The Supercell Composite Parameter (SCP)

Over the years, forecasters have analyzed individual parameters (measures of wind, temperature, and moisture) in assessing the threat of severe weather, and visualized how these parameters come together by constructing composite charts. More recently, composite parameters have been developed that combine individual parameters using simple mathematical relationships. An example is the Energy-Helicity Index (EHI), which combines 3 km environmental helicity and surface-based CAPE. However, the EHI can show misleading values if significant vertical wind shear exists above 3 km AGL. Many times, strong low level veering winds occur beneath rather weak mid level flow - resulting in high EHI values but a lessened supercell and tornado threat. Inversely, weak low level winds may limit the EHI, but strong mid level winds and favorable deep layer shear could enhance the supercell and tornado threat.

The Supercell Composite Parameter (SCP) is defined as:

SCP=[CAPE / 1000j/kg] * [3kmSREH / 150m2s2] * [BRNShear / 40m2s2]

Similar to the EHI concept, each predictor in the SCP equation is normalized by a baseline value of that parameter. The scheme also attempts to build on the concept of the EHI by adding a deep-layer shear term, Bulk Richardson Number (BRN) Shear . Several examples will be shown to illustrate the increased value of this parameter over the EHI, and to examine its strengths and weaknesses.