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Reader Comments on the July 19 '25-Year Rain'
Posted on Sunday, September 25 @ 09:00:00 EDT by jfbailey
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WPCNR MR. & MRS. & MS. WHITE PLAINS VOICE. September 25, 2005: A 25-year rain. Lately we have heard a number of references to terms like ’25-year storms’. What is that? A 25-year storm is one whose intensity is seen, on average, only once every 25 years. Or slightly differently, the highest intensity storm that you are likely to see in any given 25-year period. As with other averages and probabilities, you could see two 25-year storms in consecutive years, or they could be 30 or 40 years apart.
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When talking about a 25-year storm we are talking about the total rainfall for the entire storm. However, the same amount of rain falling over 12 or 24 hours would be much less damaging than if it fell in 15 minutes, and so it is often necessary to qualify the amount of precipitation with the duration of the storm.
Note also, that the amount of rain delivered by a 25-year storm varies by location, so that a 25-year storm in White Plains might be a 5-year storm in parts of Florida or a 50-year storm in Nebraska.
In designing roads, gutters, roofs, storm drains and such, engineers need to estimate the maximum amount of water that they will have to handle, and will use numbers derived from charts prepared by agencies such as the National Weather Service giving rainfall amounts for various locatins and various storm intensities. For normal structures, they may design for 25-year storms, while for critical structures such as bridges or levees; they may be required to design for 100-year or 500-year storms. If they design for a 25-year storm, say, and you experience a 50-year event, them you will have too much water for the system to handle, and something will flood. If it is the gutters around your roof, the excess just sloshes over the sides and splashes on the plants, probably no big deal. If it is the levee surrounding your town, it probably is a big deal.
Terms like 25-year storm are too imprecise for Meteorologists who tend to talk about Annual Exceedance Probability or Average Recurrence Intervals.
On July 18th we had a heavy thunderstorm that overwhelmed a lot of our infrastructure. Various people have referred to that storm as a 25-year or 50-year or even 100-year storm. The entire storm dropped 2.64 inches of rain. As a storm total, that was not that much, and would barely make a 2-year event. However, since most of the rain (2.15 inches) fell in just an hour around 4:30, that brings it to just shy of a 10-year storm.
As an aside, 2.64 inches of rain falling over a 200’ x 300’ lot would amount to about 98,700 gallons of water!
These numbers are from the National Climatic Data Center (NOAA). Other agencies, such as the Westchester Soil and Water Board, collect similar data and may have slightly different numbers.
Don Hughes
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