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However, that is far from certain and not official.

One expert I talked to suspected that the new 100-year floodplain could be close to where the 500-year flood plain is now. What does it mean that the 100-year rainfall has increased 4-5 inches?įirst and foremost, it means that all of the floodplain maps will be revised. It shows the 24-hour, 100-year rainfall to be about 13 inches. I believe it became the basis for the current flood-plain maps redrawn after Tropical Storm Allison that were released in 2007. See the USGS Rainfall Maxima Guide for Texas (Warning: 40 meg PDF). USGS also published a precipitation frequency study in 2004. Twelve inches in 24-hours represented the old 100-year rainfall for our area for decades. Look on page 58 for the 100-year/24 hour data from 1961. Comparison to Previous Studiesįrom this data, we can see that – for the gage at the San Jacinto and US59 – the new, official 100-year rainfall is 17.3 inches in a 24-hour period.Ĭompare a previous dataset published. This may be the information you want to keep handy for ready reference. Then review the new data for different time periods and recurrence intervals. Note that the Houston to Beaumont area is in the bulls-eye. Here’s what the 100-year/24-hour rainfall map looks like. The new NOAA estimates include data from Harvey and all of the huge storms we have had since 1994 including Tropical Storm Allison, the Tax Day Flood and the Memorial Day Flood. The new precipitation frequency estimates supersede the NOAA estimates published in 1961, 1964, and 1977, and the USGS estimates published in 2004. They are published as NOAA Atlas 14 Volume 11: Precipitation-Frequency Atlas of the United States, Texas. Today, the Hydrometeorological Design Studies Center of the NOAA’s Office of Water Prediction released updated precipitation frequency estimates for Texas. NOAA Atlas 14, Volume 11: The New Go-By for Everything Related to Rainfall Basically, what we used to think of as a 100-year storm is now almost a 25-year storm. This is why flood mitigation and reducing sedimentation are so important. New data shows the 100-year rainfall for this area has increased 4-5 inches since the NOAA study in 1961 or 2-3 inches since the USGS study in 2004.