Why Can 1000-Year Storms Occur More Than Every 1000 Years?

(Photo Credit: NWS/NOAA)

DISCUSSION: Ellicott City, Maryland was devastated Memorial Day weekend by what was referred to as a 1000-year flood. In recent years, but there have been multiple storms that have produced 100-year, 500-year, and even 1000-year floods. But what do these terms actually mean and how can there be multiple of these historic floods within the given timespan?

Contrary to the name, a 1000-year flood does not necessarily mean this type of flooding will occur exactly once every 1000 years. Weather does not occur on a timed pattern so it is not possible to say that a certain severity of flooding will only occur exactly every 1000 years. What is actually meant by the name is that there is a 1 in 1000 chance of a storm producing flooding of that severity occurring within a year. It is not unheard of to have multiple historic floods within a relatively short time period. In 2017, flooding from Hurricane Harvey was reported as the third 500-year flood to affect Houston, Texas in three years. There was some confusion from the public as to how it was possible to have multiple 500-year floods less than 500 years apart from each other within one area. Since the naming of the category of these floods is based on probability, we can’t exactly say for certain there will or won’t be more than one flood of that magnitude within the 100/500/1000 etc. year timeframe.

When you flip a coin, there is a 1 in 2 chance of the coin landing on heads. Say you flip the coin twice. The only possible outcomes are the coin landing on heads twice, the coin landing on tails twice, or the coin landing on each once. Two of these scenarios do not represent this 1 in 2 chance probability. That is because the probability does not mean that for every 2 coins one of them must be heads.  More accurately what this probability represents is that for a single coin toss, there is a 1 in 2 chance that the coin will land on heads. When the coin is tossed again, there is once again a 1 in 2 chance that the coin will land on heads since each coin toss is treated as an individual event. Since the 100/500/1000 year floods are based on the probability that type of flooding will occur within a given year, each year would be treated as an individual event. This means that for each year, there is a 1 in 100 chance that a 100-year flood will occur. The occurrence of that type of flooding in one year does not affect the probability of it happening again the next year, which is why it is entirely possible to have more than one flood of the same probability within that given time frame.

Ultimately, weather does not occur like clockwork. We cannot predict that a severe flood will occur exactly every 1000-years. We can, however, deduce that there is a 1 in 1000 chance that particular type of flood can occur in any given year. While the terminology may seem confusing, it is important to remember it refers to the probability of a flood, not an exact timeline of occurrence.

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©2018 Meteorologist Stephanie Edwards