The Southwest Monsoon
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  The summer monsoon in southern New Mexico and far western Texas generally starts in late June or early July and ends in mid September. There is wide variation in rainfall with monsoon thunderstorm activity from year to year and place to place, depending on meteorological factors and geographical effects.  Thus looking at "official" rainfall totals at El Paso airport may be totally unrepresentative of what happened at other locations.

  For example, in 2002 El Paso Airport measured
1.3 inches of rain for the entire month of July. In contrast, on July 16, 2002, over 4 inches of rain fell at Radium Springs, New Mexico in a single evening. Similarly in June 2000, rainfall at El Paso Airport totaled 2.45 inches while Columbus,		 image of a weather map showing the southwest monsoon flow
 
   The summer monsoon begins when a desert "heat 
   low" develops over the southwestern United States.
   As winds become more southerly, moisture from 
   the Gulf of Mexico or Gulf of California streams
   into the region.

 N.M. received a record 8.66 inches with flooding. Thus while you may not have any downpours at your location, don't assume it won't happen somewhere else in the vicinity.

 During the monsoon season thunderstorms can develop anywhere but they often form or become enhanced near meteorological boundaries such as cold fronts or outflows from previous storms. In addition, disturbances or low pressure systems in the upper atmosphere can act to both increase and intensify thunderstorm activity.  Thunderstorm initiation is especially favored over the mountains, including the Sacramentos, the Gilas, and the Franklins.

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 The drainage patterns imposed by the steep slopes and arroyos of the mountains, combined with any urban development, can lead to flash flooding. An example of this was on August 2, 2002, when thunderstorms developed  south of El Paso and then moved north and strengthened over the southern end of the Franklin Mountains. These storms dumped 1.5 inches of rain in less than an hour over portions of eastern El Paso. This proved sufficient  to create havoc in the city with one young boy drowning, numerous cars washed off the road, and Interstate 10 closed for several hours due to high waters.


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