Case 035: San Antonio Flood Lab Exercise

Case Summary: 17-18 October 1998 - Excessive Rainfall in South & Southeast Texas

Excessive rainfall of >20 inches in 24 hours (>17 inches in 6 hours) resulted in historic river flooding along many rivers in southeastern Texas (particularly the Guadalupe and Colorado Rivers). Several classic indicators were in place including:

deep, moist low-level flow from the southeast,

a tropical connection allowing for very efficient precipitation growth through a deep layer,

a warm cloud layer more the 4 km deep (San Antonio RUC 18 October 0000Z F000 Sounding),

and nearby subsidence (over the Gulf of Mexico) allowing rich low-level moisture to flow into a narrowly defined area of precipitation production.

Upper level jet dynamics to further enhance upward motion were present; in fact a subtropical jet streak can be seen propagating north toward Texas (WV imagery) and was likely associated with the timing of storm initiation.

Even in Texas orography can have an impact. You can see in the included images that the terrain elevation gradient had an impact on the rainfall distribution and was likely associated with the location of storm initiation.

Once the storm generation was established, rain-cooled outflow air established meteorological boundaries (a mesohigh is implied in the included hand analyses and can be seen in radar velocity imagery) which became a focus for regenerating storm cells.

To Summarize:

• Initial development: Tremendous moisture flux into the area, a terrain gradient, and upward support form jet dynamics (as seen in satellite imagery) was present.
• Regenerating storms: Although the terrain gradient continued to play a role throughout the event, the combination of the frontal boundary and cool outflow boundaries along the terrain and the front became a focus for the deep moisture surging north and westward.
• Both duration and intensity contrubuted to the excessive accumulations. The intensity in particular was enhanced by a deep tropical air mass ( the above-freezing cloud layer exceeded 4 km) combined with a low level boundary to focus low-level upward motion.
• Flash flooding dominated the early parts of this event as intense convective rains struck small, fast-response basins in the urban areas around San Antonio and Austin, and in small, relatively steep basins north/west of San Antonio.
  

  At least 26 of the 31 fatalities were from flooding, and at least 17 of those involved vehicles.
  A total of 163 flash flood warnings were issued by the San Antonio office, the first warnings more than 4 hours before the first fatality.

• Were the warnings specific enough? If some lead times were 4 hours, and the average was 48 minutes, that suggests some lead times were quite short (some were likely extended warnings).
• Were they issued ahead of the flash flooding? (fatalities don’t necessarily indicate the onset of a flash flood).
• What tools do the forecasters need for more specific warnings?

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