Plasmaspheric hiss plays a major role in the scattering and loss of electrons from the Earth's radiation belts, thereby contributing to the maintenance of the slot region between the inner and outer electron belt. As such, models of radiation belt dynamics require accurate estimates of hiss amplitudes. We present a straightforward empirical model, based on THEMIS data from June 2008 through December 2010, which uses multiple regression to predict observed equatorial hiss amplitudes on the three magnetospheric THEMIS probes as a function of L and MLT. The model utilizes solar wind measurements (e.g., dynamic pressure and IMF) and geomagnetic indices (e.g., AE and SYM-H) as model inputs. This model explains on average 26% of the variance of hiss amplitude observed on THEMIS. We present example output of the model for a minor geomagnetic storm that reveals, for the first time, the evolution of plasmaspheric hiss amplitude as a function of storm phase.