Borderland flash floods often develop within certain meteorological or weather patterns. One particular pattern favorable for flash floods is often referred to as the "Back Door Frontal Pattern" . At the surface, this scenario is characterized by a modified Canadian air mass which spreads southward into the southern high plains. As the Canadian air pushes southward, a weak associated cold front will move west or southwest through New Mexico and western Texas, sometimes becoming stationary along the western mountains or along the Mexican border. To the west, a well-defined heat low is also in place, acting to pull moisture into the deserts.
East or southeast winds behind the cold front may also transport moist air into the region, creating an unstable air mass. Consequently lifting along the cold front combined with the moist air being forced over the higher terrain will generate strong thunderstorms with heavy rains. On August 19, 1978, over 10 inches of rain fell at White Sands Missile Range near a weak back door front.
A favorable weather pattern for flash floods includes a weak cold front moving slowly into the borderland from the east or northeast. Stippled areas indicate where surface dewpoints exceed 50 degrees F.
Another pattern associated with borderland flash floods also includes a summer thermal trough or low pressure area over the southwest while a broad area of high pressure extends from the lower Mississippi Valley into the plains states. In addition, in the middle and upper levels of the atmosphere a slow moving trough or low pressure system is located over southern California or through western Arizona and northwestern Mexico. This pattern induces a deep southerly flow with abundant tropical moisture from the south streaming into southern New Mexico and far western Texas. Thunderstorms can become intense with heavy rain likely when a weak disturbances embedded in the flow move into the borderland from Mexico. In fact, the threat of flash flooding may last for several consecutive days when this pattern persists over the southwest. ( next page)
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The presence of a heat low at the surface over Arizona or southern California ( top) and a trough or low pressure system in the upper levels of the atmosphere ( below) can induce a deep southerly flow with very moist tropical air streaming into the southwestern United States.