"San Antonio Flood"
17 October 1998

During the weekend of 17-18 October, 1998, heavy rains fell over south and southeast Texas. 20 to 30 inches of rain fell locally near San Antonio, with more widespread areas receiving 10 to 20 inches of precipitation. This heavy rainfall resulted first in flash flooding from San Antonio to Austin, followed by record-breaking river flooding along several southern Texas rivers in the days to follow. To make matters worse, many tornadoes were spawned by storms in the affected areas, with strengths ranging from F0-F2 on the Fujita Scale.

The conditions were present to produce heavy rain in the southern Texas area. A trough was moving in from the north and west, with it's associated surface cold front. Moisture was being transported in from the Gulf of Mexico at low levels on southeast winds, as dewpoints were in the 70s in southeast Texas. At mid-levels, Hurricane Madeline was providing moisture from it's outflow from the southwest into southeast Texas. A marked theta-e ridge was present over south and east Texas, signifying a combination of moisture and temperature favorable for convective development.

The conditions all present, a trigger was thought to be nearing the area in the form of the cold front associated with the trough, though convection began over southeast Texas well in advance of the front. A line of disorganized convection was present around midnight on the 16th and developed slowly until around 1100Z, when it became concentrated and heavy around San Antonio. By 1615Z, precipitation spread ahead of the front, which can be seen as the squall line in north-central Texas. By 1700Z on the 18th, radar-estimated storm total precipitation measured greater than 18 inches of rain between San Antonio and Austin.

A service assessment of the affected offices showed that warning and forecasting services were good. All field offices anticipated the potential for heavy rainfall over the weekend. Although QPF amounts were significantly underestimated initially, updates were prepared which incorporated the heavier rainfall. Although loss of life occurred due to flash flooding, the NWS forecast office in San Antonio/Austin had warnings out 4 hours before the first flooding related death was reported.

Tragically, a total of 31 people died from flooding or tornado related injuries. Preliminary property damage estimates approached three quarters of a billion dollars.

Case Study 35