under a 1% chance for a given year to have a 16-inch or greater snowfall in one calendar day

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