Winter Weather Awareness Information

While hard to believe now, given the relatively warm and humid conditions that have prevailed across the Lower Rio Grande Valley into early November, cooler, and perhaps colder, weather is inevitably on the way south as the Thanksgiving and Christmas holiday seasons approach. As we begin turning cooler, this is a good time to review potential impacts from winter weather hazards that affect the area, mainly in December and January, but sometimes extending into February as well.

Unlike most of Texas, where cold fronts sweep sub freezing air masses on gusty northerly winds and can produce an occasional diet of low wind chills, frozen ground, and wintry precipitation - ranging from ice pellets and frozen rain in south, central, and East Texas to powdery snow in West Texas and the Panhandle, "winter" in the Valley is fleeting and relatively gentle. However, residents shouldn’t be fooled; the near-tropical climate of Deep South Texas can quickly make what would be a more marginal event farther north into a critical hazard for people and plants alike here.

For all your winter weather preparedness needs, click here. Information in "press release" form from the Texas Governor's Division of Emergency Management can be found here. Want to learn more about winter weather, and meteorology in general? Check out our online weather school! The following paragraphs will describe, in order of likelihood and impact, the winter weather hazards which can affect the Lower Rio Grande Valley and Deep South Texas.

When temperatures fall to 32°F (0°C) or lower for a couple hours or more, native plants and crops can be stressed or killed, depending on how low the temperatures fall, the length of the cold, and even the affect of wind on crop protection. Minor freezes occur somewhere in Deep South Texas and the Lower Rio Grande Valley each year, but are typically infrequent. In general, the average first freeze in the Lower Rio Grande Valley occurs around the New Year, and the average last freeze occurs in late January. Freezes can occur as early as November, and as late as March.

Freezes, continued
On more rare occasions, a hard freeze occurs, defined as at least two hours of temperatures below 28°F (-2.5°C) over a relatively wide area (i.e., half of a county, a large city, etc). During a hard freeze, unprotected cold sensitive plants and crops will be damaged, and some killed outright. The most critical of hard freezes are killing freezes, which feature a long duration of temperatures well below freezing for many hours (generally 10 hours or more), often combined with a continuing northerly wind that does not allow crops to retain necessary surface moisture to help insulate them from the chill.

Notable killing freezes across the Lower Rio Grande Valley, those which wiped out a large number of crops such as citrus and sugar cane, each occurred on or around Christmas. Three notable freezes, in 1981, 1983, and in 1989, brought plummeting temperatures into the upper teens to around 20 in the critical agricultural, with the duration of subfreezing temperatures for most of the overnight hours. The 1983 and 1989 freezes featured consecutive nights of frigid temperatures, increasing the amount of damage to crops and tender vegetation.

Wind Chill
Wind Chill is simply a measure of how cold people and animals feel when outside, based on the rate of heat loss from exposed skin caused by the combination of wind and cold temperatures. The wind chill index calculates the wind speed at an average human "face height", roughly five feet, and incorporates the affect of heat loss from the body to its surroundings during cold and generally breezy to windy days. The near-tropical climate of the Lower Rio Grande Valley likely results in a lower threshold of wind chill impact than a location in temperate regions such as North Texas. The National Weather Service in Brownsville is in the process of determining a best fit of critical wind chill thresholds to the acclimation of residents; these thresholds will be available on this website, most likely by early December. For full details on wind chill, including detailed descriptions and safety measures, click here.

NWS Wind Chill Chart, circa 2001

Ice, or freezing rain/drizzle, occurs when snow melts into rain well above the earth, then refreezes on contact with sub freezing surfaces on or just above the ground. Ice accretion is extremely dangerous on untreated highways, and has resulted in hundreds of injuries and deaths, nationwide, through the years. Ice is a very rare occurrence in the Lower Rio Grande Valley, given the lack of persistent frigid air masses needed to keep the ground cold enough for ice to accrete when warm air overruns what remains of any cold surface air. Ice Pellets, or sleet, form when melted snow well above the earth refreezes in a thicker sub freezing layer extending from the ground up a few thousand feet. Sleet, also a dangerous winter weather precipitation hazard, should not be confused with hail, which forms in strong to severe thunderstorms during the spring and summer. Ice pellets are only a few millimeters in diameter, and look like very tiny white, fuzzy balls.

Snow is the rarest of all winter precipitation types in Deep South Texas and the Lower Rio Grande Valley. In a nutshell, the reason for the lack of observed snow is twofold: Our latitude, and our elevation. It is extremely difficult for a sufficiently cold air mass, extending through the entire depth of the atmosphere, to plunge to the edge of the tropics. While the sinking of cold air, which is "heaviest", to the ground is common, so are pockets of above freezing temperatures in layers somewhat above the surface of the atmosphere, for instance, between 3,000 and 8,000 feet. For these reasons, snow is considered virtually unlikely. However, the atmosphere once in awhile can surprise, and even produce a miracle. On Christmas Eve and early Christmas Morning, 2004, a blanket of snow fell across all of Deep South Texas and the Lower Rio Grande Valley, with a general swath of 2 to 4 inches from Starr and Jim Hogg Counties eastward to the coast!

February, 1899: The Great Arctic Outbreak

Perhaps no winter period was more wicked in Texas than that of February 11th through 13th, 1899. The month itself was exceptionally cold, ranking 2nd all time in Brownsville (since records began in 1878). Statewide, incredible arctic cold was felt by Texas standards, lasting a full three days. On February 12th, the lowest temperature ever recorded, -23°F, was observed at Tulia, in the southern Panhandle. Closer to home, record temperatures in the single numbers and lower teens were recorded at Brownsville (12°F on the 13th), Corpus Christi (11°F on the 12th), and Galveston (8°F on the 12th)! Prior to the deep freeze, areas of snow and wind affected portions of central and southeast Texas, as an incredible plunge of artic air helped to "squeeze" moisture out of the atmosphere, beginning in central Texas during the morning and shifting to the southeast Texas coast during the evening. While Deep South Texas and the Lower Rio Grande Valley were not impacted by significant wintry precipitation, a long duration hard freeze, including a period of more than 30 hours where the temperature did not exceed 30°F, occurred. Such a freeze today would extinguish all but the heartiest cold weather plants and crops.

For a detailed article of the Great Arctic Outbreak of February 1899, click here.

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