A true (cubical) triaxial tester was built to evaluate the modified Cam-clay parameters for wheat flour [1]. A finite element model (FEM), using the modified Cam-clay equations, was used to predict incipient flow behavior of wheat flour in a mass flow hopper bin. The incipient flow regime in a hopper bin was characterized by the formation of the first dynamic arch. The first dynamic arch represented the transition between the static case, before opening the hopper outlet, and the dynamic case, when the powder was flowing out of the hopper. Experimental observation showed that an arch was formed after minimal discharge when the gate of a mass flow hopper bin was opened. This arch broke before the development of fully dynamic powder flow. To validate the FEM, a transparent plastic laboratory size mass flow hopper bin with an outlet size of 203 mm and a hopper angle of 15 degrees with respect to the vertical was used. The first experimentally observed dynamic arch location and profile were used to validate the finite element model predictions. Mean average relative difference (ARD) between the FEM predicted and the mean of the first experimentally observed dynamic arch location and profile was 14.2% and the mean absolute difference was 5.8 mm. The FEM predictions were within the 95% CI of the measured values.