Severe thunderstorm formation comes down to four key ingredients coming together (moisture, instability, lift, and wind shear) and the specific combination determines whether you get an ordinary thunderstorm or an organized severe storm capable of large hail, damaging wind, or tornadoes.
- Moisture - Warm, humid air near the surface provides the fuel. This is why severe weather in the US is so concentrated in spring/early summer over the Plains and Midwest
- Instability - This is the atmosphere's willingness to let a rising air parcel keep accelerating upward rather than sinking back down. It develops when warm, moist air sits near the surface underneath cooler, drier air aloft.
- Lift - Something has to actually kick the air parcel upward to get convection started, since instability alone is potential energy, not motion. Common lifting mechanisms:
- Cold fronts - dense cold air wedges under warm air, forcing it up.
- Drylines - a boundary between moist and dry air masses (classic in the Southern Plains), where the denser dry air undercuts the moist air.
- Daytime heating - surface heating alone can be enough to break a weak capping inversion.
- Outflow boundaries - cold air pools spreading out from existing thunderstorms can trigger new ones nearby.
- Wind shear - This is what separates an ordinary, short-lived "pulse" thunderstorm from an organized severe storm. Shear (change in wind speed and/or direction with height) tilts the storm's updraft.