Background

Geography
Texas, bounded on the southeast by the Gulf of Mexico and on the west by arid and semiarid regions characteristic of the southwestern United States, is a land of climatic and geographic diversity. The terrain is equally diverse, ranging from the featureless coastal plains along the Gulf Coast to the spectacular features of West Texas, which include the Guadalupe Mountains, the canyons of Big Bend, and the Caprock escarpment of the High Plains (fig. 1).

Sources of Moisture
The principal sources of moisture for Texas are the Gulf of Mexico and, to a lesser extent, the eastern Pacific Ocean (fig. 1). Moisture from the Gulf of Mexico is carried into the State by low-level southerly and southeasterly winds. Moisture from the eastern Pacific is carried into the State from the southwest by tropical continental air masses. In addition to the oceans, important moisture sources include local and upwind land masses, as well as lakes and reservoirs, from which moisture evaporates to the atmosphere. Typically as a moisture-laden ocean air mass moves inland, it is combined with moisture that has been recycled through the land-vegetation-air interface.

Precipitation
The average annual precipitation differs little from north to south but greatly from west to east. Average annual precipitation in El Paso is less than 8 inches. More than 770 miles to the east, average annual precipitation in the lower Sabine River valley of extreme eastern Texas exceeds 56 inches. The precipitation varies seasonally as well as geographically. Although spring and fall usually are the wettest seasons, intense rainfall can occur in late summer during the tropical storm and hurricane season. For most of the State, however, the average precipitation during summer is only slightly greater than that during the winter.

Spring is the wettest season in most of Texas, with April and May often the wettest months. Spring thunderstorms generally are caused by successive weak frontal systems that move through the State. These cool air masses from the north meet warm, moist air masses from the Gulf of Mexico. The warm, moist air is less dense than the cooler air and thus overtops the cool air mass. The moist air then condenses and forms thunderstorms along the line of contact between the two systems. Summer usually is drier than spring in most of the State, but a secondary peak of rainfall often occurs in September and October. Tropical cyclones (hurricanes and tropical storms) originate in weather systems that begin in the Caribbean Sea or the Gulf of Mexico in late summer or early fall. Rainfall amounts that result from tropical cyclones can differ greatly because of the different conditions in each storm. Remnants of some hurricanes reaching landfall have produced large depths of rainfall over wide areas of the State.

Balcones Escarpment
The hills and associated elevation increases along the north side of the Balcones escarpment in central Texas assist in the uplifting of air masses and the formation of storms. Many large thunderstorms form along the escarpment, where they can stall and produce extreme precipitation depths during a few hours or few days. Many of the largest storms in the State have occurred in this area. Some of those occurring along the escarpment during about the past 80 years are indicated in figure 2.

Largest Storms
Many Texas storms represent some of the largest storms in the world. Figure 3 shows the largest precipitation depths in the world, for durations ranging from 1 minute to 24 months. Also shown are some of the largest known precipitation depths in Texas. As indicated, many of the largest storms with durations from about 1 hour to 48 hours have occurred in Texas. Examples of these storms include a 1921 storm in Thrall that produced 32 inches of rainfall in 12 hours and a 1935 storm in D'Hanis that produced 22 inches of rain in 2 hours and 45 minutes.

Flooding
Flooding from large storms has affected Texas throughout its history, causing many deaths and much economic loss and hardship. Floods occur regularly in Texas, and destructive floods occur somewhere in the State every year. Many of these floods are destructive because they often occur in areas where extreme flooding had not occurred for many years. These floods often are perceived as unexpected or even unprecedented because their peak water-surface elevations (stages) can greatly exceed those of past floods. For example, a recent report by the U.S. Geological Survey identified, for sites throughout the State, maximum known peak stream discharges that greatly exceed peak discharges for 100-year floods. The maximum known discharges typically range from about 1.5 to about 3 times greater than 100-year discharges in the western and eastern parts of the State, but documented discharges for some sites along the Balcones escarpment have been as much as 4 or 5 times greater than 100-year peak discharges. Such peaks usually are devastating because structures and development typically exist outside the 100-year floodplain but often are within floodplains for maximum floods.