Hermine brings life-threatening flooding to central Texas
The remnants of Tropical Storm Hermine are producing significant amounts of rain, causing dangerous flooding in central Texas. As of 300 AM CDT, radar data (Fig. 1) estimates that over 15 inches of rain as fallen over Travis and Williamson county. This is a result of storms from Hermine passing over the same area, this is known as training.
Fig. 1 Storm-total precipitation from the New Braunfels NEXRAD at 300AM.
To make matters worse, the rain is falling directly over the Balcones Escarpment, a sharp transition between low coastal plains and the higher Edwards Plateau. This rapid change in elevation promotes flash-flooding. That is going on in Georgetown, TX. Figure 2 shows a graph of the height of the San Gabriel river. According to this graph, the river rose 16 feet in under 6 hours.
Fig. 2 Height of the San Gabriel River near Georgetown, TX. Image courtesy of the USGS.
The USGS webpage indicates that this is a 97.86% event, that is to say only 2.14% of time will the river flow more than it is now. Roughly speaking, this is a 50 year flood for Georgetown.
I haven't been able to find news reports, but here's what the NWS has to say:
Areas along I-35 and the Balcones Escarpment in and around the Austin Metro area will experience a life threatening flash flood event over the next several hours. Numerous evacuations and high water rescues have already been reported by the media and law enforcement personnel.
The city of Austin has a good summary of flash floods. However, the reference storm for central Texas is the Thrall rainstorm of September 9-10, 1921. Remnants from a tropical storm stalled out over Williamson County and produced record amounts of rainfall. The official records was 20 inches in a 12-hour period, but an unofficial rain gage 2 miles north of Thrall, TX recorded 32 inches of rain in a 12-hour period. The resulting flooding was catastrophic. There was $19 million in damage and officially 215 fatalities. _Road, River and Ol'Boy Politics_ by Linda Scarbrough has a chapter on the human impact of the flood in the context of discussing how Williamson County sought to get two dams built. There's a comprehensive meteorological review of the flood in Monthly Weather Review.