Tornadoes don’t happen in mountains. Or do they? Debunking the myth.

“Tornadoes don’t happen in mountains.”

Have you ever heard this statement? It is a common sentiment shared by many.  It is true that tornadoes are less common at higher elevations and mountainous terrain, but this does not mean that these landscapes are immune.

This article discusses the influence of mountainous terrain on tornadogenesis and distribution, discusses geographical patterns of tornadoes for four major mountain ranges of the contiguous United States, and provides examples of confirmed tornadoes that have occurred at high elevations.

Why are tornadoes less common over mountainous terrain?

Take a look at the maps below.  The first map shows all tornado reports courtesy of the Storm Prediction Center from 1950-2011, and the second map shows elevation (in meters) across the continental United States. Notice the gaps in tornado coverage coincide with the large mountain ranges.  The Appalachian Mountains are clearly evident, especially over West Virginia, and the Rocky Mountains provide a nice sharp western edge to the tornado reports found across the Great Plains.

So why are tornadoes less common at higher elevations?  The basic answer is simple: higher elevations typically have cooler, more stable air.

A ripe tornado environment features warm and humid “unstable” air to provide energy for the explosive thunderstorms capable of producing thunderstorms.  The golden rule when talking about higher elevations is, “the higher you go, the colder it gets.”  This is true simply due to environmental lapse rates, or the change in temperature with height.

All things being equal, for every thousand feet gained in elevation there is roughly a 3°F drop in temperature.  Colder air at the surface is inherently more stable, and thus colder high elevation landscapes are more stable.

Geographical patterns of tornado reports and the major mountain ranges

The Appalachian Mountains

The Appalachian Mountains are the oldest mountains in the continental United States at approximately 480 million years.  Due to their age, they are also the lowest elevation mountains from years of erosion, with average elevations of 3,000-6,000ft.

The Appalachian Mountains are directly in the path of storm systems that move from west to east.  When these storm systems hit the mountainous terrain, they often weaken or break down then redevelop to the east on the lee-side of the mountain range.  Looking at the map above, the gap in tornado coverage is obvious, especially through West Virginia.

Despite the lack of tornado reports, there have been a number of tornadoes in this region, including a mile-wide tornado that struck Roanoke, Virginia in April 1974, an EF3 tornado at 2,080 feet over Glade Spring, Virginia in April 2011, and a weak tornado that struck Mt. Rogers National Park at nearly 4,000 feet also in April 2011. (note: If you’re interested in learning more about Appalachian Mountain tornadoes, you can review my master’s research which investigated multiple tornado events across a small portion of the southern Appalachians in southwestern Virginia.)

The Rocky Mountains

The Rocky Mountains of the United States are likely the most recognized mountains the United States.  An impressive range, it stretches more than 2,000 miles from southern Alaska through New Mexico with elevations exceeding 14,000 feet.  These mountains are much younger than the Appalachians, at 70 million years old.  When comparing the terrain of the Rocky Mountains with spatial tornado coverage some interesting geographical patterns emerge.

Notice the sharp gradient of tornado coverage just to the east of the mountains, quite literally outlining the edge of the terrain.  The lee-side of the Rocky Mountains is prime territory for convective development and incipient tornadogenesis during storm season. The terrain effects of the eastern Rocky Mountains can often override less than desirable atmospheric conditions conducive for tornado development.

Despite the impressive elevations, there have been some notable tornadoes within the Rocky Mountains, including a F4 tornado at 10,000 feet over the Teton-Yellowstone landscape of northwest Wyoming (the orange tornado track just below the title-box) and a recent tornado in July 2012 near Mt. Evans, Colorado at nearly 12,000 feet.  These two tornado events are discussed in further detail below.

The Cascade Mountains

The Cascade Range is an impressive western mountain range that stretches from British Columbia, Canada to northern California and features elevations upwards of 14,000 feet (the popular Mt. Rainier towers at 14,411 feet).

In comparison to the tornado coverage surrounding the other mountain ranges, the Cascade Range features the lowest spatial coverage, with only a handful of F3/EFs during 1950-2011, and no violent F4/EF4s or F5/EF5s.

The relative lack of tornado coverage is most likely due to the regional climate.  Situated at the higher latitudes of Oregon and Washington State, this region is characterized by a cool, moist maritime climate.  This maritime air from the Pacific Ocean creates an inherently stable environment unfavorable for frequent tornado development.

The Sierra Nevada Mountains

Surprisingly enough, the highest peaks in the contiguous United States can be found in eastern California as part of the Sierra Nevada Mountains.  The tallest, Mt. Whitney at 14,495 feet, exceeds the highest peaks of both the Rocky Mountains and Cascade Range.

Like the Cascade Range to the north, the maritime influence of the Pacific Ocean has an influence on environmental stability resulting in less frequent tornado events.  Another climatic aspect of this region includes the hot and dry easterly desert winds that blow in from the Mojave Desert.  Hot and dry winds are not very conducive for tornado development, though land spouts have been noted nearby at times with robust convection.  (Dust devils and firenados in this region, however, are another story!)

One fascinating geographical pattern to point out: can you find the Central Valley?  Situated in between the coastal mountains and the Sierra Nevadas, the Central Valley in central California is a large and flat open valley characterized by a warm and moist Mediterranean climate.  As you can see on the map, tornadoes occur fairly frequently in this region, most often in winter or spring than at other times of the year.

Case studies

Pulaski, Virginia – April 8, 2011

 Two tornadoes from a single supercell thunderstorm occurred at almost 2,000 feet near the town of Pulaski, Virginia on the evening of April 8^th, 2011. One tornado was rated an EF1 and a second tornado rated an EF2.

What was interesting about these two tornadoes was that they were separated by a mountain known as Draper Mountain.  The potential topographic influence of this ridgeline on those tornadoes was examined in my thesis.  In short, the rapid change in elevation due to the storm crossing the ridge may have had a profound effect on the rotational strength of the rotating updraft (mesocyclone).

When the supercell mesocyclone encountered the sudden decrease in elevation on the lee-side of Draper Mountain, that may have caused the updraft to stretch and tighten and thus rotate faster; a phenomenon known as vorticity stretching.

Glade Spring, Virginia and Mt. Rogers — April 27, 2011

Like the Pulaski tornado, these tornadoes occurred deep within the southern Appalachian Mountains.  They represented two separate tornadoes that happened on the same night of April 27^th-28^th, as part of the major southeast outbreak of April 25^th-28^th.

The Glade Spring tornado (orange track) was an EF3 that occurred around 2,000 feet, and caused devastating damage in the town of Glade Spring.  That same night an EF1 tornado (bottom right-hand corner of the map) ripped through a campground near Mt. Rogers at nearly 4,000 feet.

Grand Teton-Yellowstone, Wyoming –  July 21, 1987

During a rare and incredible meteorological event, a F4 tornado occurred at 10,000 feet in northwest Wyoming on July 21^st, 1987.

The tornado traveled through the Teton-Wilderness, Yellowstone National Park, and crossed the Continental Divide.  It is still the strongest tornado ever recorded for the state of Wyoming, a state on the edge of the Plains that sees numerous tornadoes many years.

Mt. Evans, Colorado – July 28, 2012

Last summer, when a tornado formed near Mt. Evans and Roger’s Peak within the Rocky Mountains of Colorado, many were surprised. But, as outlined above, mountain tornadoes do happen!

This was one for the books. The National Weather Service in Boulder confirmed the tornado traversed across elevations of 11,800 feet, which is right around the elevation the record holder was observed at in Sequoia National Park, California.  Weather photographer Chris Kirby was on scene to capture incredible images of the event.  His awesome pictures are below.

Conclusion

This article discussed the relationship between high elevation mountain environments and tornado climatology.  The first portion discussed atmospheric stability as the main contributing factor for the relative scarcity of tornado events within mountainous environments.

The next part analyzed the spatial distribution of tornado coverage in respect to the four major mountain ranges in the contiguous U.S.  Lastly, case studies of high altitude tornadoes were reviewed.

While this article acknowledges that tornadoes are less common at higher elevations and mountainous terrain, it also debunks the myth that “tornadoes don’t happen in mountains.”

References…

Scott, Doug (Ed). (2006). Guide to Mountains. Buffalo, NY: Firefly Books, Ltd.

Teton-Yellowstone Tornado. In Wikipedia. Retrieved March 11, 2013, from http://en.wikipedia.org/wiki/Teton%E2%80%93Yellowstone_tornado

Tornado History Project. Retrieved March 13, 2013, from http://www.tornadohistoryproject.com/