On November 20, 2017 I posted about the release of the preliminary Volume 11 of Atlas 14, the wholesale update of statistically based rainfall intensity, duration, and frequency estimates for the state of Texas.  In that post I shared a screen shot of the Houston region from the statewide map published by the National Weather Service showing the new projected depths of the 1% annual chance, 24-hour duration rain event.  In this post I will discuss the changes depicted for Harris County.

First, let’s look at the official depths in use before Atlas 14.  The map below shows the three regions and the associated rainfall depths used by Harris County, Harris County Flood Control District, and all civil engineers working the area.

Source: Harris County Flood Control District, hydrology and hydraulics guidance manual. color and labeling by m. bloom.

This map shows three regions with areas closer to the Gulf of Mexico generally getting more rain than those more inland.  The rainfall depths range from 12.4 inches to 13.5 inches. These values were based on an analysis of observed rainfall events between the early 1900’s (the exact date varies by rain gauge) and 2001 (just after Tropical Storm Allison). The map above has been shaded to match the color legend used in the new Atlas 14 map, so that the changes can be more easily seen.

The map below is the preliminary Atlas 14 map zoomed and cropped to show just Harris County.

Source: National Weather Service, preliminary atlas 14 data released for peer review. zoomed, cropped, and labeled by m. bloom.

This map shows five regions with rainfall depths ranging from 14.01 inches to 18.80 inches. The table below presents the magnitude of the proposed, preliminary changes in each region.  The percentage is calculated as follows:

Percent Change = (After – Before) / Before

So, in some parts of the county the depth of the so-called “100-year” event is proposed to be increased by only 13%, while in some parts of the county it is proposed to increase by 39% percent.