In preparation for the direct assimilation of GOES EAST/WEST infrared imager channels (2-5), a fast forward model (MSCFAST) is used to compute model equivalent radiances from 6-h forecasts every six hours. Surface emissivity is considered. Statistics of observation (O) minus forecast (P) radiances over oceans allow to design a simple bias correction scheme. A very clear diurnal cycle of brightness temperatures (BT) is seen in surface channels. A 1-D variational estimate of skin temperature Ts confirms a rather large +1.5 K Ts difference of BT(18 UTC) - BT(00 UTC) for the Caribbean Sea while a +1.0 K cycle is typical over the Pacific for March 2001. Strangely enough, there is also a significant diurnal cycle of about + 0.6 K in channel 3, which is not influenced by the surface. This indicates that the model tends to be drier in the upper-troposphere at night. Assimilation results may also be available at the time of the conference.