Wednesday, May 22, 2013

Mountains and Tornadoes

Climatologically, the highest frequency of significant (F2 or greater) tornadoes in the world occurs the southern Great Plains during the spring.  That's why the region is appropriately referred to as Tornado Alley and why Norman, Oklahoma has become the nexus for severe weather research, forecasting, and education in the United States (with a tip of the hat to many talented people who helped to make it so).  

Source: Storm Prediction Center
We don't usually think of Tornadoes as a mountain meteorology problem, but it turns out that the severe storm environment of the Great Plains is partly a consequence of topography and land-sea contrasts.  During the spring, the confluence of dry, downsloped air from the deserts and high terrain of the southwest US and northern Mexico (airstream SP below) with cooler, moister air originating over the Gulf of Mexico (airstream M) frequently leads to the development of the dryline.  The dry, downsloped air often continues to move eastward over the moist Gulf air, leading to the development of what meteorologists call an elevated mixed layer.  The confluence of these airstreams and the formation of the dryline is related to the development of lee troughing downstream of the Rockies.  In addition, the location and movement of the dryline can be influenced by the gradual slope of the Great Plains.  

Source: Carlson et al. (1983)
This elevated mixed layer is sometimes quite warm and dry and forms a cap or lid that can prevent the formation of thunderstorms.  However, in areas where the cap is weaker, or where a surface boundary or other feature can lead to strong, organized vertical motion, violent thunderstorms can form. 

Many of these characteristics can be seen in the regional analysis of 1000-mb dewpoint (contours below), wind, and radar/satellite imagery for ~2100 UTC 20 May, minutes after the tornado moved through Moore Oklahoma.  Note the confluence of dry air from the southwest with moist air from the Gulf of Mexico, which generated a pronounced dryline running from central Oklahoma to northern Mexico.  

In addition, the sounding from Norman, Oklahoma at 1700 UTC shows how dry, warm air from the southwest has moved over the cooler moister Gulf air, resulting in a temperature inversion just above 850 mb and an elevated mixed layer further aloft.  Imagine if you will in the analysis above that the dry air over and near the Texas Panhandle is moving over the Gulf air to the east to form the elevated mixed layer.  

The strength of the low-level jet and its impact on vertical wind shear and northward moisture transport is also influenced by the Rocky Mountains and the slope of the High Plains.  

So, the fingers of mountain meteorology are quite long, with influences that extend even into "flatland" regions.  


  1. Interesting. I had always thought of the "classic cold front", ie - warm moist air being lifted and displaced by cold dense air. However, the sounding shows a dry adiabatic layer above what could pass as a cool maritime air mass. Apparently, all you need is some lifting mechanism to get the maritime air to penetrate deep enough into the dry adiabatic layer and it will explode upward.

  2. I think the mountaious terrain of Mexico as a whole is a big factor. Not only does it help to generate dry, well-mixed air masses which overrun the moist air; but the terrain configuration also channels much of the low-level easterly tropical trade wind air mass from the Atlantic, the Caribbean, and Gulf of Mexico northward into the Plains region (even though the Gulf tends to get all the credit for being the moisture source). Also, I have sometimes wondered why Tornado Alley does not have a more pronouced severe weather peak in the fall (around October) although I am sure they are glad for that. The water sources for the moist air mass are actually warmer then, although sun angle is much lower and jet stream patterns are a bit different. Anyone have any other ideas on this?

  3. Sorry for the double comment... but I think that one could add the terrain of Central America to the list also as a potentially significant factor. A good chunk of the easterlies over the northern Caribbean (from about Honduras northward) seem to get channeled northwestward across the basically flat Yucatan Peninsula, western Gulf of Mexico and into south Texas this time of year. I think this particular track might be responsible for some of the juiciest low-level air that can contribute to the severe storm outbreaks, and why some of the highest dew point values can often be found over TX/OK not too far east of the dryline.

  4. Jim,

    Here is reference for recent paper regarding mountain tornadoes in northern Arizona.


    Weather and Forecasting, April 2013, Vol. 28, No. 2 : pp. 408-417

    A Comparison of Wind Speed and Forest Damage Associated with Tornadoes in Northern Arizona
    David O. Blanchard
    (doi: 10.1175/WAF-D-12-00046.1)

    1. Thanks Bob. A good followup to this post would be to do some sort of review of tornadoes in area of complex terrain, including the article you note above.