The climate  of the southwestern United States, including southern New Mexico and far western Texas, is considered semi-arid or desert, since compared to most areas of the country, this region has abundant sunshine and little rainfall. El Paso, Texas is called the "Sun City" for good reason; according to solar radiation statistics it is the fifth sunniest city in the United States, receiving 83% of the possible sunshine. And in most years, from October through the middle of June, the threat of heavy rainfall is low because during this period the prevailing wind flow across the southwestern United States is from west to east. As a result, most of the moisture from the Pacific Ocean is blocked by the mountains to the west while moisture from the Gulf of Mexico is literally blown far to the east. This circulation pattern keeps the weather mostly sunny, warm and dry from early autumn through early summer.

However as the deserts heat up during the late spring and summer, important changes take place in the pressure field and  wind pattern which can rapidly increase the threat of heavy rainfall. The reasons for these changes can be understood if we remember that in most circumstances, warmer air is lighter than relatively cooler air and thus exerts less pressure on the earths surface. So during the late spring and into the summer, the hot temperatures across the deserts of the southwestern United States cause the development of a broad area of low pressure over southern California, Arizona, western New Mexico and Nevada. Further east where the air is cooler, high pressure will often extend across the Mississippi Valley into eastern Texas.

This pressure field generates southeasterly and southerly winds in the lower and sometimes middle layers of the atmosphere, which transport moisture from the Gulf of Mexico and Gulf of California into southern New Mexico and far western Texas. The increase in moisture, combined with the usual warm temperatures, produces an unstable air mass favorable for thunderstorms which may generate flash flooding, hail, damaging winds and even an isolated tornado.

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