A stationary complex of thunderstorms dumped an estimated 8 to 10 inches, with pockets of a foot or more (12 inches) of rainfall, near and just east of Roma between daybreak and noon, producing widespread flash and areal flooding beginning during the mid morning of the 18th, and continuing through mid to late afternoon. The complex was the result of the combination of an upper level disturbance acting on a plume of deep tropical moisture, which enhanced the lifting of this moisture through a low level southeasterly jet streak moving from the Lower Rio Grande Valley northwest into the Rio Grande Plains and Rio Grande Highlands.  Conditions began to improve by late afternoon, well after the rains had ended.
Farther east, development of intense thunderstorms in northwestern Tamaulipas province, Mexico, eased north into southern Hidalgo County during the evening, dropping at least 4 to 6 inches in a 3 hour period.

Flood in STARR, TEXAS
From 18-AUG-08 12:00:00 to 18-AUG-08 15:00:00 (CST-6)
Location: ESCOBARES
Flood cause: Heavy Rain
Property damage: 700.00K
Crop damage: 0.00K
Reported by: County Official

Though the rains tapered off shortly after noon, and sunshine peeked through the clouds during later in the day, floodwaters continued to cover much of Roma, but particularly Escobares and Garceno, as Arroyo Quiote continued to rise and remain in flood for several hours after the rains ended. Farm-to-Market Road 650 was closed due to the high water covering the roadway, and numerous local roads and some major thoroughfares were closed due to flooding.
Federal Highway 83, the primary east-west highway connecting the Upper Rio Grande Valley with the Lower Rio Grande Valley, was closed with up to three feet of water covering portions of the highway into the evening of the 18th. Total structural damage and repair costs were estimated to be in excess of one million dollars from the entire flood episode.  At the peak of the flash flooding and areal flooding, an estimated 500 water rescues were undertaken by emergency management personnel.

Heavy Rain in ZAPATA, TEXAS
From 18-AUG-08 09:15:00 to 18-AUG-08 12:00:00 (CST-6)
Location: FALCON
Reported by: Public

Radar data suggested that at least 3 inches of rain fell in southwestern Zapata County, with perhaps 6 inches closer to Falcon Lake, between late morning until just after noon, associated with a northwest extension of the more intense convection from Starr County.
These storms likely resulting in minor flooding of some poor drainage areas. A citizen report of some flooding was received at 1220 CDT from the city of Zapata, along 9th Avenue.
Much heavier rains of more than 8 inches created severe flash flooding just east of Roma, associated with the core of the thunderstorm complex.

Flash Flood in STARR, TEXAS
From 18-AUG-08 08:45:00 to 18-AUG-08 11:59:00 (CST-6)
Location: ESCOBARES
Flood cause: Heavy Rain
Property damage: 500.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Torrential rainfall of at least 8 to 10 inches, but perhaps locally 12 inches,  produced significant flash flooding from mid morning through early afternoon of the 18th in a pocket of Starr County along Federal Highway 83, stretching from the east side of Roma through Garce��o and Escobares. Initial urban flooding as far east as the Los Lomas colonia near Rio Grande City soon evolved into a flash flood.  The Roma Police Department reported one to one and a half feet of water throughout the city, with locally higher amounts, which has been confirmed by a number of home videos.
Major flash flooding developed in these areas when several Arroyos, including the Arroyo Quiote (east of Rosita), Arroyo Los Morenos (near Escobares), and Arroyo Grande temporarily became raging rivers and aggravated the already high water. Starr County Emergency Management officials estimated at least 1100 homes had some degree of flooding ranging from 2 to 3 inches up to 5 feet at the peak of the flooding. A number of these homes were in newer communities near the Arroyo. First responders evacuated 234 or more people from the flooding areas.
Structural damage and repair costs were estimated to be in excess of one million dollars.

Heavy Rain in HIDALGO, TEXAS
From 18-AUG-08 16:53:00 to 18-AUG-08 21:11:00 (CST-6)
Location: (MFE)MILLER INTL ARP
Reported by: CoCoRaHS

Developing multi-cellular clusters of intense thunderstorms moved slowly across Hidalgo, Mission, Sharyland, and west McAllen during the early to mid evening of the 18th, dropping copious rainfall in a 3 to 5 hour period, but not producing anything more than nuisance minor flooding as of this writing.
The evening rain dump, combined with moderate to locally heavy amounts during the early morning hours, left up to 8 inches for the calendar day.  Daily rainfall totals include a record 5.38 inches at McAllen/Miller International Airport, and between 4.69 and 5.10 near Mission.  Of the 5.38 which fell at Miller Airport, 4.46 of it occurred roughly between 6 PM and 10 PM CDT.