Classic Supercell: "Cascade" to Tornado
1. Buoyancy and deep layer shear (0-6 km; vector difference between surface and 500 mb winds about 35 knots or greater) in range favorable for supercells. In the Great Plains, this combination is often found in association with late spring and early summer wave cyclones. These cyclones circulate high dew point air northward at the surface under the jet stream.
2. Thunderstorm develops, and becomes tilted (due to 1), thus ensuring that precipitation does not fall into the updraft area.
3. Thunderstorm updraft develops lower midlevel and midlevel rotation (mesocyclone) by tilting horizontal flow which has adequate inflow layer (0-3 km) Storm Relative Helicity [SRH or SREH] (horizontal spin) into the vertical. Values of around 150 or greater suggest rotating updrafts strong enough to be detected by radar and which stimulate the continuation of the cascade. (See severe weather reports for this day).
4. Mesocyclone develops vertical continuity (about 1/3 the depth of the radar echo).
5. Deep mesocyclone circulates precipitation around rear flank of storm (development of "hook echo").
6. Mesocyclone develops downward towards ground. Hook shows up at lower elevations.
7. Interaction of environmental air at lower midlevels of the storm with deep hook in lower parts of storm is associated with the development of the Rear Flank Downdraft (RFD).
8. RFD strikes ground, part of it flowing away from the storm, and part of it flowing back towards the updraft.
9. If low level shear (in 0-1 km layer; measured in a number of ways) is sufficient, interaction of RFD with updraft is associated with Tornado Vortex Signature— (TVS) on radar and eventually the development of a tornado IF the RFD is as warm or warmer than the updraft. (Supercells that have cold RFDs generally do not produce tornadoes) (Note: If Steps 1 through 9 occur, but LCLs/LFCs are higher than about 1000 feet AGL, tornadoes will not occur, despite the impressive hooks that show up in reflectivity radar information).
If the process is interrupted at any stage, Steps 8 and 9 may never occur. Examples:
· Outflow from neighboring thunderstorm ingested into the updraft.
· Low level vertical shear is favorable for supercell tornado development but deeper shear (measured by Sfc-500 mb Shear Vector) not quite right.
· Low level vertical shear is too weak even though deeper shear is favorable.
a. Deep layer shear not quite right, so that some precipitation falls into the updraft area of the storm (outflow dominated supercell).
b. Deep layer shear is slightly too large, so that precipitation area too far away to be circulated to rear side of storm by mesocyclone (Low Precipitation Supercell—chief threat giant hail and winds).
c. Deep layer shear slightly weak so that too much precipitation is circulated around to the rear side of the storm by the mesocyclone and may completely encircle updraft (High Precipitation Supercell—chief threat giant hail, strong winds, flash flooding).