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September-October 2016

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Class Aptent Taciti Sociosqu ad LitoraThe Weather and Climate of Texas: A Big State With Big Extremes

Overheard in West Texas: “We received eight inches of rain last year … but the other 364 days were pretty darned dry.”

Wild, bizarre, and rapidly changing weather is typical across Texas in most seasons. Texans are not surprised to see heavy snow falling over the Texas panhandle, dust storms blanketing Southwest Texas, severe thunderstorms and tornadoes marauding across North Central Texas, or 90–100°F sun-drenched warmth baking the Rio Grande River in South Texas. They are not even surprised when all of this weather occurs at the same time on the same day, linked to an intense springtime low-pressure storm system with associated warm and cold fronts and a dry line!

Texas is the second largest state in the United States, with a total land area of 261,914 square miles. The opposite corners of the state are about 800 miles apart. Texas contains the largest acreage of farming land and rangeland in the United States, and is second only to Alaska in the amount of the state covered by inland water. It is also the second most populous state and contains the largest amount of urban land of any state.

The state has variations in geography fitting with its size and diverse character. Perhaps the most distinctive region of the state is the portion west of the Pecos River, which is known as the Trans-Pecos or Far West Texas. Far West Texas, a part of the Chihuahuan Desert, has a Basin and Range topography similar to much of the southwestern United States. The tallest mountains are found in the Guadalupe Range. Guadalupe Peak is 8,749 feet above sea level and is the highest point in Texas.

The climate of Far West Texas is far more similar to New Mexico than to the rest of Texas. Winters are cold and generally dry, except for rain and snow that fall mostly in the higher elevations. Summer is the rainy season, as moisture from both the Gulf of Mexico and the Pacific Ocean's Gulf of California contribute to afternoon thunderstorms. In this area of the state, annual precipitation depends on elevation more than location.

Other parts of Texas have neither the topographic relief nor the wide variations of climate of Far West Texas. The terrain changes steadily and continuously from one end of the state to the other, interrupted only by such features as the Panhandle's Caprock Escarpment and the Balcones Escarpment along the southern and eastern margin of the Edwards Plateau. The terrain descends from northwest to southeast, draining to the Gulf of Mexico by a series of mainly parallel rivers. The climate changes are even more gradual than the changes in elevation. Annual mean temperatures are coolest to the north and warmest to the south. Annual mean precipitation is heaviest in the east and lightest in the west. Precipitation changes are more substantial than the temperature changes, as the near-desert in the west gradually gives way to annual accumulations close to 60 inches along the Texas/Louisiana border.

Climate Overview

The climate of Texas is determined by geographical features beyond the state borders as noted in Figure 2. To the southeast, the Gulf of Mexico provides a source of warm, moist air throughout the year. During the summer, the entire state comes under the influence of the Gulf, as southeasterly and southerly winds prevail. Except for the Trans-Pecos, most of the water that falls as precipitation in Texas has its origins in the Gulf of Mexico.

While temperatures across the state vary in a fairly uniform manner, there are situations in which anomalous warm to hot and very dry air affects the state. The most obvious involves the dryline, a frontal-like boundary that separates air of desert origin from warm and moist Gulf of Mexico air. This boundary is often a breeding ground for supercell thunderstorms.

In strong, springtime wind patterns, the dryline may advance across much of the state. In lighter wind settings, the dryline may oscillate around a more or less fixed position. In addition to density differences due to temperature and moisture, the dryline position is linked to terrain.

Farther east, the hot, dry air from the nearby Mexican Plateau is carried aloft over the warm and moist air from the Gulf of Mexico, forming a capping inversion. Richard Berler, a television meteorologist at KGNS-TV in Laredo, Texas, notes that on rare occasions, roughly every 10 years or so, and mostly in the late winter to spring, this air is able to mix downward from its three- to five-thousand-foot source altitude, causing temperatures across South Texas to soar and dew points to tumble. On a shorter return period, mixing is not as complete and warming arrives, but there is not a drop in dew points.

A Year of Texas Climate

Winter

Winter in Texas is a season of extremes, both in space and time. Average January low temperatures range from 20°F along the northern border of the Panhandle to 50°F in the Lower Rio Grande Valley. Rapid temperature changes occur in response to a weather phenomenon known as a “Blue Norther,” a cold front that moves from north to south, propelled by cold air that has been channeled along the east side of the Rocky Mountains. These cold fronts are among the strongest in the United States; 20°F temperature drops in one hour are not uncommon. The temperature drops are accompanied by winds from the north of 20–30 miles per hour. Low stratus clouds behind the front often linger for a day or two before skies clear.

Figure 1.  Two examples of south Texas heat linked to transport of Mexican Plateau air northeastward. First map (Feb. 20, 1986) is the same day as the Figure 3 map.

Figure 1.

Two examples of south Texas heat linked to transport of Mexican Plateau air northeastward. First map (Feb. 20, 1986) is the same day as the Figure 3 map.

Ten to 20 Blue Northers occur during a typical year. Perhaps the most dramatic was February 6, 1933, when the High Plains city of Clarendon reported a high temperature of 69°F and a low temperature of −6°F, for a one-day temperature range of 75°F. Two days later, Seminole tied the all-time minimum temperature record for Texas at −23°F. More recently, Rio Grande City reached 102°F on February 9, 1960, and three days later a snowstorm hit central and south Texas with eight to 10 inches of snow.

Figure 2.  The geography and moisture source regions for Texas. Map also shows many of the locations mentioned in the article.

Figure 2.

The geography and moisture source regions for Texas. Map also shows many of the locations mentioned in the article.

The coldest day in weather records in Texas was February 12, 1899. On that day, over half of the state reached temperatures below 0°F. Another massive cold wave took place in late January 1949, when temperatures dropped below zero from Dallas to San Antonio.

Warm temperatures are the norm in the Lower Rio Grande Valley, even in wintertime, and the area supports cold-sensitive crops such as grapefruit. When the occasional cold wave hits the Valley, agricultural damage can be severe.

Figure 3.  U.S. Daily weather map for 6:00 a.m. C.S.T. on Feb. 20, 1986 showing the arrival of a “Blue Norther” across Oklahoma and the Texas Panhandle. Later in the day, as the front continued to advance southward, temperatures across south Texas, ahead of the front, topped 100°F.

Figure 3.

U.S. Daily weather map for 6:00 a.m. C.S.T. on Feb. 20, 1986 showing the arrival of a “Blue Norther” across Oklahoma and the Texas Panhandle. Later in the day, as the front continued to advance southward, temperatures across south Texas, ahead of the front, topped 100°F.

Monthly average precipitation ranges from less than half an inch in parts of West Texas to over four inches in parts of East Texas. Significant amounts of snow are confined almost entirely to the mountainous Trans-Pecos region and the High Plains. Measurable snow falls south of the High Plains but usually melts almost as fast as it falls. Average annual snowfall is one foot in Amarillo, six inches in Abilene, three inches in Dallas, and one inch in Austin. The record accumulation for one calendar year is 65 inches in Romero in 1923.

From central Texas southward, most winters bring no accumulation of snowfall. Freak snowstorms occur only once every few decades, and no corner of the state is immune. Memorable freak snowstorms include the February 1895 snowstorm that brought 20 inches of snow to Southeast Texas from Houston to Orange, the December 1929 snowstorm that brought over 24 inches of snow to Central Texas, the April 1983 snowstorm whose 16.5 inches in El Paso set the city's all-time record, and the January 1985 snowstorm that dumped 13.5 inches on San Antonio. Brownsville and the lower Rio Grande Valley go long periods of time without any snowfall. However, on December 24–25, 2004, the Valley recorded 3.5 inches of snow—the most there in recorded history.

While snowstorms are the primary winter threat in the High Plains, the other parts of northern and central Texas are subject to occasional ice storms. Like snow, ice can occur almost anywhere in the state, but ice storms are most common across north central and northeastern Texas.

Despite the threat of the rare snow or ice storm, winters in most parts of Texas are mild and pleasant. In most years during the past few decades, the temperature in Brownsville has not dropped below 32°F. In Dallas, the growing season is 235 days long, and in Amarillo, it is 205 days. In less than half the state, the latest freeze typically occurs after April 1.

Spring

Springtime is the most pleasant season of the year in most parts of Texas. However, spring is also the peak season for severe weather here. Significant severe weather events are often triggered by an upper-level disturbance approaching Texas from the Four Corners area of the United States. Ahead of the upper-level trough, low-level southerly winds from the Gulf of Mexico advance moisture northward across Texas. To the west, winds descend from the Continental Divide. In West Texas, a dryline forms between the moist air from the Gulf of Mexico and the drier air from the higher elevations to the west. The dryline, which typically runs from north to south, serves predominantly as a windshift line and a moisture discontinuity rather than a strong temperature contrast zone.

Figure 4.  U.S. lightning deaths for the period 2006–2015. Texas is ranked 2nd nationally in deaths, but 27th nationally when deaths are normalized for population.

Figure 4.

U.S. lightning deaths for the period 2006–2015. Texas is ranked 2nd nationally in deaths, but 27th nationally when deaths are normalized for population.

Atmospheric lift along the dryline can cause thunderstorm development. Often a line of thunderstorms will form along the dryline in mid-afternoon, and by sunset coalesce into a squall line that advances eastward across Texas. This type of evolution is most common in May, the wettest month of the year in many parts of Texas. Tornadoes are most prevalent in May as well. Earlier in the season, tornadoes are most common in North Texas, between Dallas and Wichita Falls, but by May, the peak area for tornadoes moves westward to the Texas High Plains. Tornadoes can occur in any part of Texas, but they are relatively uncommon in the western Trans-Pecos and extreme south Texas. The most devastating tornado in Texas history occurred on May 11, 1953. On that day a tornado struck Waco, killing 114 and injuring 597. Over 1500 homes and other buildings were destroyed. The Waco tornado tied the May 18, 1902, tornado that struck Goliad for the most deadly tornado in Texas history.

Figure 5.  Almost all the weather you'd want at an instant. This map of Texas features a 71-degree temperature variation from the Texas Panhandle, to the lower Rio Grande Valley, snow and blowing dust in the Panhandle and strong winds statewide. The cold front would likely be referred to as a “Blue Norther.”

Figure 5.

Almost all the weather you'd want at an instant. This map of Texas features a 71-degree temperature variation from the Texas Panhandle, to the lower Rio Grande Valley, snow and blowing dust in the Panhandle and strong winds statewide. The cold front would likely be referred to as a “Blue Norther.”

In addition to tornadoes, weather threats can include hail, heavy rainfall, and strong straight-line thunderstorm winds. These thunderstorm “downdraft” winds can create much more widespread damage than tornadoes, since their damage pattern is larger.

Hail often causes the greatest amount of monetary damage. Hail forms when supercooled water in the middle and upper part of a thunderstorm deposits on other hydrometeors high up in thunderstorm clouds. Depending upon the strength and persistency of the updraft, hail can grow to very large size. In urban areas, hail dents cars and damages roofs; in agricultural areas hail destroys crops. On August 24, 1979, a hailstorm in the High Plains destroyed 150,000 acres of crops and caused damage to 550,000 additional acres. The largest hailstone in Texas was recorded on May 10, 1996 near Big Spring (Howard County). There, 5-inch diameter hailstones caused 48 injuries.

Figure 6.  The march of precipitation across Texas. Far East Texas receives, on average, more than five times the rainfall of far West Texas.

Figure 6.

The march of precipitation across Texas. Far East Texas receives, on average, more than five times the rainfall of far West Texas.

Summer

Unlike the other three seasons, during which Texas receives frequent incursions of polar air, summertime finds Texas stuck primarily in a humid subtropical air mass, with only infrequent, weak cold front incursions. Typical high temperatures are in the 90s everywhere, except in the higher elevations of western Texas and along the immediate coastline, where temperatures in the 80s are more common. The normally high humidity keeps low temperatures warm also; lows range from the 70s in the eastern part of the state to the 50s in the high-altitude, arid regions of the Trans-Pecos. The warmest temperatures in summertime are typically found near Laredo, Wichita Falls, and Midland. Readings over 100°F occur frequently there, challenging riders in Wichita Falls' annual Hotter'N Hell 100-mile bicycle race. The record high temperature for Texas is 120°F, set in 1936 in Seymour and tied in 1984. During unusually warm summers, the temperature occasionally exceeds 110°F. Afternoon thunderstorms occur in the state, but the movement of the jet stream to the north means that severe thunderstorms become a rarity.

Autumn

The turning of the leaves that heralds the arrival of fall in the northern United States takes place near the end of fall in Texas. In October or November, one or two outbreaks of severe weather commonly occur each year, but the primary late-summer through early-fall weather phenomenon is the tropical cyclone. The most deadly weather disaster in United States history was the Galveston hurricane of September 8–9, 1900. With only a few hours' warning, this Category 4 hurricane produced a storm surge that inundated most of the city and killed an estimated 8,000 people. Rather than abandoning the city, after the storm the residents built a seawall and elevated the main part of town by several feet.

Some 108 years later, Hurricane Ike made landfall in nearly the same location. Though its winds only qualified the storm as a strong Category 2 on the Saffir-Simpson scale, its large size and interaction with underwater topography produced one of the largest storm surges ever recorded in the United States. The barrier island just north of Galveston was virtually wiped clean.

The occasional tropical disturbances, tropical storms, and hurricanes combine to make September the wettest month on average in much of East and South Texas. Parts of West Texas also experience their heaviest rainfall on average during September, due to the influence of the remnants/middle- and upper-level moisture associated with eastern Pacific tropical cyclones making landfall in western Mexico. This rainfall is not as reliable as the spring rains, because it tends to come in one or two major events. Because of this, fall can also see prolonged dry weather. Some of the driest months in Texas history have occurred in October and November.

Figure 7.  Lightning density map showing cloud-to-ground lightning flashes annually per square mile. When matched with the Texas rainfall graph, shows dramatic west-to-east moisture gradient across the state.

Figure 7.

Lightning density map showing cloud-to-ground lightning flashes annually per square mile. When matched with the Texas rainfall graph, shows dramatic west-to-east moisture gradient across the state.

Droughts

From the early days of Texas history, recorded by Spaniards who were exploring the Southwest, drought has been a recurring problem. A drought in central Texas dried up the San Gabriel River in the early 1750s, forcing the abandonment of a settlement of missionaries and Native Americans. Stephen F. Austin's first colonists were also hit by drought. Their initial corn crop was snuffed out in 1822, turning the once ambitious farmers into desperate hunters. The most severe drought in Texas during the past century began in 1950 and continued until March 1957. Other major droughts included the periods 1886–1887, 1893, 1917–1919, 1933–1935, 1962–1966, 2005–2006, 2008–2009, and 2010–2015. Because much of the agriculture and all of the forage for cattle is non-irrigated, even just a few consecutive months of substantially below-normal precipitation can cause an emergency situation. The 1930s drought was only serious in northern portions of the state, which form the southern end of the infamous Dust Bowl region. Most dust storms occur during the first few months of the year, when strong winds from the west or north are common in the Panhandle and parts of West Texas.

Floods

Floods can occur in any season in Texas, but they are most common in the warmer months. Causes of floods include stalled fronts, thunderstorms that form and/or move repeatedly over the same location, and slow-moving tropical cyclones after landfall. Rainfall rates and storm totals can often border on excessive.

The most significant flood in Texas history was an area flood that took place during September 8–10, 1921. The torrential rains from this event rival the most extreme floods anywhere in the world. Thrall, just northeast of Austin, measured 32 inches of rain in a 12-hour period and 38.4 inches in 24 hours. The costliest flood in Texas history was caused by Tropical Storm Allison, which made landfall on June 4, 2001, and remained in the state for several days. On June 8–9, torrential rains fell directly over Houston at rates of up to four inches per hour. In one evening, many parts of central Houston received over a foot of rain, and storm totals just east of the city approached 35 inches. The resulting floods caused 22 deaths, destroyed or damaged over 48,000 homes, and caused $5 billion in damage.

Flash floods affect a much smaller area but can cause significantly greater loss of life because of their suddenness. These often highly localized, heavy rainfall events can turn normally lazy rivers and creeks into rampaging torrents in a matter of minutes. Of note, on October 17, 1998, up to 30 inches of rain were documented in the area between San Antonio and San Marcos, leading to 26 drownings and causing property damage of about $750 million.

Some flood events are notable not for their damage or death toll, but for their impressive precipitation totals. The United States record for precipitation in a 24-hour period is held by the remnants of tropical storm Claudette, which dumped 43 inches of rain in one day near Alvin, south of Houston.

Lightning

Given the myriad weather situations in which thunderstorms and lightning can occur in Texas, and the size of the state, it shouldn't be surprising that Texas ranks second nationally in lightning deaths (2006–2015), with an average of 20 deaths per year. However, when normalized for population, Texas drops to 27th.

Perhaps more dramatic is the newly created lightning density map (Vaisala). Shown here, the map for Texas shows cloud-to-ground lightning flashes across the Lone Star State. The gradient of lightning density (see Figure 7) approximates the gradient for precipitation (see Figure 6).

Wind

There is no doubt about it: Texas is a windy state. The windiest spot in the state with regular weather observations is Guadalupe Pass, a major east–west gap in the Trans-Pecos Mountains. Sustained winds over 40 mph are common during springtime. Many areas in West Texas and on the mid-Texas coast are being utilized for wind power generation due to their frequent moderate to strong winds. In fact, Texas produces the most wind energy of any state in the nation. During the summer months, due to land areas heating more rapidly than nearby ocean surfaces, the coastal areas of Texas experience a sea breeze, most notably during the afternoon hours.

Big Weather for a Big State

Texas is a big state with “big” weather and “big” weather extremes. From incredible statewide variability during a single day to seasonal and year-to-year extremes, Texas has it all. Throw in excessive rainfall, excessive dryness, excessive heat, excessive wind, and occasional dust storms, lightning, and tropical storms, and there is no wanting for big weather in the Lone Star State.

TROY M. KIMMEL, JR. is a broadcast meteorologist and senior lecturer in the Department of Geography and the Environment, University of Texas, Austin, Texas.

DR. JOHN NIELSEN-GAMMON is the Texas State Climatologist and a Regents Professor in the Department of Atmospheric Sciences at Texas A&M University, College Station, Texas.

BOB ROSE is chief meteorologist at the Lower Colorado River Authority (LCRA), Austin, Texas.

H. MICHAEL MOGIL is a certified consulting meteorologist and science writer based in Naples, Florida.       

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