“Colorado Magic”- How Terrain Influences Supercell and Tornado Development East of the Rockies
If you’ve never heard of the phrase, “Colorado Magic,” it simply means that Colorado has shown to be quite the hot spot in storm and tornado development. It all starts with the Rocky Mountains. Based on a 2017 article by The Weather Channel, from 1950 to 2016 the county ranking number one in the most tornadoes occurring over this 67-year period is Weld County, Colorado (see the first link below the article for more details on how this statistic was obtained).
This is where the “magic” begins. The reason this location including Eastern Colorado is so prone to such frequent storm development is based on the mid to upper level winds and how these winds cross over the mountains. As westerly winds cross over the Rockies the air sinks and warms due to subsidence where air descends, compresses, and warms in the process. This leads to the development of lee cyclogenesis in which the lack of cooler air at lower levels combined with the descending upper level-air causes the leeward side of the mountain to become less stable than its surroundings. This sets the stage for storm development as a cyclonic circulation develops downwind of the mountain range.
The formation of such storms begins with warm, moist, unstable air (typically from flow off the Gulf of Mexico) being forced upslope along higher terrain, which is a common process towards the Western Plains where orographic lift is especially enhanced. There exists a specific region of higher terrain, oriented West to East across central and eastern Colorado known as the Palmer Divide. It stretches approximately 80 miles from the front range of the Rockies into central Colorado towards Limon, Colorado and is known for its significant synoptic and mesoscale impacts.
A phenomenon also known to have effects on storm development in this area is known as the Denver Convergence Vorticity Zone (DCVZ). This is yet another topographically induced mesoscale feature, oriented North to South and categorized by convergent winds in northeastern Colorado due to the interaction of southerly, low-level flow with the Palmer Divide. These convergent winds often help initiate storm development especially during the convective season as convergent boundaries such as these often create natural zones of vorticity, or localized areas of spin to create rotation.
If you remember that the basic ingredients for a thunderstorm are lift, instability, and moisture (while adding in the right amount of shear for supercells specifically), it’s no doubt that this area provides all the right ingredients to create such explosive development, and this merely scratches the surface as to what this terrain can do!
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@2019 Weather Forecaster Christine Gregory