1. Instability through a great depth of the troposphere. Once the air starts rising it will keep rising up to the tropopause.
2. Abundant moisture in the lower levels. The rising air needs lots of water vapour as this will condense to form the cloud releasing latent heat of condensation and, higher up, latent heat of freezing. This heat maintains the instability of the air.
3. A trigger. Something has to get the air rising. This could be a front, orographic lifting of air over mountains, differential heating of the surface. There are a number of triggers
TubaRiver
Weather lvl 5