Severe Convection Checklist
Severe convection usually forms when atmospheric ingredients come together shown (above). These severe weather ingredients allow ordinary thunderstorms to become severe producing winds over 58mph, large hail > 1-inch diameter, and tornadoes. Now lets take a look at key features associated with severe thunderstorms and supercells in the mesoscale environment.
Wind shear is very important for thunderstorms to become severe. Simply, without wind shear thunderstorms cannot keep their updraft separate from their downdraft. In other words, wind shear allows thunderstorms to have updraft-downdraft separation allowing storms to last longer, become more intense by tapping strong winds aloft, and travel faster. This is all thanks to the jet stream which is a band of relatively strong winds in a narrow stream in the upper-levels of the atmosphere that allows storm systems to move across the U.S. including air masses as well.
A pulse thunderstorm is a thunderstorm that usually develops in a hot-summer air mass due to high instability and surface convergence. However, these thunderstorms environment(s) usually lack shear which allows the downdraft to cut-off the updraft. Heavy downpours, vivid lightning, and gusty winds can be expected though. We may have high instability, a front/trigger, and sufficient moisture in this case, but we are missing the key ingredient to allow a thunderstorm to become severe which is wind shear...
This is an example of what forecasters and storm chasers observe to determine what kind of "environment" is present for thunderstorms. This is an overlay skew-t diagram (pictured left) to illustrate what a thermodynamic diagram looks like in an actual convective environment.
Another feature in supercells and severe thunderstorms is the wall cloud. A wall cloud is a persistent lowering under the rain-free base of an updraft. Wall clouds can precede tornadoes. A wall cloud can become a tornado if enough inflow is being ingested into the updraft as well as enough low-level shear is present in the storm's environment.
A supercell (pictured above) is a severe thunderstorm with a rotating updraft (mesocyclone). Supercells can last for hours and hours and can track across several states in some cases. Supercells often produce torrential rainfall, large hail, vivid lightning, damaging straight-line winds, and tornadoes (including strong tornadoes). Supercells usually form when enough shear is available 0-6km > 30kts while low-level shear (SRH) Storm Relative Helicity is also present and sufficient allowing severe thunderstorms to rotate.
A severe thunderstorm usually has a strong updraft while at same time a strong downdraft. Severe thunderstorms often produce strong winds, large hail, and if the conditions are right...tornadoes.
A (LP) low-precipitation supercell has a rotating updraft (mesocyclone), but usually has a relatively dry Rear Flank Downdraft (RFD) and/or Forward Flank Downdraft (FFD). These supercells can produce tornadoes, but often spin for hours and hours never producing a tornado. These supercells, however can be the most photogenic for storm chasers since they usually allow the entire structure of the supercell to be observed.
Here's an example of a (LP) supercell (above). This is a time-lapse of a rotating mesocyclone.
A (HP) high-precipitation supercell has a rotating updraft (mesocyclone), but usually has a wet RFD. These supercells usually produce tornadoes. These tornadoes can sometimes be rain-wrapped as the wet RFD rain curtains wrap around the mesocyclone which makes these tornadoes that HP supercells produce quite dangerous as they are sometimes very hard to discern.
An animation using my still photographs can be found here (above) of a severe thunderstorm at dusk in eastern Iowa. Vivid lightning, large hail, and heavy rain accompanied this severe thunderstorm.
Another supercell that can be found in the Great Plains and the Midwest is the northwest-flow supercell. This supercell usually forms in an environment conducive for storm rotation. Most supercells move northeast or take a hard right-turn along a warm front, but northwest-flow supercells differ in that these supercells track northwest to southeast and can be quite common in the Northern Plains and Midwest in the late-summer. All the same classic supercell features can be observed as well.
The barber pole updraft referred by storm chasers is a common feature observed a few miles away from the supercell. It is basically the mesocyclone of the supercell, but is given this name since it resembles a barber pole in the way the mesocyclone spins and rotates.
Mammatus clouds are rounded but smooth clouds protrusions hanging from the underside of a thunderstorm's anvil. A strong updraft is responsible for these clouds undulating under the anvil and usually is a good indicator of a severe thunderstorm or supercell in the area.
Not all supercells produce tornadoes, however if the right "conditions" are present a tornado can form. Tornadoes form when the mesocyclone tightens while spinning faster allowing the rotation to become more concentrated as the RFD begins to wrap around the mesocyclone. It's still not known what is the "exact-process" of how a tornado forms. More research continues to go on year by year, however the scientific consensus thus far seems to be that a warm (buoyant) RFD is very important for tornadogenesis.
Here's an example of a (HP) supercell (above). This is a time-lapse of a rotating mesocyclone with a wet Rear Flank Downdraft (RFD).
Here's an example of a northwest-flow supercell (above). This is a time-lapse of a northwest-flow supercell near sunset in west-central Illinois.
Here's an example of a wall cloud (above). This is a time-lapse of a supercell with a persistent wall cloud below the rain-free base of the updraft in eastern Iowa.
Here's an example of a tornado (above). This is a time-lapse of a supercell that produced a tornado in eastern Nebraska.
A shear vector around 45° may allow discrete severe thunderstorms to form including discrete supercells...