A developing cold front, associated with the creation of the December Blizzard of 2010 from New Jersey to New England, pulled down dry, Canadian air with strengthening high pressure overnight on December 25th and well into the morning hours of December 26th.  Breezes through the post midnight hours kept temperatures a hair above freezing in many areas, but calming winds by 2 AM or so allowed temperatures to fall more steadily, continuing for an hour or two after sunrise.
Lows fell well into the 20s in sheltered locations of rural Deep South Texas, and even reached 32 or lower in populated areas of the Lower and Mid Rio Grande Valley.  The coldest temperatures of the winter thus far were felt from McAllen to Harlingen and Brownsville, where temperatures fell to a hair above freezing.  For the lower Valley, all non urban areas west of highway 77 saw freezing temperatures on the 26th.

Frost/Freeze in WILLACY, TEXAS
From 26-DEC-10 05:00:00 to 26-DEC-10 08:15:00 (CST-6)
Property damage: 0.00K
Reported by: COOP Observer

The cooperative observer at Raymondville reported a minimum temperature of 29 degrees early on December 26th.  The nearby Remote Automated Weather Station at the Lower Rio Grande Valley National Wildlife Refuge near San Manuel (less than 10 miles west of the County line) dropped to 25 degrees, with more than four hours of freezing temperatures on the 26th.  reaching 33 degrees.
The combination of Raymondville's low temperature, proxy observations from the King Ranch and San Manuel, and the likelihood that more than half of the area of inland Willacy County reached freezing temperatures for several hours argues for a freeze event.

Frost/Freeze in CAMERON, TEXAS
From 26-DEC-10 06:00:00 to 26-DEC-10 08:15:00 (CST-6)
Property damage: 0.00K
Reported by: ASOS

The coldest recorded temperatures of the 2010/2011 winter so far occurred in western Cameron County on the 26th. While minimum temperatures fell just shy of the freezing point in Brownsville and Harlingen (36 at Brownsville and 33 degrees at Harlingen), proxy observations from western Willacy County and eastern Hidalgo County, each where temperatures fell to or below 32 degrees for several hours, argue that locations west of Federal Highway 77 in Cameron County, an area covering half of the inland zone, did likewise.
The combination of Raymondville's low temperature, proxy observations from the King Ranch and San Manuel, and the likelihood that more than half of the area of inland Willacy County reached freezing temperatures for several hours argues for a freeze event.

Frost/Freeze in BROOKS, TEXAS
From 26-DEC-10 04:00:00 to 26-DEC-10 08:30:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

The Automated Weather Observing Station at the Brooks County Airport just southeast of Falfurrias recorded more than 4 hours of freezing temperatures, falling to 27 degrees just after daybreak on the 26th. Proxy observations near Sarita and Armstrong in Kenedy County/King Ranch fell to 27 and 25 degrees, respectively.  The Lower Rio Grande Valley National Wildlife Refuge Remote Automated Weather Station near San Manuel also reached 25 degrees, with freezing temperatures for just under 5 hours.

Frost/Freeze in ZAPATA, TEXAS
From 26-DEC-10 03:45:00 to 26-DEC-10 08:30:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

The Automated Weather Observing System at the Zapata County Airport dipped to 29 degrees just after daybreak on the 26th, with duration of freezing temperatures lasting from just before 4 AM until around 815 AM.  The Remote Automated Weather Station at Falcon Lake, in Starr County just south of the Zapata County line, fell to 26 degrees with more than four hours of freezing temperatures. Even Laredo (approximately 25 miles north of the Zapata/Webb County line), which is normally moderated by nearby urban and water features, dipped to freezing, though only briefly.

Frost/Freeze in HIDALGO, TEXAS
From 26-DEC-10 04:00:00 to 26-DEC-10 08:15:00 (CST-6)
Property damage: 0.00K
Reported by: COOP Observer

The Remote Automated Weather Station at the Lower Rio Grande Valley National Wildlife Refuge near San Manuel dropped to 25 degrees, with more than four hours of freezing temperatures on the 26th. For the first time this season, locations closer to the larger population centers of Edinburg, McAllen, Mission, and Weslaco also reached 32.  At the Edinburg International Airport just north of town, the low reached 30; the cooperative observer in town fell just shy, reaching 33 degrees.
More than one half of the surface area of Hidalgo County was considered freezing for more than 2 hours.

Frost/Freeze in JIM HOGG, TEXAS
From 26-DEC-10 02:00:00 to 26-DEC-10 08:15:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

The Automated Weather Observing System at the Jim Hogg County Airport dipped to 27 degrees just after daybreak on the 26th, with duration of freezing temperatures lasting from just before 2 AM until around 815 AM.  Based on proxy observations in Brooks and Zapata County, interpolation suggests all of Jim Hogg County had several hours of freezing temperatures on either side of daybreak.

Frost/Freeze in KENEDY, TEXAS
From 26-DEC-10 04:00:00 to 26-DEC-10 08:30:00 (CST-6)
Property damage: 0.00K
Reported by: COOP Observer

The cooperative observer at the Lebh Shomea House of Prayer east of Sarita recorded a minimum temperature of 27 degrees on December 26th; the location near Armstrong fell farther, reaching 25 degrees.  The Lower Rio Grande Valley National Wildlife Refuge Remote Automated Weather Station near San Manuel, just 10 miles southwest of the Kenedy/Hidalgo County line, also reached 25 degrees, with freezing temperatures for just under 5 hours.

Frost/Freeze in STARR, TEXAS
From 26-DEC-10 04:00:00 to 26-DEC-10 08:30:00 (CST-6)
Property damage: 0.00K
Reported by: RAWS

The Remote Automated Weather Station at Falcon Lake, in Starr County just south of the Zapata County line, fell to 26 degrees with more than four hours of freezing temperatures. The cooperative observer in Rio Grande City recorded 31 degrees for the calendar day low temperature, while the cooperative observer at Falcon Dam (not too far from the Falcon Lake RAWS) fell just shy of freezing, reaching 33 degrees.  Proxy observations from Hidalgo, Jim Hogg, and Brooks County suggest most of Starr fell to or below 32 degrees for several hours either side of daybreak.