Part Two: “Tornadoes DO Happen in Virginia’s Mountains”
Today is the statewide tornado drill which will be conducted at 9:45 a.m. I hope many of you are participating!
I am privileged to present part two of Kathryn Prociv’s guest post which is based upon her master’s thesis research at Virginia Tech. Remember what the title says…
“Investigating my first objective, I found storms that occur predominantly in the mountains are significantly different than those storms that occur in the Piedmont region. One reason could be the obvious difference in terrain between the two regions, with the complex topography of the mountains compared to the lower and flatter elevations of the Piedmont. Fredericksburg, located in the Piedmont region of Virginia, is located in a relatively flat area with close proximity to the Atlantic Ocean and the Chesapeake Bay. The prolific warmth and moisture more characteristic of the Piedmont compared to the cooler more stable environment of the mountains could influence storm evolution. The Piedmont region, including Fredericksburg, also experiences the redevelopment of storm systems caused by leeside convergence as the systems re-intensify on the eastern sides of the Appalachian and Blue Ridge Mountains. Regression analysis also revealed that storms in the Piedmont are more “predictable” than those in the mountains; this is likely due to the absence of the complex terrain.
My second objective revealed three main themes: lower elevations augment storm strength, higher elevations and westward-facing slopes inhibit storm strength. These conclusions make sense: higher elevations decrease storm intensity, as do westward facing slopes which are inherently cooler and more stable due to the blocking influence of warm and moist air from the Atlantic. Additionally, westward facing slopes would be those that cause a decrease in the conservation of angular momentum, or the opposite of vorticity stretching. Here the storm updrafts would be compressed as the storm traversed up the westward slopes, causing a decrease in rotation speed over time.
The third and perhaps most interesting objective included examining instances of vorticity stretching; the principle of vorticity stretching states that as a storm encounters an abrupt/steep decrease in elevation, this could enhance the storm by stretching the updraft thereby causing rotation to tighten and increase. Eight out of the fourteen storms exhibited visual evidence of vorticity stretching along the storm track. It is interesting to note that original hypotheses stated that easterly-facing slopes would be important because storms moving in a west to east direction would encounter vorticity stretching on the eastern and leeside slopes.
Southeast moving supercell across Botetourt and Bedford counties. Notice the apparent intensification on the eastern lee sides of the ridgelines.
However, by visually examining the storms some showed intensification due to vorticity stretching on northwesterly facing slopes. This was apparent in the northeast moving storms; as storms tracked northeast, they often paralleled ridges and depending on small deviations in the storm track the supercell encountered a decrease in elevation on the southeast slopes, but also on the northwest slopes.
The Montgomery County, VA storm from April 28th, 2011. Notice the track of the storm parallel to the ridgeline, and also apparent intensification and vorticity stretching on the northwestern facing slopes.
This finding represents a clear dichotomy between northeast moving and southeast moving storms; southeast moving storms, by moving perpendicular to and crossing the mountain ridges, always encountered vorticity stretching on the southeasterly slopes. Northeast moving storms, in contrast, could encounter vorticity stretching on either southeasterly or northwesterly slopes depending on the relationship between storm track and movement along parallel mountain ridges.
The evidence found through research proves that tornadoes do happen in mountains, and in some cases the complex terrain can actually enhance tornadogenesis. This evidence debunks the myth that mountains protect against destructive tornadoes. I believe that one reason why there is the myth that tornadoes can’t happen in mountains is due to the lack of knowledge and research on the topic. Knowledge is power. I plan to continue research on this topic in the coming years, with hope that others jump in and add to the bank of knowledge. Knowledge will improve understanding, and experience will generate awareness and combat complacency. Unfortunately, sometimes we must learn through experience: the residents of Pulaski and Glade Springs know that tornadoes happen in Virginia’s Mountains, and will never forget it. Knowledge and experience passed down through generations, supplemented with documented research of tornadoes in the Virginia Mountains must continue if we’re to flip that popular saying from, “Tornadoes don’t happen in Virginia’s Mountains!” to “Tornadoes DO happen in Virginia’s Mountains!”
Thanks Kathryn! Keep the research going on this timely topic!!
