A deep trough of mid level low pressure, extending from the Upper Midwest southwest into Texas brought a period of cold rain and snow to South Texas on the 4th.  When the clouds cleared at the end of an unusually cold afternoon on the 4th, Canadian high pressure overspread the region overnight on the 4th and into the 5th, bringing the coldest surface temperatures in nearly 5 years to all of Deep South Texas.
Morning lows on the 5th dropped to the mid 20s to around 30 in all counties, with freezing or lower temperatures lasting up to 6 hours in typically colder pockets.  The freeze was frosty for many critical agricultural areas of the Rio Grande Valley; temperatures hovering just below 32 combined with calm winds limited notable damage to protected crops and plants.

Frost/Freeze in STARR, TEXAS
From 04-DEC-09 22:30:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

Clearing skies and a chilly air mass (temperatures in the upper 40s) during the afternoon on December 4th set the stage for a widespread freeze overnight on the 4th into the morning of the 5th as winds calmed, allowing temperatures to fall steadily from the 30s into the 20s in Starr County.  By daybreak on the 5th, lows fell to the mid or upper 20s, including 24 at Falcon Lake near the Zapata County line, and 26 at the Rio Grande City Cooperative Observer location.  Estimated and measured duration of 32 degree or lower temperatures was 7 to 10 hours.
The freeze was accompanied by several hours of frost, which combined with calm winds minimized notable damage to temperature sensitive crops.  Unprotected plants at individual residences may have received some damage, but costs were unknown.

Frost/Freeze in KENEDY, TEXAS
From 04-DEC-09 22:30:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

Cloudy, drizzly, cold conditions (temperatures in the upper 30s to lower 40s) during the afternoon on December 4th set the stage for a widespread freeze overnight on the 4th into the morning of the 5th as skies cleared, winds calmed, and temperatures fell steadily from the 30s into the 20s in Kenedy County.  By daybreak on the 5th, lows fell to the mid or upper 20s, including 26 at the Sarita Cooperative site just east of Federal Highway 77, and 25 at Armstrong (southern half of the county).
Estimated duration of 32 degree or lower temperatures was 7 to 10 hours.

Frost/Freeze in COASTAL CAMERON, TEXAS
From 05-DEC-09 05:30:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: COOP Observer

After a cloudy, cold afternoon on December 4th (temperatures in the upper 30s to lower 40s), skies rapidly cleared during the evening as winds gradually calmed, setting the stage for a freeze during the morning hours of the 5th.  Temperatures were estimated likely fell to, or below, freezing around or a little after 530 AM on the 5th toward the immediate coast.  By daybreak, the measured low temperatures on the west side of the Laguna Madre ranged from 30 degrees in Port Isabel and Bayview to 32 degrees at the Laguna Atascosa/Lower Rio Grande Valley National Wildlife Refuge near the Laguna Madre southeast of Arroyo City. Slightly lower temperatures may have occurred in normally colder locations away from the bay.  In the Town of South Padre Island, warming influence from the Bay and Gulf kept temperatures from dropping below the mid 30s.
The freeze was accompanied by frost, which combined with calm winds minimized notable damage to temperature sensitive crops.  Unprotected plants at individual residences may have received some damage, but costs were unknown.

Frost/Freeze in CAMERON, TEXAS
From 05-DEC-09 00:00:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: ASOS

A drizzly, chilly afternoon on December 4th featured temperatures falling into the upper 30s to around 40 before nightfall.  Clearing skies and calming winds right after dark set the stage for temperatures to steadily fall to or below freezing overnight.  By daybreak on the 5th, readings ranged from the upper 20s to around 30 area wide.  A daily record low was tied in Harlingen (31 degrees) and broken at the Valley International Airport (29 degrees).
Duration of freezing or lower temperatures varied by location.  Rural locations in southwestern and northwestern Cameron County likely had 8 hours or more of freezing temperatures; Valley International Airport, just east of populated Harlingen, had more than 6 hours of freezing temperatures.  Farther south and east, Brownsville International Airport had an hour of freezing temperatures; Los Fresnos had only two hours of freezing temperatures.
The freeze was accompanied by several hours of frost, and, with calm winds, there were no reports of notable crop damage in Cameron County.  Residential damage to temperature-sensitive plants likely occurred, but the extent was unknown.

Frost/Freeze in WILLACY, TEXAS
From 05-DEC-09 00:00:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: COOP Observer

After a cloudy, cold afternoon on December 4th (temperatures in the upper 30s to lower 40s), skies rapidly cleared during the early evening as winds calmed, setting the stage for a widespread freeze overnight on the 4th into the morning of the 5th.  Temperatures were estimated to fall to or below freezing around or a little midnight in rural locations north west of Federal Highway 77 corridor, probably a bit later at night east of the highway toward the coast. By daybreak, low temperatures ranged from the upper 20s to around 30 degrees.  Measured low temperatures were 28 at Raymondville (Cooperative), and 26 degrees 4 miles southeast of Linn (in Hidalgo County just west of the Willacy County line).
Duration of the freeze west of Federal Highway 77 was estimated at 5 to 8 hours; duration east of Federal Highway 77 was estimated at 2 to 5 hours.
The freeze was accompanied by several hours of frost, which combined with calm winds minimized notable damage to temperature sensitive crops.  Unprotected plants at individual residences may have received some damage, but costs were unknown.

Frost/Freeze in HIDALGO, TEXAS
From 04-DEC-09 22:00:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

After a cloudy, cold afternoon on December 4th (temperatures in the lower 40s), skies rapidly cleared during the early evening as winds calmed, setting the stage for a widespread freeze overnight on the 4th into the morning of the 5th.  Temperatures fell to or below freezing before midnight in rural locations north and west of the urban Federal Highway 83 corridor, as well as in rural pockets along the border.  By daybreak, low temperatures ranged from the mid and upper 20s north of Edinburg to 30 to 32 in urban areas, including Weslaco, McAllen, and Pharr.  Aforementioned rural pockets near the Rio Grande fell to the mid 20s, as well.
Duration of the freeze across northern Hidalgo County, as well as rural pockets near the river, was measured and estimated to range from 7 to 10 hours.
In urban areas, away from known heat islands, 32 degree or lower temperatures lasted from 1 to 3 hours, except 3 to 5 hours in the urban/rural interface in such areas as Edinburg and Alton.
The freeze was accompanied by several hours of frost, which combined with calm winds minimized notable damage to temperature sensitive crops.  Unprotected plants at individual residences may have received some damage, but costs were unknown.

Frost/Freeze in BROOKS, TEXAS
From 04-DEC-09 22:30:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

Daytime temperatures fell into the upper 30s to lower 40s during the afternoon of a chilly, cloudy December 4th, setting the stage for a widespread freeze overnight on the 4th into the morning of the 5th as skies cleared, winds calmed, and temperatures fell steadily from the 30s into the 20s in Brooks County.  By daybreak, lows fell to the mid or upper 20s, including 25 at the Falfurrias Cooperative station.   Estimated duration of freezing or below temperatures was 7 to 10 hours.

Frost/Freeze in ZAPATA, TEXAS
From 04-DEC-09 22:30:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

Clearing skies and a chilly air mass (temperatures in the mid to upper 40s) during the afternoon on December 4th, set the stage for a widespread freeze overnight on the 4th into the morning of the 5th as winds calmed, allowing temperatures to fall steadily from the 30s into the 20s in Zapata County.  By daybreak on the 5th, lows fell to the mid or upper 20s, including 24 at Zapata County Airport in Medina, 27 at Falcon Dam in the southwest corner of the county, and 27 near Escobas.  Estimated and measured duration of 32 degree or lower temperatures was 7 to 10 hours.

Frost/Freeze in JIM HOGG, TEXAS
From 04-DEC-09 22:30:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: AWOS

Daytime temperatures held in the lower to mid 40s even as skies cleared during the late afternoon of December 4th, setting the stage for a widespread freeze overnight on the 4th into the morning of the 5th as clear skies and calm winds allowed temperatures to fall steadily from the 30s into the 20s in Jim Hogg County.  By daybreak on the 5th, lows fell to the mid or upper 20s, including 26 at Jim Hogg County Airport northwest of Hebbronville and 24 at the Hebbronville Cooperative observing site.   Estimated and measured duration of 32 degree or lower temperatures was 7 to 10 hours.

Frost/Freeze in COASTAL WILLACY, TEXAS
From 05-DEC-09 04:00:00 to 05-DEC-09 08:00:00 (CST-6)
Property damage: 0.00K
Reported by: COOP Observer

After a cloudy, cold afternoon on December 4th (temperatures in the upper 30s to lower 40s), skies rapidly cleared during the early evening as winds calmed, setting the stage for a freeze during the morning hours of the 5th.  Temperatures were estimated to fall to or below freezing around or a little after 4 AM on the 5th toward the immediate coast.  By daybreak, the measured low temperature in Port Mansfield was 30 degrees. Slightly lower temperatures may have occurred at locations surrounding Port Mansfield; temperatures likely remained above freezing on Laguna Madre and the uninhabited South Padre Island to the east.
The freeze was accompanied by frost, which combined with calm winds minimized notable damage to temperature sensitive crops.  Unprotected plants at individual residences may have received some damage, but costs were unknown.