Climate

The South Central Texas Region lies in three climatic divisions in Texas: the Edwards Plateau division; the South Central division; and the Upper Coast division. The climate of the region is classified as humid subtropical. Summers are usually hot and humid, while winters are often mild and dry. The hot weather is rather persistent from late May through September, accompanied by prevailing southeasterly winds. There is little change in the day-to-day summer weather except for the occasional thunderstorm, which produces much of the annual precipitation within the region. The cool season, beginning about the first of November and extending through March, is typically the driest season of the year as well. Winters are typically short and mild, with most of the precipitation falling as drizzle or light rain. Any accumulation of snow is a rare occurrence. Polar air masses, which penetrate the region in winter, bring northerly winds and sharp drops in temperature for short periods of time.

In the coastal region, the climate is dominated by proximity to the Gulf of Mexico and characterized by prevailing southeasterly winds. During the long humid summers, high daytime temperatures, which are common in inland areas, are moderated in coastal areas by the Gulf breeze.

Mean annual precipitation in the region ranges from a high of 38 inches per year in DeWitt County, in the eastern part of the region, to a low of 23 inches per year in the Nueces River Basin, in the west (Table 2-2). There is a general trend of decreasing precipitation from the eastern portions of the region to western portions. There is also a general trend of increasing precipitation from inland areas to coastal areas.

Although mean annual temperatures are basically uniform throughout the region, there are some marked seasonal variations, which lead to widely varied values for annual net lake surface evaporation. The values for annual net lake surface evaporation range from a high of 65 inches per year, for the portion of Dimmit County located in the Rio Grande River Basin, to a low of 24 inches per year, for the portion of DeWitt County that lies in the Lavaca River Basin (Table 2-2).

Table 2-2 - Climatological Data for South Central Texas Region

Precipitation

Mean Daily Minimum (° F)

Mean Daily Maximum (° F)

Annual Net Lake Surf Evap (in)

River Basin	Mean Ann (in)	Wettest Month(s)	Driest Month(s)	Mean Annual	Jan	July	Jan	July
Rio Grande	25	Sept	Mar	74	48	74	71	96	65
Nueces	23	May, Sept.	Mar	71	40	72	65	98	45
San Antonio	30	Sept.	Mar, Dec	70	41	74	64	96	31
Guadalupe	32	May, Sept	Mar	79	37	71	60	95	37
Colorado	34	May, Sept	Jan	68	39	74	60	96	35
Lavaca	38	May, Sept	Mar, July	70	41	72	65	98	24
Lavaca-Guadalupe	37	Sept.	Mar, July	70	44	76	64	94	25
San Antonio-Nueces	33	Sept.	Mar	71	43	73	65	96	30
Colorado-Lavaca	41	Sept.	Mar, July	70	43	78	91	91	20

Source: TWDB, "Continuing Water Resources Planning and Development for Texas," May 1977.

The South Central Texas Region is subject to the threat of hurricanes each year from mid-June to the end of October, and in those parts of the region along and near the coastline, the hazard of hurricane tides is prevalent. Although hurricane winds and tornadoes spawned by hurricanes cause extensive damage and occasional loss of life, surveys of hurricanes reaching the Texas Coast indicate that storm tides cause by far the greatest destruction and largest number of deaths. Elsewhere in the inland areas of the region, the greatest concern with regard to hurricanes is the damage that results from winds and flooding. Records dating back to 1871 show that, on average, a tropical storm or hurricane has affected the region once every 3 years.