Every spring/summer time, lee troughing occurs at the Sierra Madre Oriental foothills in Mexico, causing showers and thunderstorms.  Typically, these storms are short lived due to the fact that they develop when diurnal heating is the strongest, and then begin to die once the sun sets.  Therefore, any chance that these storms reach the RGV are very minimal, but can occur every so often.  The low pressure that these troughs produce interact with high pressure in the Gulf of Mexico.  This is why it is always so windy during the spring and early summer months in the Rio Grande Valley.

Figure 1. Thunderstorms developing at the foothills of the Sierra Madre Oriental from lee troughing (5/28/2011).

What is Lee Troughing?

In order for any type of precipitation to form anywhere in the world, air needs to be lifted, and cooled to form clouds and eventually precipitation.  Air flow near the surface typically flows from west to east in the Northern Hemisphere.  As air flows up a mountain from the west, it causes the air to cool and form clouds near the top.  Eventually, the air aloft saturates enough for precipitation to fall.  So, air being lifted up a mountain acts like an updraft for precipitation. This process is called orographic lift.  Lee side troughs are caused by the stretching of a column of air as it descends on the lee side of a mountain.  Positive vorticity increases on the lee side and upper-level divergence is a product of this vorticity.  The downslope flow causes adiabatic warming, which then causes temperatures and heights to increase.  Though the sinking air should be stable, the air flowing down the mountain is still cooler than the air it is replacing.  In addition, the friction over the mountains in the mid-levels of the atmosphere causes a short-wave trough, depicted in the diagram below.

Figure 2. Example of lee troughing occuring east of the Rockie Mountains. The same effect takes place in the mountains of Mexico. Source: clem.mscd.edu


About Brian

University of Oklahoma graduate with a degree in Meteorology. Follow me on Twitter: @WeatherInformer
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