Harmful algal blooms caused by Karenia brevis result in large fish kills, human respiratory irritation, and shellfishing closures in affected areas. Most previous work on bloom formation in the Gulf of Mexico has focused on the west coast of Florida. To investigate the origin of bloom-forming cells along the coast of Texas, potential distributions of cells during two bloom years (2009, 2011) and one non-bloom year (2010) were examined using a spatially explicit, individual-based model of K. brevis. The model incorporates a previously developed model of dinoflagellate vertical migration and utilizes observed data (field samples of cell concentrations, photosynthetically active radiation) and modeled environmental output (salinity, temperature, current velocities) from a hydrodynamic model. Running the model in reverse showed that cells near the coast of Texas during early fall originate from the southern Gulf of Mexico in bloom years and from the northern Gulf of Mexico in the non-bloom year for the three years studied. Identification of a southern origin for bloom-forming cells provides a target area for increased sampling in order to provide early warning of potentially harmful algal blooms of K. brevis.