Credit: Richard Thompson, NOAA SPC.
These are two variants of the Supercell Composite Parameters, contributed to the library by Richard Thompson of the NOAA SPC. One is based on surface based CAPE, the other based on the most unstable CAPE from 0-255hPa above ground level. About which one to use, Richard wrote:
I think the easiest way to handle the SCP is to treat it like the STP, where you are focusing on surface-based CAPE and shear parameters. The downside to this approach is you can miss some environments that support supercells rooted above a shallow stable layer near the ground (like north of a warm front). However, there is also a downside to using the most-unstable CAPE - there are plenty of situations (in the Great Plains of the United States, at least) where the CAPE is rooted above the layer where most of the strong low-level shear is location, such that the combination of 0-3 km SRH and most-unstable CAPE over-estimates the supercell potential. It comes down to your personal preference - do you want to highlight the potential for surface-based supercell environments, or do you also want to include the possibility of elevated supercells and greater false alarm area?
The SCP can be over-estimated when you use most-unstable CAPE in combination with 0-3 km SRH, which was the motivation for the "effective SRH" parameter.
capesfc = surface based CAPE
cape255_0mb = most unstable CAPE, 255-0hPa above ground
srh3km = storm-relative helicity, 0-3km
shear06 = 0-6km bulk shear