P4.8 A MULTI-PLATFORM ANALYSIS OF THE CENTRAL TEXAS FLOODS OF MAY 13, 2004 Lon Curtis, KWTX-TV, Waco, TX; and Alan R. Moller, NOAA-NWS-Fort Worth 1. INTRODUCTION & SYNOPTIC SITUATION The area forecast discussion (AFD) mentioned that warm advection would develop over most of Deep, moist convective storms developed the CWA by midday Thursday and heavy rainfall during the early morning hours of May 13, 2004 would be possible with the approach of a cold front, over portions of central Texas southeast of Waco in mainly from late Thursday into early Friday (the an area of strong onshore flow of tropical air from 14th). The evening AFD update, issued around 01 the nearby Gulf of Mexico. In less than twelve UTC on the 13th, opined that no precipitation was hours, some areas of Milam and Robertson expected “overnight” . counties received 11 to 17 inches of rainfall. Widespread disastrous flooding resulted. Indeed, the operational numerical models were not at all alarming with respect to QPF output, at Preliminary estimates of property damage least through noon on the 13th, as neither the ETA totaled $2.7 million in each county and an additional nor the GFS produced precipitation that exceeded $2 million in crop losses were estimated for flash flood guidance across the area. The GFS Robertson County, where 107 farm dwellings and model (from the 00 UTC run) did produce a large service buildings were destroyed, and a number of area of very intense precipitation after 18 UTC on small dams failed. the 13th, but over far eastern Texas. Following the passage of a strong cold front In this study we will utilize a variety of more than 10 days prior to the event, return flow platforms, some in situ and some remote, to from the Gulf commenced on the 4th and continued document the development and evolution of this without interruption through the morning of the 13th. event. These will include automated surface Moisture return was gradual, but by the evening of observations, products from the WSR-88D radars, May 12th dew points were ~22° C across the area wind profiles from profilers operated by NOAA and and had reached ~ 25° C along the immediate by the Texas Commission on Environmental coast, ~95 miles to the south. Surface analyses Quality, rawinsonde observations, and satellite from NCEP did not depict, nor did objective and imagery from the eastern GOES unit. We will also manual analysis of surface observations reveal any make use of the 0-hour (analysis) output from the surface boundaries near the area. Rapid Update Cycle (RUC) model as a source of pseudo-real-time gridded data above the surface. Objective analysis of 00 UTC upper air data Figure 1 shows the area of the study, including revealed a mid and upper level synoptic scale county lines, radar and profiler sites, and selected trough west of the area over the Rockies. A weak surface observation sites. mid-level trough was advancing eastward ahead of the primary trough. Streamline and isotach analysis 2. SITUATION AT 06 UTC revealed a jet stream maximum exceeding 70 knots over the Great Plains and a subtropical jet Examination of reflectivity data from WSR-88D maximum exceeding 70 knots over northern sites around the area (not shown) and IR imagery Mexico. Between the two jets, a broadly diffluent from the eastern GOES satellite (Figure 4a) southwesterly flow covered much of Texas. indicated scattered deep convection developing around 06 UTC in a small area between Austin and The area that would become the center of the Houston. Cells initially moved toward the north- excessive rainfall event on the 13th lies in the northeast, gradually forming a broken line that was southeastern corner of the County Warning Area oriented west-northwest to east-southeast. As the (CWA) of the NWS office located in Fort Worth. The broken line moved across the area, additional cells afternoon zone forecast package (ZFP) on the 12th developed in a broad triangular area between San placed the probability of measurable precipitation at Antonio, Waco and Houston, with a gradual 60% for the daylight hours on the 13th. increase in intensity and coverage. _________________________________________ The RUC 0-hour (analysis) output (not Corresponding author address: Lon Curtis, KWTX- shown) revealed a deep southeasterly flow across TV, P.O. Box 2636, Waco, TX 76702-2636; email: the Texas coastal plains toward Waco and the [email protected] Dallas-Fort Worth area. Of potential significance, Fig. 1. Map depicting area of interest on May 13, 2004. Shaded counties are Milam and Robertson. the 250 mb analysis revealed an area of weak The most intense and rapidly expanding deep divergence over the area where the first storms had convection was within this stagnation zone. developed, with a stronger area of divergence upstream over northern Mexico, southwest of Just before 10 UTC, the Hydrometeorological Laredo. Prediction Center (HPC) updated the Excessive Rainfall Discussion (QPFERD) to outline an area 3. SITUATION AT 09 UTC over southeastern Texas into Louisiana where isolated excessive rainfall would be possible. The Areal coverage of deep convective cells was southwestern corner of the area was Hearne (LHB), continuing to expand at 09 UTC. Figure 2a shows a in Robertson County. The issuance mentioned that composite of 0.5 degree base reflectivity at the latest model guidance pointed to a 12 to 16 hour WSR-88D site at Granger (KGRK) for 0858 UTC period of isolated heavy rainfall across the region, and 09 UTC surface observations plotted in the and noted that the latest satellite and radar usual manner. observations were “very supportive”, with increasing convection through south central Texas Infrared imagery from the eastern GOES that was directly related to subtropical shortwave satellite (Figure 4b) depicted the initial convective energy lifting northeastward. area moving away to the northeast over Huntsvile (UTS) and Conroe (CXO) while a new (and 4. SITUATION AT 12 UTC stronger) area of storms was developing from east of Temple to east of Austin. By 12 UTC, it was obvious that a serious situation was unwinding across the area bounded The RUC 0-hour (analysis) output for 09 UTC roughly by Austin, Waco and Bryan-College revealed a continuation of the deep southeasterly Station. About 1120 UTC, the NWS office at flow of warm, moist air into the area. Perhaps of League City (HGX) issued a tornado warning for greater significance, divergence at 250 mb had Brazos County, including the Bryan-College Station increased and the isotach analysis at that level area as an intense storm cell moved across that depicted a zone of stagnating flow over the area area. Storm cells were developing and repeating between Austin and Houston. Fig. 3 shows a zone over areas of Milam and Robertson counties, which of less than 30 knot flow oriented southeast to prompted the NWS office at Fort Worth to issue a northwest from approximately Galveston to Waco. Significant Weather Update shortly after 1115 UTC, that cited strong thunderstorms with very heavy rain pattern in a single boundary. Looping of the product in Milam, Robertson and Falls counties. over time indicated that the cells were forming into a line and beginning to move toward the southeast. The 3-hour accumulated precipitation Immediately ahead of the line, cool outflow was product from the WSR-88D at KGRK at 1215 UTC noted at College Station while warm, moist inflow (not shown) indicated a large area of rainfall continued at Giddings. exceeding 2 to 3 inches northeast of the radar site, with a small area near and west of Hearne showing Both the one-hour and the three-hour more than 4 inches. Very heavy rainfall was accumulated precipitation products from KGRK are continuing across the area, as seen in Figure 2b, a missing or unusable (from the NCDC archive) for composite of 0.5° base reflectivity from KGRK at the period from approximately 13 UTC to almost 16 1203 UTC with the 12 UTC surface observations UTC. However, review of reflectivity data from plotted in the usual manner. KGRK and other WSR-88D sites leaves no doubt that very heavy rainfall persisted during that entire Base velocity products (0.5 degree) from period. KGRK between 11 UTC and 13 UTC (not shown) indicated the development of very strong east to The RUC 0-hour (analysis) output for 15 west outflow from the area of intense rainfall over UTC (Fig. 5) revealed a continued expansion of the eastern Milam and western Robertson counties area of divergence at 250 mb. The isotach analysis toward the KGRK radar site. Around 12 UTC a depicted a zone directly over the area of deep significant area of >50 knot inbound velocity was convection where flow was less than 20 knots, and shown from near Cameron to near the KGRK site. depicted the area of maximum divergence to be In loops of the velocity product, this strong outflow centered west of Lufkin. can be seen passing across the GRK site, with the flow only gradually subsiding. Then, by 13 UTC, Infrared imagery from the eastern GOES strong convergence developed very near the KGRK satellite at 1515 UTC (Figure 4d) depicted a mature site associated with yet another (but more limited) MCC, with a very large area (>110,000 km2) in burst of deep convection. which cloud tops were at or below –52° deg C. An interesting feature seen in animation of the IR The RUC 0-hour (analysis) output for 12 imagery between 11 UTC and 17 UTC was a UTC revealed a continuation of the deep cyclonic looping of the coldest cloud tops over and southeasterly flow of warm, moist air into the area, west of the zone of most intense rainfall.