Weather Forecasters Are Warning Americans to Prepare for a '100-Year Flood.' Here's What You Need To Know

Every expertise has specific terminology, and weather forecasting is no different. Sometimes, it gets difficult for civilians to understand these terminologies. There are some specific wordings forecasters employ to describe storms that could produce unprecedented rains, stated The Independent.

The terms forecasters employ for such intense events are 100-year or 500-year floods. For a layman, these terms may imply that the forecasters are predicting a flood that could last for centuries. However, these terms don't mean that. These terms reflect how unusual the event is, rather than its time period. 

Weather scientists utilize maths to understand the peculiarity of a flood and to contrast its intensity with other severe floods. The U.S. Geological Survey explained that a 500-year flood reflects how likely that weather phenomenon is to take place in a year. The term implies that there is a 1 in 500 chance of this flood happening in a year.

For any event named a 100-year flood, it means that there is a 1 in 100 chance of that phenomenon occurring in a year. "The 100-year flood or x-year flood refers to the probability of those events occurring. That is, for a 100-year flood, there is a 1% chance in any given year of having a flood of that magnitude. For a 500-year flood, there is a 0.2% chance of having a flood of that magnitude occurring," the National Weather Service stated. "It should be stressed that the 100-year and 500-year events are independent events, from the perspective of probability. That means that if one of those events occurs, it has no effect on future events occurring. In other words, if a 100-year flood event occurs, that does NOT mean that people are 'safe' for 99 years." 

The methodology of calculating a 100-year flood or a 500-year flood was set up by government authorities, stated Vox. The calculations brought together the standards levied by the Army Corps of Engineers for dams and levees and flood prevention standards accepted by most communities. The authorities hoped that the methodology would help the National Flood Insurance Program in providing appropriate aid to vulnerable populations. 

Several University of Chicago researchers claimed that Houston faced three 500-year flood events within 24 months from 2015 through 2017, Independent stated. One of those events was Hurricane Harvey, which produced the heaviest recorded rainfall ever in the U.S. Many homes and businesses were destroyed in the event, with visuals of cars floating in water flooding the internet.

At present, most forecasters utilize this method of depicting the yearly probability of a flood. Another method they can use is associated with the average recurrence interval. In the latter methodology, weather forecasters inform people about the expected frequency of a particular flood in the future. This methodology has experts describe a flood based on the likelihood of it happening once in ten years, twenty-five years, and so on. The yearly probability method has been considered more reliable by experts because it better reflects that rare floods can happen within a few years of each other.