Sunday, July 13, 2014

Why Are Cumulus Clouds and Thunderstorms More Common over the Mountains?

It is very common in Utah in the summer for the forecast to feature either a greater chance of thunderstorms in the mountains or a chance of thunderstorms mainly in the mountains.  Even if storms don't develop, one can typically find more cumulus clouds over the mountains than over the valleys and basins.

Satellite images for this afternoon show this nicely.  Note the presence of shallow cumulus clouds over most of the higher mountains and plateaus of Utah, with fewer clouds over the valleys and basins.  In a few places over the high terrain, the clouds are deeper and producing anvils.  This is especially apparent in northeast Utah where some deeper clouds are found over the Uinta Mountains, whereas it is cloud free over the Uinta Basin to the south.  The storms over the eastern Uinta Mountains might be producing some showers and maybe even a bolt or two of lightning.

This tendency for convective (cumulus) clouds and storms to form over the mountains occurs because the air is more strongly heated during the day over the slopes and higher terrain of mountains and plateaus than the atmosphere at comparable elevations more removed from the mountains.  This leads to an thermally driven upslope flow with rising air and convergence that triggers cumulus clouds and thunderstorms over the mountains.

Source: Houze (2012)
This effect is most pronounced during periods when the large-scale winds are light, as is the case today.  It's also most pronounced during the initial triggering of clouds and thunderstorms.  Once storms develop, they can sometimes move off the mountains or generate outflow boundaries that propagate away from the mountains and trigger storms over the surrounding lowlands.

There are other thermally driven flows that can contribute to cloud and thunderstorm formation.  One example is the Great Salt Lake breeze, which is basically a sea breeze that forms because the air over the Great Salt Lake is heated less strongly than the surrounding land surface.  Convergence at the leading edge of the Great Salt Lake breeze, sometimes called a lake-breeze front, can contribute to cloud and storm formation.  In fact, it contributed to the development of the storm that spawned the August 11, 1999 tornado over Salt Lake City.

Of course, one needs to be cautious about over applying these generalizations.  For example, there are situations, especially in big terrain (e.g., Himalayas and Andes) where the upslope flow initiates storms not over the highest terrain but over the lower mountain slopes and foothills.

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