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A supercell thunderstrom in Oklahoma. Image courtesy of NSSL/NOAA.
The National Weather Service defines a severe thunderstorm as a thunderstorm that is producing:
- Hail that is at least quarter size (1 inch diameter or larger)
- And/or wind gusts 58 mph or greater
- And/or a tornado
If a storm that meets these criteria are possible for an area, the Storm Prediction Center will issue a severe thunderstorm watch or a tornado watch. If a storm with these criteria is imminent, your local Weather Service office will issue a severe thunderstorm warning or a tornado warning.
Lightning and heavy rainfall are dangerous elements of a thunderstorm, but they are not in the severe thunderstorm criteria for a few reasons. If lightning were a prerequisite, all thunderstorms would be severe. Also, flash flooding is handled by a separate set of watches and warnings which are issued by your local Weather Service office.
A supercell is a highly organized thunderstorm with some components that set it apart from other "garden variety" thunderstorms. Supercells have the capability to produce tornadoes, damaging hail and strong downdrafts (which translate into straight-line winds at the surface). All tornadoes are spawned from a parent supercell, but not all supercells produce tornadoes.
In addition to the standard necessary ingredients for a thunderstorm (instability, moisture, source of lift), supercells require strong "veering" of the winds, which means the winds are turning clockwise with height. So, for example, the surface wind could be out of the southeast, wind at the mid-levels will be out of the southwest, and winds at the upper levels will be out of the northwest. The turning of the winds with height helps the thunderstorm develop its most essential component: the mesocyclone.
A mesocyclone (or "meso" for short) is formed when a thunderstorm updraft meets veering winds. As the air rises in the thunderstorm, the winds will begin to twist the updraft until the whole column of air is rotating. Although each storm is different, the meso is usually found in the right rear flank of the supercell and is typically 2-6 miles wide. Technically, the mesocyclone is defined as the radar signature that appears if one is present (a yellow circle on Doppler velocity products), but you can often see the rotation with your bare eyes.
While each storm is different, most supercells usually have the following parts:
- Mesocyclone - Strong, rotating updraft
- Forward-Flank Downdraft - Cold, dense air descending through the front of the storm
- Rear-Flank Downdraft - Cold, dense air descending through the back of the storm
- Flanking Line - A line of towering cumulonimbus connected to and extending outward from the rear of the supercell
- Rain Shaft - The area in which rain and/or hail falls to the ground
- Overshooting Top - The area of clouds that "punch through" the jet stream into the lower stratosphere (occurs when updraft is particularly strong)
- Anvil - The flat layer of high cirrus clouds at the top of the storm that is shaped like an anvil, formed as the jet stream shears the updraft clouds away from the core of the storm
Types of Supercells
There are three types of supercells: low precipitation, classic and high precipitation. The definitions of these types are exactly what you would expect, but they have some different consequences.
Low Precipitation (LP) Supercells
LP supercells usually form in dry regions, where there might be just enough moisture to form the storm, but not enough moisture to rain very hard. You can usually find the updraft on the rear flank (back) of the storm, and the meso will be more defined and obvious. On radar, an LP will not show up as a hook echo because there's not enough precipitation within the storm to provide the reflectivity. These storms might not look that strong, but they can pack a punch. LP supercells often produce tornadoes and large hail.
The classic, textbook supercell looks much like the figure above. The storm will have a flat updraft base and potentially a wall cloud underneath the updraft. The precipitation (rain and hail) will fall adjacent to the updraft, usually underneath the forward flank downdraft (FFD). If the conditions are right, a tornado will form underneath the wall cloud.
High Precipitation (HP) Supercells
HP supercells usually have the updraft on the forward flank (front) of the storm, and the precip surrounds the updraft, from the FFD to the rear flank downdraft (RFD). The wall cloud and potential tornado will be "rain-wrapped" (within the "bear's cage") and difficult to observe. Rain and hail is extreme in these storms, and flash flooding usually occurs. It's possible that HP supercells are the most dangerous because of their ability to hide the warning signs of an approaching tornado.