Six rare ‘once-in-a-thousand-year’ rainfall events in a month? Climate change may force NOAA to update standards

July 26th, 9 inches of rain in 24 hours St. Louis broke all-time records for more than a foot of measured rain in several suburbs.

2 days later Dozens dead in eastern Kentucky When more than 10 inches of rain fell, river levels rose several feet in a matter of hours, far exceeding historical records and washing away homes near the shore.

As August begins, another Heavy rain hits southeastern Illinois When Radar estimate is 10 to 13 inches It will rain in just a few hours.That same week, usually dry Death Valley A year’s worth of rain fell in three hourswhich inundated areas with floods that closed all roads within the national park, and some suffered moderate to severe damage.

just this week Dallas-Fort Worth Metroplex suffered widespread flooding after 3 inches of rain fell in an hourpart of a series of thunderstorms that dropped more than 9 inches in 24 hours at Dallas-Fort Worth International Airport, with more than 15 inches of rain reported in several eastern Dallas suburbs.

Not to be outdone, parts of Mississippi received more than eight inches of rain in just three hours Wednesday morning.sudden flood Caused dozens of rescue needs and caused a flash flood emergency.

heavy rain in mississippi

We had more than 8 inches of rain in 3 hours on Wednesday morning.

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All six extreme events hit the regional threshold for being classified as a “once in a thousand years” rainfall event by the National Weather Service, but all occurred within the same month.Last year, that deadly flood Hit New York City and New Jersey From the wreckage of Hurricane Ida devastating flood It is also recognized as a very rare 1-in-1,000-year rainfall event in Midwest Tennessee.

the term “Once in 1,000 years“A rainfall or flood event may sound like something that is predicted to occur only once in 1000 years. .

That’s not how it works. Instead, the term is given to an event that the forecaster considers as likely as 0.1%, or 1 in 1,000, likely to occur in a given year. A 0.5% chance is considered a “once in 500 years” event, and a 1% chance is considered a “once in 100 years” event.

st.Louis and the Kentucky Flood: His Two Different 1-in-1,000-year Rainfall Events in the Same Week

National Oceanic and Atmospheric Administration water forecasting bureau supervise Investigating past rainfall events To reach each probability threshold, classify how much rain would have to fall in a given time period (such as an hour, hours, or days) compared to the national climate average. increase. For example, Death Valley (just under 1.5 inches) reached the 0.1% probability threshold with much less precipitation than St. Louis (over 9 inches).

You might wonder if your calculations are off because many of what you think are very rare events are occurring much more frequently.

According to a study by the United States Global Change Research Programclimate change is Increase in heavy rain events in the USespecially in the Northeast and Midwest, both in intensity and frequency.

As ocean temperatures rise, evaporation increases, making more water vapor available. Warm air, on the other hand, can hold more water vapor. As global temperatures rise with climate change, storms will have access to even more moisture when heavy rains occur. According to USGCRP.


According to the US Environmental Protection Agency, 9 of the top 10 years of extreme daily precipitation from 1910 to 2020 have occurred since 1996. after that. “

The Hydrometeorological Design Research Center of the Water Forecasting Bureau Already in the process of years of research and updating Nationwide precipitation frequency threshold and Created an interactive map Allows the user to calculate the estimated frequency of a given rainfall in the area where the study is completed. (Note: Studies are ongoing in his five states in the Northwest to determine these thresholds.)

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However, these maps may be updated in the coming years.

of Recently Enacted Infrastructure Law including funds Allow NOAA to further analyze changes and trends Observed rainfall events due to climate change incorporate them into national updates of precipitation frequency estimates.the research already in progress.

With these extreme and rare rain and flood events making the headlines, NOAA emphasized that a 1-in-1,000-year rainfall event is not the same as a 1-in-1,000-year flood event.

Just as a 1-in-1,000-year flood event may not have been caused by a 1-in-1,000-year rainfall event, an extreme rain event reaching that threshold may not lead to a 1-in-1,000-year flood event. There is a nature. Therefore, according to NOAA, the terms should not be used interchangeably.

NOAA is responsible for determining extreme rainfall probabilities, and the US Geological Survey is responsible for determining such probabilities when it comes to flooding.