We analyzed pumice from the February 11, 2010 Vulcanian explosion that immediately followed a large dome collapse at Soufrière Hills volcano. We obtained pre-explosive values of porosity, pressure, and depth by combining textural analyses and glass water content determinations. Our data suggest that the February 2010 explosion evacuated the upper 3 km of the conduit from the dense magma (≤10 vol% porosity) it contained. The low porosity distribution in the volcanic conduit implies that the magma rising from the reservoir had time to extensively degas during emplacement. We use a conduit flow model to characterize the effects of permeability on ascent conditions. Model input parameters were fitted so as to match our pre-explosive porosity data, which yielded first-order constraints on conduit radius, mass flux, outgassing efficiency, and permeability. This parametric study suggests that efficient lateral gas escape is necessary to explain the low pre-explosive porosities. Steady-state solutions fitting the observed range of dome extrusion rate in the month preceding the February 11 event suggest permeabilities < 10−13 m2 deeper than 500 m, which are values typical of crack-supported permeability. Conversely, solutions with parameters consistent with bubble-supported permeability imply transient flow conditions prior to the February 11 event. The transient conditions imply that our data represent a snapshot of the porosity distribution within the conduit that does not preclude the temporary presence of much higher porosities in the conduit. Such unsteady conduit flow conditions are consistent with the irregular but active dome growth in the month prior to the February 11 event.