How Large is the 1% Annual Chance 24-Hour Storm?

I’ve written many times about the depth of the current, statistically-derived, 1% annual chance, 24-hour storm in the Houston area. Its currently defined by the Harris County Flood Control District (HCFCD) as ranging from 12.4 to 13.5 inches in 24 hours, depending on the location. As you move closer to the coast (towards the southeast) the depth of the 1% event increases. This variation is addressed in HCFCD’s Hydrology and Hydraulics Guidance Manual by dividing the county into three rainfall regions as shown below:

The National Oceanic Atmospheric Administration (NOAA) is updating the nation’s storm statistics to include a longer period of rainfall records.  This effort is known as “Atlas 14.” The work is being accomplished on a state by state or region by region basis. The Texas analysis is currently underway, however, the work won’t wrap up for a year or two.

To get an idea of what the updated 1% event might be for Houston I browsed to the NOAA Atlas 14 website. This website has a map that let’s you pick any point in the United States. The website then provides the storm event statistics for that point based on the most recent data available. Since the update for Texas is not yet complete, you’ll get a message below the location picker map pointing you to the three older documents that provide information on Texas. (For the Houston area, folks should rely on the HCFCD Hydrology and Hydraulics Guidance Manual, which is more detailed and up to date.)

I picked a location along the Sabine River on the border of Texas and Louisiana, about 50 miles in from the coast (about equivalent to the distance from the Galveston coast to Memorial Park) to get an idea of what the Houston area Atlas 14 rainfall amounts might be when the NOAA study is finished.

After clicking on the map near the right-hand star above I was presented with the following table:

The Atlas 14 table for Starks, Louisiana shows that the 1% annual chance, 24-hour rainfall (equivalent to the 100 year recurrence interval in the table) is 14.3 inches, with a 90% confidence interval ranging from 10.9 inches to 19.0 inches.  Here’s what that looks like in graphical format.

The rainfall information from this point in Louisiana should be a pretty good predictor of what the rainfall statistics will show for the Houston area.  Based on this, I’d expect the 1% annual chance, 24-hour event for Houston will increase from 12.4 to 13.5 inches to something closer to 14 to 16 inches in a 24-hour period.

If the Atlas 14 analysis was updated to include rain events from 2016 and 2017 these 1% annual chance depths might increase a bit more.

One important outcome of this pending change: HCFCD will need to re-run our bayou models to determine the water surface elevation that results from the new, larger, 1% annual chance event and update all the regulatory floodplain maps to match this new elevation. This will likely enlarge our regulatory floodplains and increase the number of structures in the floodplain.

4 thoughts on “How Large is the 1% Annual Chance 24-Hour Storm?

  1. Thanks Mike, informative article. Much bluster, but little facts out there as to just how much the numbers are changing in the last decades. Does anyone know what Harvey’s 24-hour peak was?

    • The maximum 24-hour total was 28.6 inches. The weighted range for the entire county was between 16 and 20 inches.

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