Thunderstorms (TS) occur in well developed cumulonimbus (Cb) cloud, though not all Cbs produce thunderstorms.

Three basic ingredients are required for a thunderstorm to form:

1. Sufficient water vapour to form and maintain the cloud;
2. Unstable air (an ELR greater than the SALR); and
3. A lifting mechanism/trigger (action to produce early saturation, thus enhancing instability.)

The triggers/lifting forces are: Convection, Orographic uplift, Convergence, Frontal uplift

With regards to this question, the above conditions are fullfiled as described below:

1. The air advecting over a moist surface (water), will pick-up some moisture => which will "fuel" the development of a Thunderstorm.

2. Instability increases when lower layers are warmed, or upper layers cooled. Hence advection of cold air over a warm surface leads to instability.

3. Finally, the cold air will start to get heated from below and, consequently, will rise into colder air above where it adiabatically cools down, condenses and releases latent heat - which fuel the convective process providing a lifting action.