The Cooperative Institute for Meteorological Satellite Studies has built a forward RT system for rapidly computing TOA thermal radiances in order to simulate the HES and certain ABI channels under all weather conditions. This study will examine the quality of the synthetic radiances produced by the system through comparisons to satellite observations and to more detailed RT model calculations. Atmospheric and cloud fields used as input for the RT models are provided by Weather Research Forecast (WRF) model simulations. Initial results for the HES show that for clouds with visible optical depths less than 10 (where errors are generally greatest) the fast RT model under nadir viewing conditions has scattering bias errors of up to 0.5 K and RMS errors below 1.5 K across the spectrum (587-2350 cm-1). Errors increase, however, for oblique viewing angles. At a local zenith angle of 55o the bias errors increase to less than 1 K with RMS errors of up to 2 K.