Between 26–29 January 2021 an atmospheric river (AR) triggered numerous debris flows within the 2020 Dolan wildfire burn scar in Big Sur, California. Here we modify WRF-Hydro to simulate both overland and channelized flows and assess the potential for runoff-generated debris-flow hazards in burn scars. High-resolution weather radar-derived precipitation is used to drive baseline and burn scar sensitivity experiments. Compared to the baseline, the burn scar simulation yields dramatic increases in total and peak discharge, as well as shorter lags between rainfall onset and peak discharge. At Rat Creek, where California Highway 1 was destroyed, discharge volume increased eight-fold and peak discharge tripled relative to the baseline. Our WRF-Hydro-based hazard assessment indicates that over 1/3 of Dolan burn scar catchments were under “very high” debris-flow hazards. Our work demonstrates that a modified version of WRF-Hydro provides a compelling new physics-based tool to investigate and potentially predict postfire hydrologic hazards.