The human body cannot survive being in temperatures over 200 degrees centigrade for more than a few moments, especially in the fast moving current of a surge. Trying to breathe in the dense cloud of hot ash in the absence of oxygen would lead to unconsciousness in a few breaths, as well as causing severe burns to the respiratory tract... On the other hand, survival is possible in the more distal parts of a surge if there is adequate shelter to protect against the surge flow and its high temperature, as well as the missiles (rocks, building materials) entrained in the moving cloud of material.