Weather Journal: Tornadoes do occur in the mountains, but not often

There is a large amount of falsehood in that idea, but a seed of truth.

It is obvious from our history, or the long-term experience of anyone who has lived here and paid attention to such things, that there are far fewer tornadoes in Western Virginia’s mountainous areas than in areas to the west and south stretching from the Deep South to the Ohio Valley, and also fewer than flatter areas to the east that include Southside Virginia and much of the central and eastern part of the state.

We saw that again just last week, with an EF-3 tornado that stayed on the ground for 17 miles in Appomattox County, a rare and extreme event for Virginia by any measure.

But sometimes, this observation gets stretched to the point that some will say tornadoes are impossible in the mountains, and should not be considered at all something to be concerned about from the Blue Ridge westward.

There are some folks in Glade Spring and Pulaski who can quickly dispel the notion that tornadoes can’t cause misery and hardship amid the mountains. Those communities experienced much destruction in April 2011 tornadoes.

The Roanoke Valley has not been immune. Three people were killed in an 1896 tornado in Salem, and five people were injured along a path of a 1974 EF-2 tornado that started in Salem, crossed western and northern Roanoke and ended near Bonsack. Weaker tornadoes have caused some damage as recently as 2003 near Hollins and 2008 in south Roanoke.

And on May 2, 1929, there were the Rye Cove tornado in far Southwest Virginia’s Scott County that killed 12 schoolchildren and a teacher, and on the same day a destructive tornado that cut a path at least 17 miles long through rugged Alleghany and Bath counties.

In other parts of the nation, tornadoes have been observed above 10,000 feet in elevation near Yellowstone National Park, in the Sierra Nevada of California and the Rockies of Colorado. Tornadoes have stayed on the ground for scores of miles on multiple occasions moving through the Ozarks of Arkansas and Missouri, climbing 1,500-foot ridges and diving into valleys of similar depth with no break in the damage path.

So the historic truth is that tornadoes, even significant ones, sometimes can and do occur in mountainous areas, but as it relates to Western Virginia, they do occur less frequently in the mountains than in regions to the east, west and south.

This can be boiled down to two principles.

The first is that terrain does not physically disrupt a tornado if the atmospheric conditions on both sides of a mountain or ridge are supportive of tornadoes.

In fact, there is some research that suggests that rotating updrafts with supercell thunderstorms and perhaps tornadoes actually intensifies going downhill on such a ridgeline, as the column of air lengthens and tightens, causing it to spin faster, much like a figure skater pulling in her arms.

The second is that the orientation of the Appalachian ridgelines from southwest to northeast often tends to trap cooler, more stable air to the east and southeast, pushed southwestward by high pressure over New England and southeast Canada.

This cooler and more stable air, when it is present, dampens low-level atmospheric instability, which can keep storms moving in from the west and southwest from becoming severe, and in some cases can even weaken them or dissipate them entirely.

This was strongly in evidence during last week’s severe storms outbreak, as a pool of cold air remained wedged at the surface into the Roanoke Valley, and storms that had briefly strengthened to near severe levels over the New River Valley weakened slightly moving into Roanoke.

The problem, though, is that somewhere there is an eastern boundary to this cool air, and it was very sharp in the Feb. 24 outbreak, with temperatures rising from the mid-40s to the low 70s in just a few miles.

Appomattox County suffered extremely when a storm moved over this sharp temperature gradient, gaining instability and ingesting rotating air that develops along such a boundary. The same feature that protected the Roanoke area was a key culprit factor in our neighbors’ misery.

So, the healthy approach to tornado risk in Southwest Virginia, supported by history and meteorology, is that they do sometimes occur. Most are brief and weak, but a few stay on the ground for many miles and become destructive.

It is a good idea to know basic tornado safety — mainly, going to an interior room on the lowest floor with no windows — when a tornado warning is issued, or even when you just instinctively feel that the storm outside doesn’t seem normal.

But we should not pretend this is tornado alley. More often than not, our region’s tornado response will be lending a helping hand to people who have been hit much harder to our east, south or west.

The day may come again, sooner or later, when we need their help to deal with an unusual but not unprecedented level of tornado damage scattered in our own hills and valleys.

Weather Journal runs on Wednesdays.