The combination of upper level energy, overnight remains of outflow boundaries from storms near the Lower Rio Grande River, and the approach of thunderstorms from south Texas toward the Rio Grande Highlands along with a very unstable atmosphere helped intensified these storms into a mesoscale convective system which raced through the Rio Grande Valley between 8 AM and 1045 AM CDT (7 AM and 945 AM CST).  The storms produced urban and flash flooding, minor wind damage, pockets of large hail, and frequent cloud to ground lightning. An earlier cluster of storms east of the Port Isabel/Laguna Vista produced a brief landspout tornado.

Thunderstorm Wind in CAMERON, TEXAS
From 18-MAY-10 09:25:00 to 18-MAY-10 09:40:00 (CST-6)
Location: PORT ISABEL
Magnitude: 56.00 MG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Coast Guard

The thunderstorm complex continued to generate strong, damaging winds as it moved across southeast Cameron County, affecting Bayview, Port Isabel, and South Padre Island.  The strongest wind core appeared to rip through Port Isabel across the southern portion of the Town of South Padre Island.  At Port Isabel, numerous power outages were the result of power lines and poles blown down at several locations throughout the city.
As the storm reached South Padre Island, the U.S. Coast Guard Station located in the Bayside recorded a 64 mph wind gust (at 936 AM CST, a six minute recorded observation).  At nearly the same time (937 AM CST), a 60 mph gust was recorded by a Citizen Weather Observer Program sensor on Sheepshead Street. In the same area, palm trees were blown down along Coral Street; large metal signs were blown from their mounts along the Queen Isabella Causeway Bridge, and more power lines and a transformer were blown down at the intersection of Constellation Dr. and Laguna Boulevard, all between 930 and 940 AM CST (end time estimated by radar).

Hail in HIDALGO, TEXAS
From 18-MAY-10 07:45:00 to 18-MAY-10 07:45:00 (CST-6)
Location: SAN CARLOS
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Nickel sized hail was reported in San Carlos, 4 miles east of Edinburg.

Thunderstorm Wind in CAMERON, TEXAS
From 18-MAY-10 09:05:00 to 18-MAY-10 09:20:00 (CST-6)
Location: BROWNSVILLE
Magnitude: 50.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Newspaper

A 20 year old cottonwood tree, renowned and visited often for having a purported image of the Virgin Mary on its bark, was blown down by strong thunderstorm winds estimated to have occurred between 905 and 910 AM CST at the intersection of East Levee and 10th Streets in downtown Brownsville.  Numerous, varying sized limbs were blown down in Greater Brownsville, with peak gusts recorded at the Brownsville/South Padre Island Airport reaching 54 mph at 928 AM CST, 5 miles east of downtown.

Thunderstorm Wind in WILLACY, TEXAS
From 18-MAY-10 08:10:00 to 18-MAY-10 08:20:00 (CST-6)
Location: RAYMONDVILLE
Magnitude: 52.00 EG
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Trees and large limbs were blown down on various streets in Raymondville.  Time and duration are estimated by radar, based on core of estimated winds between 50 and 52 knots a few thousand feet above the surface.

Thunderstorm Wind in HIDALGO, TEXAS
From 18-MAY-10 07:55:00 to 18-MAY-10 07:55:00 (CST-6)
Location: EDCOUCH
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Broadcast media reported a roof was blown off a mobile home during a thunderstorm that produced high winds across the area.  Radar indicated a couplet of winds, as well as a reflectivity overhang, had moved across the area, suggesting a possible tornado.  There were no confirmed reports of a tornado, however.

Tornado in CAMERON, TEXAS
From 18-MAY-10 07:41:00 to 18-MAY-10 07:45:00 (CST-6)
Location: PORT ISABEL
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

An off duty Central Region National Weather Service Employee reported a waterspout that briefly moved onshore near Port Isabel.  No damage was reported.

Flash Flood in HIDALGO, TEXAS
From 18-MAY-10 07:45:00 to 18-MAY-10 08:30:00 (CST-6)
Location: MISSION
Flood cause: Heavy Rain
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

The squall line that tore through the heart of populated Hidalgo County dropped very heavy rainfall in an hour or less, creating numerous reports of urban flooding with embedded, more dangerous flash flooding especially in poor drainage locations of Mission, McAllen, and Edinburg. Four inches fell at the McAllen Cooperative (water plant) location, 2.82 inches at McAllen/Miller Airport, and 2.95 inches just southeast of McAllen.  In Mission, the Fire Department reported several roads throughout were under two to three feet of water.  In McAllen,  the City transportation department reported 7 to 9 automobiles stalled in floodwaters halfway up the doors at 10th and Pecan.  A video submitted via Twitter showed the depth of the water, at least 2 feet in some places, on the sides of the road.  Attendees of the Texas Hurricane Conference in McAllen reported significant high water along the Expressway 83 Frontage Road during the same time.
In Edinburg, University Drive was blocked by high water just east of the courthouse due to high water, which threatened businesses but appeared to have remained outside of them.

Hail in HIDALGO, TEXAS
From 18-MAY-10 07:25:00 to 18-MAY-10 07:25:00 (CST-6)
Location: MC ALLEN
Magnitude: 1.50 
Crop damage: 0.00K
Reported by: Emergency Manager

The Hidalgo County Emergency Manager reported hail up to one and one half inches in diameter fell at the La Quinta Inn on south 10th Street during a thunderstorm which moved through the area.

Thunderstorm Wind in HIDALGO, TEXAS
From 18-MAY-10 08:06:00 to 18-MAY-10 08:12:00 (CST-6)
Location: WESLACO
Magnitude: 39.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Emergency Manager

The Hidalgo County Emergency Manager reported strong winds occurred during a thunderstorm which knocked down some power lines in Weslaco. A funnel cloud was reported by the emergency manager approximately two miles west of Weslaco as the thunderstorm moved through the area. Peak winds of 43 mph were reported at the Mid Valley Airport, 2 miles east of Weslaco, estimated at around 812 AM CST.

Thunderstorm Wind in CAMERON, TEXAS
From 18-MAY-10 08:20:00 to 18-MAY-10 08:35:00 (CST-6)
Location: SANTA ROSA
Magnitude: 52.00 MG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: ASOS

Winds within and ahead of the squall line increased to severe levels as they swept across Cameron County.  Racing from from west northwest to east southeast across the County, the winds, both measured and estimated at 60 mph, knocked down hundreds of different sized tree limbs and caused power outages from Santa Rosa through Harlingen and San Benito.  The ASOS (KHRL) at the Valley International Airport recorded a 60 mph gusts, and the public estimated a 60 mph gust moments later in San Benito.