The Supercell Composite Parameter, or SCP, multiplies normalized values of most-unstable CAPE, effective storm-relative helicity, and effective bulk wind difference into one number, so a mesoanalysis map can highlight where all three supercell ingredients are present at once. It was developed at the Storm Prediction Center and updated by Thompson, Edwards, and colleagues to use effective-layer kinematics rather than fixed layers.
What goes into the formula
SCP is the product of three normalized terms. Most-unstable CAPE is divided by 1000 J/kg. Effective storm-relative helicity is divided by 50 m squared per second squared. Effective bulk wind difference is divided by 10 m/s, capped at a value of 1.5 for very strong shear and set to zero when the shear falls below 10 m/s.
The normalization values are calibrated so a typical significant supercell environment gives a product on the order of one or greater. Right-moving supercells give positive SCP in the Northern Hemisphere. Left-moving anticyclonic supercells give negative SCP when the assumed storm motion is flipped.
How forecasters use it
The most common use is spatial. An SCP map at hour-by-hour resolution shows where the three ingredients are stacking, which frames the day's threat area before any storms fire. Forecasters typically look for a coherent region of SCP greater than one that overlaps expected initiation, then drop into soundings to confirm the ingredients look real.
SCP is also useful for post-mortem context. Comparing the SCP value that was analyzed in past significant supercell environments with today's map is one way to place the current setup on a rough scale.
Where it misleads
The multiplicative structure means one very large term can carry the composite into high values even when another term is marginal. An SCP of three built on strong shear and helicity but only a few hundred J/kg of CAPE reads very differently than an SCP of three built on balanced ingredients. Always look at the individual fields.
The composite also inherits any error in its inputs. If the model background misplaces a boundary or the effective layer is thin because of a missed cap, the analyzed SCP can be too low or too high in the wrong places. And a favorable SCP still requires storms to actually initiate. A capped environment can carry a large SCP for hours without producing anything.