SPC Mesoanalysis Explained — MLCAPE, Shear, SRH, STP

SPC Mesoanalysis Explained — MLCAPE, Shear, SRH, STPWhat MLCAPE, 0–6 km bulk shear, 0–1 km SRH, and the Significant Tornado Parameter mean. The "ingredients" forecasters check first when assessing severe-weather potential.Mesoanalysis is a real-time look at the atmospheric "ingredients" for severe weather, updated hourly by SPC. The four parameters most useful when assessing thunderstorm potential are MLCAPE, bulk shear, SRH, and the Significant Tornado Parameter.MLCAPE — Mixed-Layer CAPE (the fuel)Convective Available Potential Energy. Measures how much energy is available to thunderstorms in the lower atmosphere. Higher values = stronger updrafts = bigger storms. Units are J/kg.

< 500: Limited storm potential

500–1000: Weak storms possible

1000–2500: Typical for severe weather

2500–4000: Significant severe potential

4000+: Extreme conditions; very large hail likely

0–6 km Bulk Shear (the organization)How much the wind changes from the surface up to 6 km altitude. Higher shear lets storms tilt instead of collapsing on themselves, which keeps them alive longer and lets them rotate. Units are knots (kt).

< 30 kt: Storms stay disorganized

30–40 kt: Organized multicell clusters

40–50 kt: Supercells possible

50+ kt: Strong supercells / wind events

0–1 km SRH — Storm-Relative Helicity (the spin)How much the wind direction twists from the surface up to 1 km. Storms ingesting this twist can develop a low-level rotating updraft (mesocyclone) — the source of most tornadoes. Units are m²/s².

< 100: Weak rotation potential

100–200: Moderate; supercells possible

200–300: Strong tornado environment

300+: Significant tornado potential

Significant Tornado Parameter (the composite)A single number that combines CAPE, shear, helicity, and low-level moisture. Designed to highlight environments where strong (EF2+) tornadoes are most likely.

< 1: Significant tornado unlikely

1–3: Significant tornadoes possible

3–6: Substantial tornado risk

6+: High-end tornado environment (rare)

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