What is a 1 in 1,000 year flood? National Weather Service hydrologist explains

CHICAGO (WLS) -- What is a 1 in 1,000 year flood?

It has nothing to do with years, but it is a way of expressing a percentage chance. A 1 in 1,000-year flood actually means a 0.1% chance of that amount of rain happening in any single year. So, the 0.1% chance is just written as a reference to years.

There have been at least five 1 in 1,000-year floods this summer across the country. Here's a list of those five:

ST. LOUIS / July 26

9.04" is the highest 24-hour rainfall on record. 7.68" of that fell in just 6 hours (this has less than 1 in a 1,000 chance of occurring in a given year). They received about 25% of the normal rainfall in just 12 hours.

KENTUCKY / July 27

Training thunderstorms in Eastern Kentucky caused 4"/hour rainfall rates and brought up to 16" of rain during a five-day period (most of it falling in 1 day). These rainfall values occurring in such a short time are incredibly rare (less than a 1 in 1,000 chance of occurring over a five-day period in a given year).

EASTERN ILLINOIS / Aug 1

Got 8-13" of rain in 12 hours

DEATH VALLEY, CA / Aug 5

Got 1.46" of rain, which doesn't sound like a lot, but that's 9-month's worth of rain for the area and 0.01" shy of setting a record for all-time daily high.

DALLAS, TX / Aug 22

9.19" is the highest 24-hour rainfall in 90 years and the 2nd highest on record. 3.01" of rain in 1 hour is the new record for hourly rain. This is now the wettest August on record. Other locations near Dallas saw up to 15" of rain.

With so many happening in a short amount of time, Meteorologist Larry Mowry spoke with W. Scott Lincoln, Senior Service Hydrologist with the National Weather Service in Romeoville about what would happen to Chicago if it happened here.