To those not deeply engulfed in meteorology, the term “supercell” can seem like a hype-inducing buzzword akin to “bomb cyclone” or “polar vortex.”
It can seem like an esoteric piece of weather geekdom.
You will never see a “supercell warning” issued, but you will hear meteorologists talk about supercells, especially when they point to velocity scan radar images with clashing colors next to each other or reflectivity echoes shaped with odd “hooks” protruding out the backside, giving the appearance of so many multi-colored scorpions crawling across the screen.
This is not something relegated to some distant part of the country far away from the Southwest and Southside Virginia coverage area of Cardinal News. We have supercells, too, though usually fewer and less often than many of the states west of us. A week ago Monday, on April 14, a series of supercells slipped out of West Virginia along the Interstate 64 corridor in Virginia, bringing copious hail and ominous rotating lowerings.
· Weather outlook: Supercell thunderstorms aren’t the biggest concern for our region this week. But a wagging front will keep a chance of showers and maybe a few thunderstorms around much of this week and weekend, though rainfall amounts look likely to be less than we still need after several mostly dry weeks. Temperatures will continue to tilt warmer than normal as high pressure reigns over the Eastern U.S. and the jet stream dips more in the central and western U.S.
It may seem there is a bit of a “we know when we see one” for meteorologists and storm chasers in regard to supercells, but there is a standard expert definition, offered by the National Severe Storms Laboratory:
A supercell is a long-lived (greater than 1 hour) and highly organized storm feeding off an updraft (a rising current of air) that is tilted and rotating. This rotating updraft — as large as 10 miles in diameter and up to 50,000 feet tall — can be present as much as 20 to 60 minutes before a tornado forms. Scientists call this rotation a mesocyclone when it is detected by Doppler radar. The tornado is a very small extension of this larger rotation. Most large and violent tornadoes come from supercells.
To be clear, not all supercell thunderstorms spawn tornadoes, but as the definition above notes, most of the worst tornadoes come from supercells.
Most of the largest hail, the baseball-, softball-, and grapefruit-sized stuff, also comes from supercell storms.
Earlier this month, we covered what makes a storm severe, which is based on storm output like size of hail (1 inch or larger in diameter) and wind speed (58 mph or greater). These effects can come from a variety of storm structures. But supercells are more likely to stay severe for longer, to produce higher-end severe weather, and to spawn tornadoes.
Supercell thunderstorms develop when relatively strong winds shift direction from the surface to aloft — and the lift, moisture, and instability are sufficient for thunderstorms to form as moist updrafts rise into these shifting winds.
The most idealized supercell environment is a “veering profile” in which winds slightly turn in direction with each successive layer — east at the surface, southeast a little higher, south a little higher than that, then southwest and west, something like that. This develops a helix or spiraling vertical column of air. “Helicity,” or the ability of updrafts to rotate in a spiral motion, is a parameter that is modeled and projected for the potential of supercell thunderstorms and tornadoes to develop.
But any juxtaposition of wind layers moving in different directions or different speeds can create spin that can help supercells develop. Often this spin is horizontal along an atmospheric boundary, and can then get tilted upward by a storm’s updrafts.
The rotating updraft is important in organizing a storm’s inflow and outflow. Updrafts and downdrafts that can conflict with each other are moved into positions where they can continue to feed and vent the thunderstorm, like a machine.
Winds blowing mostly in one direction at similar speeds from the surface to aloft are more likely to result in storm lines, whereas relatively weak winds result in less organized storm clusters or “pulse” storms that quickly rise and fall. These other storm types can produce severe impacts with hail and wind, even occasionally tornadoes, but they’re unlikely to produce the extremely large hail or especially long-track or violent tornadoes.
So this is why meteorologists are looking for rotation in storm cells and the potential development of supercells.
In our region, supercells are not extremely common, but temporary supercell structures — those that last less than an hour, we’ll say — are more common within other types of storm structures as favorable helicity conditions exist for a relatively short time in a local area.
Thunderstorm frequency picks up in our area as we move toward summer, but typically, winds aloft also weaken as we get deeper into the warm months, making supercells a bit more difficult to develop. Still, anytime a storm can rotate, even for a short while, it can spin out some serious impacts.
Journalist Kevin Myatt has been writing about weather for 20 years. His weekly column, appearing on Wednesdays, is sponsored by Oakey’s, a family-run, locally-owned funeral home with locations throughout the Roanoke Valley. Sign up for his weekly newsletter: