What is a "Supercell" Thunderstorm?

• The following information on supercell

thunderstorms comes from the National

Weather Service's JetStream - Online

Weather School page.

• All photos are from the NOAA website.

Supercell thunderstorms are a special kind of single cell thunderstorm that can persist for

many hours.

Supercells

Supercells are highly organized storms characterized by updrafts that can attain speeds over 100 miles per hour, able to produce extremely large hail and strong and/or violent tornadoes, downdrafts that can produce damaging outflow winds in excess of 100 mph - all of which pose a high threat to life and property.

Downward rushing winds

Downdraft

• (Abbrev. DWNDFT) - A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft

Downburst

• A strong downdraft current of air from a cumulonimbus cloud, often associated with intense thunderstorms. Downdrafts may produce damaging winds at the surface.

The most ideal conditions for supercells occurs when the winds are veering or turning clockwise with height. For example, in a veering wind situation the winds may be from the south at the surface and from the west at 15,000 feet (4,500 meters). This change in wind speed and direction produces storm-scale rotation, meaning the entire cloud rotates, which may gives a striated or corkscrew appearance to the storm"s updraft.

Outflow Boundary

• Based on their visual appearance, supercellsare often divided into three groups;

• Rear Flank Supercell - Low precipitation (LP),

• Classic (CL), or

• Front Flank Supercell - High precipitation (HP).

Rear Flank Supercell

Classic

Front Flank SupercellFlanking LineA line of cumulus or towering cumulus clouds connected to and extending outward from the most active part of a supercell, normally on the southwest side. The line normally has a stair-step appearance, with the tallest clouds closest to the main storm

Three stages

Dynamically, all supercellsare fundamentally similar. However, they often appear quite different visually from one storm to another depending on the amount of precipitation accompanying the storm and whether precipitation falls adjacent to, or is removed from, the storm"supdraft.

Tornado AlleyA geographic corridor that receives more than the average number of tornadoes, as in the Midwest; also called storm lane.

Satellite view of leading edge

LP supercellsIn LP supercells the updraft is on the rear flank of the storm, a barber pole or corkscrew appearance of updraft is possible, precipitation sparse or well removed from the updraft, often is transparent and you can"t see it, and large hail is often difficult to discern visually. Also, there is no "hook" seen on Doppler radar.

LP StormLow-Precipitation storm (or Low-

Precipitation supercell). A supercellthunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic supercell, except without the heavy precipitation core. LP storms often exhibit a striking visual appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation. They are capable of producing tornadoes and very large hail. Radar identification often is difficult relative to other types of supercells, so visual reports are very important. LP storms almost always occur on or near the dry line, and thus are sometimes referred to as dry line storms.

The majority of supercells

fall in the "classic" category. These have large, flat updraft bases, generally has a wall cloud with it, striations or banding can been seen around the periphery of the updraft, heavy precipitation falls adjacent to the updraft with large hail likely, and have the potential for strong, long-lived tornadoes.

HP supercells will have...

the updraft on the front flank of the storm precipitation that almost surrounds updraft at times the likelihood of a wall cloud (but it may be obscured by the heavy precipitation)tornadoes that are potentially wrapped by rain (and therefore difficult to see), and extremely heavy precipitation with flash flooding.

HP Stormor HP Supercell - High-Precipitation storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone.

Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events.

Beneath the supercell

the rotation of the storm is often visible as well. The wall cloud is sometimes a precursor to a tornado. If a tornado were to form, it would usually do so within the wall cloud.

Wall cloudsWall clouds associated with potentially severe storms can:

Be a persistent feature that lasts for 10 minutes or more

Have visible rotation

Appear with lots of rising or sinking motion within and around the wall cloud

Aerial view of a supercell

The schematic plan views below show how isolated supercellthunderstorms (panel a) appear on radar reflectivity displays in comparison to various bow echo configurations (panels b-d). The arrows depict surface winds and downdrafts. Isolated supercells(a) often appear as roughly circular or kidney-shaped blobs, with a point or hook-shaped appendage on the rear side of the echo, relative to its direction of motion. ("FFD" and "RFD" refer to the storm's front flank and rear flank downdrafts, respectively). Bow echoes typically are larger in scale than individual supercells. However, supercells sometimes are embedded in bow echoes.

Gust front "shelf cloud" (or "arcus") on the leading edge of a derecho-producing convective system

What is a “derecho”?

Essentially, a "derecho" is a long-lived, rapidly moving line of intense thunderstorms that produces widespread damaging winds in a nearly continuous swath.

By definition (according Johns and Hirt, 1987), the term “derecho” applies to a complex line of thunderstorms that travels a minimum distance of 240 miles (~400 km) or more, and produces a nearly continuous and widespread swath of damaging winds over that distance, with concentrated areas of wind speeds over 58 mph (93 km/hr). Surface wind gusts accompanying a derecho can often approach or exceed 100 mph.

A derecho moving from west to east

Click on the url to see NOAA’s satellite images of the derecho of June 29, 2012.

http://www.erh.noaa.gov/lwx/events/svrwx_20120629/radar_klwx_063012.gif