Occluded front, when the cold front catches up with the warm front, as the front passes you experience changeable weather which can include stormy conditions.
The cold air arriving with this front pushes the warm air ahead of it upwards. This forms rain, bearing cumulonimbus clouds, this often leads to thunderstorms and heavy rain. Temperature reduces.
As warm front approaches clouds thicken from circus to stratus and then rain. On passing rain then becomes more sporadic (odd showers) and temperatures rise.