Heavy Rainfall Pounds Portions of Rio Grande Valley, Leaving Flooding Behind September 26-27, 2014

September 2014, already with above average rainfall in hand and plenty of lush, green grass and brush as a result, finished with a flourish as a slow moving upper level disturbance (large red "L" above) seeded the high levels of the atmosphere with moisture and helped focus/enhance tropical moisture closer to the earth’s surface along a trough of low pressure (smaller red "L"), from September 25th through 28th. The result was a combination of collisions that led to life and property threatening flooding across different locations in the Rio Grande Valley between Friday night, September 26th, and early Sunday morning, September 28th. By the time the final weekend of September 2014 ended, Valley–wide rainfall would be the largest for a single month since July 2008, when early month rains were overwhelmed by widespread flooding from Hurricane Dolly. The repeated rainfall overcame comparative months of September 2010 and June 2010.

September 26, Evening: Rio Grande City Washout
Early afternoon heating, boundaries/storms moving toward the west and southwest from the South Texas Brush Country, and a wave of upper level energy rotating from southwest to northeast each combined to create a nearly stationary band of torrential rain producing thunderstorms over Rio Grande City and nearby locations. Nuisance flooding began during the late afternoon, and significant, life and property flooding began in Rio Grande City before sunset. Flooding peaked after sunset and continued through at least 11 PM, including water levels on the normally low flowing or dry Arroyo Los Olmos exceeding bank full by 2 to 3 feet, and several feet of flowing water that flooded the Las Lomas neighborhood. Several cars were partially submerged, and at least one house had extensive inundation. Observations and radar estimated suggested 6 to 8+ inches of rain fell during the long duration event. In addition to the flooded Arroyo Los Olmos, the river stage at Rio Grande City spiked as well but remained well below the danger level – though flow rates and water level in the Rio Grande were harrowing.

September 27, Afternoon: Mid Valley Mess
A similar circumstance to the setup on the 26th arose by mid afternoon on the 27th, but this time favored the most populated metropolitan region in the Rio Grande Valley. A slow moving band of blinding rains developed in rural northeast Starr and northwest Hidalgo County soon after noon, and would soon link up with other cells forming from Reynosa/Tamaulipas into the McAllen/Edinburg/Mission/San Juan/Pharr area generally between 230 and 330 PM. The rain may have lasted an hour, but accumulated well over 2 inches in McAllen and Edinburg. Poor drainage locations were unable to keep up with the rainfall rate, and water up to 3 feet deep piled up in a few areas until a little while after the rain moved east. Farther north, in rural central and northern Hidalgo County, rains continued well after they had cleared out from the larger population centers. Nearly 4 ½ inches fell in San Manuel during the afternoon; some areas likely received between 6 and 8 inches. Locations in and near the heaviest cores reported several feet of inundation, including water reaching into houses.

September 27–28: Laguna Madre Lakes?
The longest lasting and highest total rainfall fell in a small area of the Laguna Madre and nearby Laguna Atascosa National Wildlife Refuge south of Arroyo City. High resolution radar estimates suggested 9 to 10 inches over the area. Along Farm to Market 106 (FM 106), between the La Tina Ranch and Holly Beach east of Rio Hondo, 6 to 9 inches likely fell. A local storm survey, as well as other accounts, indicated part of the road bed washed away on FM 106 with water up to or still into homes along the road, more than 12 hours after the rain had ended. Along Arroyo City Blvd. (FM 1847), several vehicles were disabled by the floodwaters, which were likely 3 feet or higher in several locations. Most of the significant flooding began around 9 PM on the 27th, and continued well past midnight into the pre dawn hours of the 28th before waters started to recede. The heavy rainfall in the Lower Valley combined with upstream rains in the Mid and Upper Valley to create a wave of high, fast flowing water on the Rio Grande at San Benito late on the 28th and into the 29th.

On South Padre Island, several rounds of showers and thunderstorms dropped between 4 and 6 inches throughout the day and evening, adding another layer of hazard to an already rough and confused surf (and on and off showers since the 25th) which pushed up to the dunes at high tide on the 26th and 27th. The heavy rain flooded most roads with 1 to 2 feet of water during the afternoon of the 27th.