The HPC and TPC both track storms while over water and come up with separate track forecasts on their own. The TPC points are the one everyone is familiar with; the HPC progs. are not as widespread but are reflected in their forecasts. They predicted Frances would travel a bit further west than the NHC did, for instance, even though they didn't pan out with that forecast.

Both groups get the chance to put out their points during the 6-hrly conference calls (4 & 10 am & pm), but the TPC has the final say for the general public. And, once inland and no longer a tropical threat, the NHC turns things over to the HPC, something they've started doing a bit more frequently (and quickly) over the past year or two.