Ingredients-based Forecasting Approach

What ingredients are necessary in order for the given forecast "issue" to occur?

Forecast Issue - Thunderstorm -- a convective storm associated with a cumulonimbus cloud producing lightning.

Forecast Issue - Severe Thunderstorm -- a thunderstorm that produces (a) straight-line wind gusts of 50 knots (57 mph) or stronger; and/or (b) hail 1 inch in diameter or larger; and/or (c) a tornado of any intensity.