def METHOD_NAME(self, step_num):
    # return a json-compatible dict of the mass_balance data
    # only applies to forecast spill_container (Not uncertain)
    sc = self.cache.load_timestep(step_num).items()[0]
    output_info = {'model_time': sc.current_time_stamp}
    output_info.update(sc.mass_balance)
    self.logger.debug(self._pid + 'step_num: {0}'.format(step_num))
    for name, val in output_info.items():
        msg = ('\t{0}: {1}'.format(name, val))
        self.logger.debug(msg)
    return output_info