The Significant Tornado Parameter, or STP, combines mean-layer CAPE, effective storm-relative helicity, effective bulk wind difference, mean-layer LCL height, and mean-layer CIN into a single number designed to highlight environments where significant tornadoes have historically been most common. It was developed at the Storm Prediction Center to add the low-level moisture and inhibition terms that pure supercell composites do not include.
What goes into the formula
The effective-layer STP is a product of five normalized terms. Mean-layer CAPE is divided by 1500 J/kg. Effective storm-relative helicity is divided by 150 m squared per second squared. Effective bulk wind difference is divided by 12 m/s, then bounded at 1.5. Mean-layer LCL contributes a term that increases as the cloud base gets lower, saturating to one when the LCL is below 1000 m AGL. Mean-layer CIN contributes a term that increases as the inhibition weakens, saturating to one when CIN is weaker than negative 50 J/kg.
The design is intentional. Multiplying the five terms means any one ingredient near zero, high cloud bases, strong inhibition, weak low-level shear, or thin instability, drives the product toward zero and knocks the environment out of the significant-tornado category.
How forecasters use it
STP is checked on the SPC mesoanalysis maps and in individual point soundings. A coherent area of STP greater than one focuses attention, and higher values into the two-to-eight range in real events tend to align with the strongest tornado environments in the historical record.
STP is also useful for reading which ingredient is doing the work. Because the terms are visible individually in point soundings, a forecaster can see whether a high value is being carried by very low LCLs and weak CIN over marginal CAPE, or by strong CAPE and shear over a cloud base that is a little higher than ideal.
Where it misleads
STP describes environments, not storms. A favorable environment still requires initiation, storm mode that supports tornado production, and often a boundary interaction. A big STP over a strongly capped airmass can persist for hours without any storms.
The composite can also be misled by the analysis it is built on. Underestimated boundary-layer moisture, a missed cold pool, or a poorly defined effective inflow layer all propagate into STP. Always cross-check the individual fields, and treat any single grid point as one piece of evidence rather than a verdict.