Hermine brings life-threatening flooding to central Texas

By: Dr. Rob Carver , 9:03 AM GMT on September 08, 2010

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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.

Historical Comparisons

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.

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7. MNTornado
9:21 AM GMT on September 09, 2010
WOW, 20 to 32 inches of rain in 12 hours. That is beyond imagination. I can't even picture that much water from a rain storm. And everyone was so worried about Gaston. Looks like Hermine caught everyone off guard.
Member Since: July 1, 2005 Posts: 154 Comments: 19315
6. StormMan
1:45 AM GMT on September 09, 2010
I can confirm a solid 12" here about 7 miles west of downtown.
Member Since: August 15, 2005 Posts: 0 Comments: 61
5. Barefootontherocks
4:30 PM GMT on September 08, 2010
More an more blue at every turn, Pat.
Member Since: April 29, 2006 Posts: 156 Comments: 18995
4. Patrap
2:07 PM GMT on September 08, 2010
NEXRAD Radar
Austin-San Antonio, Storm Total Surface Rainfall Accumulation Range 124 NMI

Member Since: July 3, 2005 Posts: 426 Comments: 129093
3. LTLROX
1:53 PM GMT on September 08, 2010
Thank you Dr Carver.

Looks like we are in for a little rain in all through the middle of the US.
Member Since: September 1, 2010 Posts: 0 Comments: 31
2. Barefootontherocks
11:02 AM GMT on September 08, 2010

036
WGUS54 KEWX 081018
FFWEWX
TXC019-029-031-091-163-187-259-265-325-081315-
/O.NEW.KEWX.FF.W.0073.100908T1018Z-100908T1315Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/

BULLETIN - EAS ACTIVATION REQUESTED
FLASH FLOOD WARNING
NATIONAL WEATHER SERVICE AUSTIN/SAN ANTONIO TX
518 AM CDT WED SEP 8 2010

THE NATIONAL WEATHER SERVICE IN AUSTIN SAN ANTONIO HAS ISSUED A

* FLASH FLOOD WARNING FOR...
SOUTHEASTERN BANDERA COUNTY...
NORTHERN BEXAR COUNTY...
SOUTH CENTRAL BLANCO COUNTY...
COMAL COUNTY...
NORTHWESTERN FRIO COUNTY...
WEST CENTRAL GUADALUPE COUNTY...
SOUTHERN KENDALL COUNTY...
EXTREME SOUTHEASTERN KERR COUNTY...
MEDINA COUNTY...

* UNTIL 815 AM CDT.

* AT 512 AM CDT...THE NATIONAL WEATHER SERVICE HAS DETERMINED THAT
RAINFALL ESTIMATES FROM DOPPLER RADAR INDICATE FLASH FLOODING FROM
A THUNDERSTORM OVER THE WARNED AREA.

* LOCATIONS IN THE WARNING AREA INCLUDE THE NORTHWESTERN HALF OF THE
SAN ANTONIO METRO AREA.


BY 6 AM...THE HEAVIEST RAINFALL AMOUNTS ARE EXPECTED TO EXTEND FROM
NEAR HONDO TO HELOTES...PIPE CREEK...AND BOERNE...WHERE 3-HOUR
TOTALS OF 2 TO 4 INCHES ARE POSSIBLE. WIDESPREAD FLOODING AND
NUMEROUS ROAD CLOSURES WILL CONTINUE OVER MUCH OF COMAL AND NORTHERN
BEXAR COUNTIES THROUGH 7 AM.


PRECAUTIONARY/PREPAREDNESS ACTIONS...

IT IS HARD TO RECOGNIZE THE DANGERS OF FLOODING AT NIGHT. IF FLOOD
WATERS RISE ACT QUICKLY. DO NOT STAY IN FLOOD PRONE AREAS.
Member Since: April 29, 2006 Posts: 156 Comments: 18995
1. Barefootontherocks
10:59 AM GMT on September 08, 2010
Statement as of 4:49 AM CDT on September 08, 2010

The National Weather Service in Austin San Antonio has extended the

* Flash Flood Warning
for...
western Bastrop County...
eastern Blanco County...
southeastern Burnet County...
northern Caldwell County...
northeastern Comal County...
north central Guadalupe County...
Hays County...
northwestern Lee County...
Travis County...
Williamson County...

* until 700 am CDT.

* At 442 am CDT... the National Weather Service has determined that
rainfall estimates from Doppler radar continue to indicate flash
flooding potential from thunderstorms over the warned area.

Areas along I-35 and the Balcones Escarpment in and around the
Austin Metro area will continue to experience a very dangerous flash
flood event over the next few hours. Numerous evacuations and high
water rescues have already been reported by the media and law
enforcement personnel.


Moderate to heavy rainfall rates of 1 to 2 inches per hour will
continue through at least 630 am over much of the warned area.
Ground reports and radar estimates suggest that overnight rainfall
amounts of 6 to 12 inches have occurred over an area from Lakeway to
Florence... extending east to Jarrell... Round Rock... and Sunset
Valley.

Many roadways will remain cut off for several hours. Some rivers and
stream will continue to rise rapidly and warrant additional
evacuations. Overnight travel is discouraged unless absolutely
necessary.

Precautionary/preparedness actions...

Runoff from heavy rain will flood creeks, streams, street
intersections, culverts and underpasses. Low lying places that
normally flood could become death traps. Do not cross any flooded
area. Turn around don't drown.

It is hard to recognize the dangers of flooding at night. If flood
waters rise act quickly. Do not stay in flood prone areas.


Lat... Lon 3092 9783 3089 9764 3075 9727 3048 9715
3054 9702 2989 9742 2969 9793 3009 9845
3103 9790
Member Since: April 29, 2006 Posts: 156 Comments: 18995

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About rcarver

Rob is the Research and Development Scientist for Weather Underground. He has a Ph.D. in meteorology from Penn State University.

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