SR SSD 99-18

Technical Attachment
OF OCTOBER 13, 1997

Gregory Flatt
National Weather Service Forecast Office
Brownsville, Texas

1. Introduction

Eastern Cameron County, Texas has a flat, generally featureless terrain. The elevation is less than 500 ft. During heavy rain events the low-lying areas are usually flooded, since runoff is minimal and maximum infiltration occurs rapidly. Very heavy rains on October 13, 1997 caused significant flooding in the greater Brownsville metropolitan region, as well as in the surrounding communities of Los Fresnos, Rancho Viejo and San Benito. Numerous roads were closed and several houses were evacuated due to rising water. This flood is noteworthy in that it resulted from heavy rains produced by a strong fast-moving cold front, a rarity for deep South Texas. The goal of this paper is to illuminate the meteorological conditions and preconditions that surrounded this event in order to assist meteorologists and hydrologists to accurately predict and warn for a similar weather and flooding scenario in the future.

2. An Overview - "Setting the Stage" for a serious flood event

This flood event began during the first week in October, 1997. A surface trough developed along the coastal areas of deep South Texas. The upper-level flow was light and variable, which kept the surface trough pretty much stationary along the coast. During this same period, Hurricane Pauline, in the Eastern Pacific, provided the region with an additional source of moisture. The circulation pattern associated with the surface trough over the South Texas coast drew the abundant moisture from Pauline northeastward into the region. The combination of the two weather systems made for a very wet two weeks. Figure 1 shows that for the period from October 2-12, heaviest rain was concentrated over the coastal counties from the towns of Port Mansfield to Bayview and Brownsville, then along the Rio Grande toward Mercedes and Weslaco.

The most significant rainfall during the 10-day period prior to October 13 occurred on the afternoon and evening of October 11. On that afternoon the Brownsville WSR-88D indicated a line of moderate to heavy showers and a few thunderstorms along a quasi-stationary trough extending from Port Mansfield to just south of Brownsville. Showers and thunderstorms developed just south of Brownsville and moved north in a train echo fashion over Brownsville. As a result the heaviest rains were concentrated in the Brownsville area. The showers and thunderstorms were moderate to heavy, and as they moved through Brownsville toward the north they decreased in intensity.

More than 4 in of rain fell in Brownsville, 3 in fell in Los Fresnos, and 1.5 in fell in Rancho Viejo. The heavy rain caused some flooding in the Brownsville area and minor flooding in Los Fresnos. The rains on October 11, together with the accumulation of rainfall during the October 2-12, period, set the stage for the floods on October 13 by thoroughly saturating the ground in the metropolitan area. Conditions had become quite favorable for a major flood in eastern Cameron County if additional heavy rainfall were to occur over the next day or two. That is what happened; October 13 was another day of heavy rainfall (Fig. 1).

3. Synoptic Situation and Meteorological Conditions

The 1200 UTC Brownsville sounding on October 13 (Fig. 2) measured a precipitable water content of 2.40 in (190 percent of normal!) and a lifted index of -6C. The 500 mb trough axis (Fig 3) was over western Texas, while the surface front extended from the middle Mississippi Valley into eastern Texas to west of Corpus Christi and McAllen (Fig 4). On October 12, as the front approached, the region experienced strong south to southwest winds from the surface to 500 mb. With a strong south to southwest wind, the Brownsville county warning area (CWA) often experiences a significant warming and drying of the air mass due to adiabatic downslope flow over the nearby Sierra Madre Oriental mountain range in Mexico. However, in this case a continual moisture flux into the CWA counteracted the adiabatic drying and allowed for an increase in atmospheric moisture, as reflected in the precipitable water measurement.

The airmass was becoming quite unstable, but a trigger was needed for strong storms. That trigger was the lift provided by the vigorous cold front moving quickly into the CWA. A 20 deg temperature and dew point gradient was observed in the baroclinic zone, providing a good indicator that indeed a strong cold front had made it into deep South Texas. The segment of the cold front over the CWA began to slow as its upper-level steering flow diminished. Thus, the potential for severe weather was high and the threat of very heavy rainfall increased.

The Brownsville WSR-88D data (not shown) indicated an area of mostly moderate showers and thunderstorms developing ahead of the front over eastern Hidalgo and western Cameron Counties. By 1400 UTC radar indicated the cold front had slowed to near 10 mph. For the next several hours a train echo pattern was observed over eastern Cameron County, with heavy rain showers repeatedly developing and moving over the same area of the county as the front moved slowly through. Rainfall rates of near 2 in/hr were measured in eastern Cameron County. The cold front finally moved offshore into the Gulf of Mexico around noon, leaving in its wake severe flooding over most of eastern Cameron County.

4. Discussion

Climatology shows a flood similar to the one on October 13 occuring once every 7 to 10 years in the Brownsville CWA. While the accumulated rainfall from October 2-13, 1997 was significant (October 1997 was the 5th wettest October on record with 13.03 in), the October 13 rainfall was not among the most extreme single day amounts ever recorded. The single day record is 12.09 in, which occurred on September 20, 1967, and was attributed to Hurricane Beulah. On September 21, 1886 an unnamed hurricane dropped 10.32 in on Brownsville and another 11.91 in the next day, making a two-day total of 22.23 in.

Within the climatological context of previous single day heavy rainfall events, the rain on October 13 was not in itself remarkable, but it became extremely significant because it occurred after a string of wet days. As a result, the Brownsville CWA, especially eastern Cameron County, was primed for serious flooding should such heavy rainfall occur. That indeed is what happened.

Climate records show flooding often takes place in September or October when a quasi-stationary front interacts with a tropical wave to produce heavy rain in a localized area. These meteorological situations tend to produce moderate to heavy rain showers or thunderstorms which develop and move over the same area for as many as three to four days. This was the case with the September 1984 flood which produced the second wettest September ever, with a four-day total of 15.18 in and a monthly total of 20.18 in.

The eastern Cameron County flood of October 13, 1997 was different and is of interest because a rather vigorous early season cold front moved through Texas, accelerated by a strong high pressure system and pressure gradient to its west. In addition, there was plenty of tropical moisture ahead of the front from Hurricane Pauline in the eastern Pacific, and the soil was saturated due to rains earlier in the month. One reason large amounts of rain were produced over eastern Cameron County was because the front slowed considerably as it moved into Cameron County, producing a train-echo effect over the Brownsville, Los Fresnos, Rancho Viejo and San Benito area.

Fortunately, no lives were lost, however, several homes were evacuated or damaged in eastern Cameron County (Fig. 5). Hundreds of people took refuge in shelters, in some cases for several days. Roads were closed due to flooding and remained impassable for up to three days after the flood event. Electrical service was slow to be restored in some communities; in a few instances it took as long as a week to restore service due to damaged infrastructure.

5. Concluding Remarks

Although the Brownsville CWA encompasses a rather arid climate zone as this event shows, flooding is a distinct possibility when consecutive days of light to moderate rainfall saturate the ground, and then is followed by a moderate to heavy single-day rainfall. The cumulative effects of individual rain events from October 2-12 made the heavy rainfall on October 13 so important.

Meteorologists at NWSO Brownsville were alert to the possibility of flooding and severe weather, as indicated by the warnings, watches and statements which were issued during the event (Table 1). An NWS goal of providing at least 41 minutes lead time with a Flash Flood Warning (FFW) was met in this case. The bulk of the heavy rain began about an hour after the FFW was issued at 1330 UTC. This successful warning also contributes positively to the overall NWS goal of at least 85 percent accuracy for flash flood warnings.

Flood waters receded slowly in eastern Cameron County because of the nature of the topography and the saturated condition of the ground. It is important for WFOs to continue to monitor situations in flooded areas in concert with the county emergency preparedness officials, even though the rain has ended. Public statements and special weather statements should continue to be issued after the flood event until the waters have completely receded to normal levels.


Warnings, Watches and Statements issued before and during the event

Flash Flood Watch 1200 (10/11) to
0300 (10/12)


Cameron, Willacy, Brooks, Hidalgo and Kenedy Counties
Flood Warning

1730 (10/11) to
2330 (10/11)


Cameron and Willacy Counties
Flood and Flash Flood Statement

1945 (10/11)


Deep South Texas
Flood Warning

2330 (10/11) to
0230 (10/12)


Eastern Cameron County
Marine Weather Statement

1200 (10/13)


Deep South TX Coast
Flash Flood Warning

1330 (10/13) to
1630 (10/13)


Cameron, Hidalgo, Kenedy and Willacy Counties
Severe Thunderstorm Warning

1430 (10/13) to
1530 (10/13)


Cameron County and The Adjacent Waters
Flash Flood Statement

1530 (10/13)


Deep South Texas
Flood Warning

1630 (10/13) to
2300 (10/13)


Cameron and Willacy Counties
Flood Statement

1920 (10/13)


Deep South Texas

6. Acknowledgments

Thanks to NWSO Brownsville staff members Shawn Bennett (SOO), Alfredo Vega, and Timothy Speece for providing thoughtful reviews of the manuscript that greatly improved its clarity, and to Don Ocker (WCM) who provided photographic evidence of the results of the flooding. Thanks to the Rio Grande Valley Chapter of the Red Cross for their efforts and time in providing me with information regarding this flood event.