STP — Significant Tornado Parameter

STP = (MLCAPE/1500) × (SRH₀-₁/150) × (SHR₀-₆/20) × ((2000 − MLLCL)/1000) × ((MLCIN + 200)/150), each term capped at 0/1.5.

STP > 1 is where significant tornadoes start to become a meaningful environment; >3 is a red-flag day. It's calibrated on US climatology — UK equivalents need care because our MLCAPE is typically smaller even when kinematics are equivalent, so STP in the 0.5–1 range here is worth paying attention to.