The very quick fire spread that occurs in some forest fires has been the cause of many fatalities in the past among fire fighters throughout the world, A theoretical model describing the convective interaction between the fire front and the surrounding air is proposed to explain the blowup phenomenon that is observed in nature. This model is based on a set of laboratory experiments of fire blowup in canyons that was used to validate it. The model predicts quite well the general fire behavior observed during two fatal accidents that occurred in the United States and one in Portugal.