From SKYbrary Wiki
A fire-storm is a large fire, which achieves such intensity that it creates and supports its own wind system that creates tremendous turbulence and also cause the strong surface inflow winds to change direction erratically. It is most commonly a natural phenomenon, created during some of the largest bush fires and wildfires. The windshear and turbulence associated with the firestorm create a particular hazard for aircraft involved in firefighting operations.
A fire-storm is created as a result of the stack effect (the movement of the air due to a difference in air density resulting from temperature and moisture differences) as the heat of the original fire draws in more and more of the surrounding air. This draft can be quickly increased if a low-level jet stream exists over or near the fire. As the updraft expands, strong inwardly directed gusty winds develop around the fire, supplying it with additional air.