A composite parameter is a single number built by multiplying or otherwise combining several individually meaningful ingredients, so a forecaster can screen a large area for environments where those ingredients are all present at useful magnitudes. The Supercell Composite Parameter, the Significant Tornado Parameter, and the Energy-Helicity Index are all examples.
The ingredients-based idea
The underlying logic is that most convective hazards require several ingredients to co-occur. Instability, shear, moisture, and lift are the classic four for severe convection, and specific hazards add further requirements such as low-level rotation or a low cloud base for tornadoes.
A composite parameter encodes that co-occurrence directly. Each ingredient is normalized against a typical significant-event value, and the normalized terms are multiplied together. If any ingredient is near zero, the product is near zero even when the others are large. If all ingredients are near or above their reference values, the product grows quickly.
How forecasters use them
Composite parameters are most useful as spatial screens. A mesoanalysis map of SCP or STP shows where the ingredients are stacking at the same time and place, which narrows the area a forecaster needs to inspect in detail with soundings and hodographs.
They are also useful for pattern recognition across cases. Because the composites are normalized, a forecaster develops a rough sense for what a two, a four, or a ten looks like in a given part of the country, and can compare today's setup with what past significant events looked like on the same maps.
Where they mislead
A composite is only as good as the ingredients it combines. If one of those ingredients is poorly analyzed, for example a cold pool that a model missed, the composite inherits that error. A high value from a bad SRH analysis is not a real high value.
Multiplicative composites also weight extremes strongly. A very large value of one term can push the product into a high category even when a second term is only marginal, and vice versa, so any high composite deserves a check that no single term is doing all the work. Finally, no composite proves that a storm will fire. Ingredients can align in a capped environment that never produces a thing, and the composite will still light up.