FLASH FLOODS ACROSS THE BORDERLAND Although the climate of of southwestern New Mexico and far western Texas is considered to be semiarid or even desert, during the summer the region frequently experiences showers and thunderstorms with attendant heavy rainfall. This is due to the seasonal changes in the circulation across the southwestern United States. Usually in late June or early July, the prevailing dry westerly flow retreats northward as warm surface temperatures induce a broad area of low pressure at the surface across southern California, western Arizona and northwestern Mexico. This feature, often referred to as the "desert heat low " pulls moisture northward from the Gulf of California or Gulf of Mexico into the Southwest.

During most summers, strong thunderstorms can cause torrential rains over far western Texas and southern New Mexico. This supercell produced heavy rains as it moved across Santa Teresa N.M. in the summer of 2000.  ( Photographed by John Fausett)

Flash floods involve a rapid rise of water, usually in 6 hours or less, and the majority are produced by either a slow moving thunderstorm or multiple thunderstorms moving across the same area. In most instances thunderstorms which produce heavy rains occur within a moist unstable environment with warm moist air in the lower levels of the atmosphere and relatively cool air aloft. Surface dewpoints are typically in the 50s's or even 60's during flash flood events, indicating  humid conditions. Another factor favorable for flash floods is weak wind flow aloft. When cloud-layer wind speeds are comparatively light, thunderstorms move more slowly allowing them to dump large amounts of rain over a given location. While floods are possible any time of the year over the borderland, almost all flash floods occur from May through early October with the majority of events reported from the middle of June through the middle of September. Southwestern New Mexico and far western Texas flash floods are also most likely to occur   between  3 PM and midnight with morning heavy rain events rather  infrequent. ( continued on page 4 )                                                                                            3

During the summer months, a "desert heat low " develops across the southwestern United States, pulling moisture northward into the region and contributing to strong thunderstorm formation.