One of the most commonly used weather terms during the spring and summer months is "supercell thunderstorm." Most weather enthusiasts (myself included) use the term with the assumption that people know what a supercell is, but many people don't. Supercells are the miniature engines of Earth's atmosphere. They're fascinating to watch both on radar and in person, but they're also responsible for the most destructive tornadoes in history.
So, what exactly is a supercell, anyway?
Life Cycle of 'Normal' Thunderstorms
Most people are familiar with typical daytime thunderstorms during the summer months. These storms are generally small — usually the size of a small city — and only last for 30-60 minutes before they dissipate.
Thunderstorms need warm, moist (read: unstable) air to sustain themselves. The inflow of this unstable air into a thunderstorm is called the updraft. The updraft feeds the thunderstorm the energy it needs to grow in size and strength.
Once the rain starts falling, it starts the downdraft, or cool, dense air that sinks out of a thunderstorm towards the ground. The storm continues until the downdraft spreads away from the storm (called an outflow) and cuts off the updraft, starving the thunderstorm of the energy it needs to survive, and the storm dissipates.
These "popcorn"/"garden-variety"/summertime thunderstorms are known as "single-cell thunderstorms."
The story of the supercell starts with wind shear and large amounts of instability.
Vertical wind shear occurs when wind changes direction and speed with height. When there's enough wind shear in the atmosphere, it can create horizontal tubes of rotation within the first one or two miles above the earth's surface.
When there's strong enough instability in the atmosphere, an updraft can form that actually bends these tubes of rotation upward in the shape of a horseshoe. This leaves two vertically-oriented tubes of air — one spinning clockwise, the other counterclockwise.
Since we're in the northern hemisphere, the now-vertical tube of rotating air that's spinning counterclockwise wins out, and this becomes the updraft for the supercell.
This rotating updraft tilts diagonally due to the strong winds aloft, causing the downdraft to form far enough away from the warm, moist air that the storm doesn't choke and die. This effectively allows the supercell to become a miniature engine that can run uninterrupted until cool, stable air cuts off the instability and starves the storm of energy.
This rotating, tilted updraft becomes a feature known as a "mesocyclone." The mesocyclone can be up to several miles wide and is usually used by the media to refer to the broad rotation within a supercell.
It's usually within or immediately next to the rotating updraft that tornadoes form from a supercell.
It's Essentially a Mini Low Pressure System
This is what a classic supercell thunderstorm looks like on weather radar. This is the infamous May 3, 1999 storm that produced the F5 tornado that tore through Bridge Creek, Newcastle, and Moore, Oklahoma.
The most well-known characteristic of supercells on radar is the "hook echo" that forms on the southwestern side of the storm, often accompanied by a bright purple circle in the center. This circle is actually debris from the tornado being detected by the weather radar.
On the eastern side of the hook echo is usually a deep notch that cuts into the thunderstorm — this is called the inflow notch, which is caused by the warm, moist (unstable) air rushing into the storm to provide it energy. This is where the updraft is located.
Immediately to the north of the updraft we find the heaviest rain and largest hail within the supercell, with precipitation lightening up as you move further east towards the far end of the storm. Often before a major tornado hits, people in the path of the storm see the strongest winds, hail, and rain pound them with an intense fury, only to abruptly stop seconds before the twister moves overhead.
Supercell thunderstorms are essentially miniature low pressure systems, complete with cold and warm fronts.
Supercells have two downdrafts — the forward flank downdraft (FFD) and the rear flank downdraft (RFD).
The forward flank downdraft occurs as a result of sinking air within the 'bulk' of the supercell, and the rear flank downdraft occurs as a result of the winds in the mid-levels of the atmosphere crashing into the rotating updraft and bending down towards the ground. The RFD gets wrapped around the rear of the supercell, and facilitates the formation of the hook echo. The RFD can be so strong that it causes wind damage that's often confused for tornado damage during the worst storms.
If the rear flank downdraft advances too quickly, it can choke off the inflow and the supercell will begin to die. This is usually how supercells die off — the RFD cuts off the inflow and the supercell quickly begins to fall apart.
From Supercell to Squall Line
I've used the following phrase in posts here on The Vane for almost every severe weather outbreak that's occurred this spring:
The supercells will begin to merge with one another later in the day and form a squall line, at which point the severe weather threat will transition to damaging winds.
Individual supercells like the one I posted above are known as "discrete supercells." They form independent from all other storms in the area, and their structures start to collapse when they interact with other thunderstorms (especially other supercells).
When multiple supercells form in close proximity to each other, the precipitation from the southernmost storms starts to disrupt the storms to their north, causing the supercellular structures to break down and the storms begin to merge.
Over time (usually an hour or two), the supercells will completely break down and form into a strong line of thunderstorms. Since supercells usually form in the central Plains during the late afternoon hours, the supercells merge into a line by the late evening and nighttime hours and the line starts to race eastward. This is why parts of the south (especially Arkansas) often see a good portion of their severe weather in the middle of the night.
Where Do Supercells Form?
Supercells form pretty much everywhere severe weather occurs, provided there's enough wind shear and instability in the atmosphere. They're most common in the middle of the United States, but they can occur in 49 of the 50 states (almost never in Alaska) — as well as Canada — and elsewhere around the world.
Australia, South Africa, Bangladesh, and central Europe (especially around Germany and Poland) are known to have some pretty bad tornado outbreaks, but nothing like what we see in the United States.
The main risks with supercells are destructive tornadoes, very large hail, and damaging winds often in excess of 75 MPH.
One of the most overlooked threats in supercells is the prolific hail that they can produce, which can prove deadly if you're caught outside when it starts falling. The above video shows one of the worst hailstorms in recent history, which pummeled the Oklahoma City area with hail up to the size of softballs (4.25" in diameter) and caused over one billion dollars worth of damage.
What is a shelf cloud? — March 7, 2014
What is a derecho? — April 8, 2014
[Images via: Niccolò Ubalducci via Flickr / Wikimedia Commons / Wikimedia Commons / NOAA / NOAA / Gibson Ridge / Gibson Ridge / Gibson Ridge / College of DuPage / NCDC]