Tornadoes Need a Ratio, Not a Fuel Supply

01The evidence

• Idealised simulations showed that environmental low-level shear and cold-pool strength jointly control tornadogenesis, with the outcome depending on their balance rather than on either alone.Journal of the Atmospheric Sciences, 2014
• The role of rear-inflow jets and vorticity in mesocyclone genesis has been documented as a distinct mechanism from the thermodynamic environment, supporting a dynamical rather than an energetic control.NOAA repository

02The tension

Forecasting is organised around CAPE and bulk shear as separate ingredients, and the combination is used as a checklist. It produces many false alarms: high-CAPE days with strong shear often produce no tornadoes, and some tornadoes occur in modest CAPE. The checklist ranks environments but does not explain the failure of a given storm.

03The theory

A tornado needs a vortex to be stretched near the ground. Stretching requires upward motion, which is generated at the boundary between the storm's cold pool and the ambient air. The strength of that boundary is set by the cold pool, and the vorticity it tilts is set by the low-level shear.

If the cold pool is too strong, the updraft is lifted away from the surface and the vortex cannot be stretched at ground level. If it is too weak, there is no boundary to concentrate vorticity. The optimum is a ratio, and the ratio is dimensionless, which is why it is more portable than either ingredient.

The prediction is that tornado potential collapses at both extremes of the ratio at fixed CAPE, producing a ridge rather than a monotonic relationship. That is a testable shape, and it would explain the false alarms on high-CAPE days.

The practical consequence is a forecast parameter that is computed from the same model output already used, but combined multiplicatively rather than additively, and calibrated as a ratio rather than as a threshold.

04What this is not

05Predictions

• P1Tornado probability should peak at an intermediate shear-to-cold-pool ratio and decline at both extremes, at fixed CAPE.
• P2High-CAPE days with a ratio far from the optimum should produce few tornadoes despite strong supercells.
• P3A ratio-based index should outperform additive indices on held-out severe-weather seasons.

06What would refute it

A monotonic increase in tornado probability with CAPE at fixed shear would restore the fuel-limited view.

07What it would change

It would replace a checklist with a balance parameter, and reduce false alarms without lowering detection.