Tornado Risk Calculator

The Significant Tornado Parameter (STP), developed by Thompson et al. (2003) and used operationally by the NOAA Storm Prediction Center, combines four separate ingredients that all have to line up for a strong tornado to form — this calculator implements it as a straightforward product of four normalized terms, each clamped to zero if it would otherwise go negative. CAPE is divided by 1500 J/kg, storm-relative helicity (SRH) by 150 m²/s², and 0-6 km bulk shear (converted from knots to m/s at 0.5144 m/s per knot) by 20 m/s — each ratio represents how that ingredient compares to a rough "climatological favorable" benchmark for significant tornadoes. The fourth term rewards a low Lifting Condensation Level: (2000 − LCL height in meters) / 1000, clamped between 0 and 1, because a lower cloud base means less dry air undercutting the storm's inflow, which historically correlates strongly with tornado intensity.

Multiplying all four together means STP collapses toward zero if even one ingredient is weak — a storm can have huge CAPE but negligible tornado risk if there's no wind shear to organize rotation. The calculator also derives a simplified Supercell Composite Parameter (SCP) by multiplying just the CAPE, SRH, and shear terms (dropping the LCL term), since storm organization matters more than tornado-specific ingredients for that index. Both STP and SCP are empirically-derived pattern-matching tools built from historical tornado and supercell soundings, not physical certainties — they flag favorable environments, not guaranteed outcomes, and real forecasting always layers them with radar trends, mesoscale boundaries, and short-term model guidance.