The FIM is being developed as a candidate to eventually supplant the Global Forecast System, the United States's current medium-range forecast model. The FIM was originally slated to become operational some time in 2014 (as of spring 2014 the model is still testing); the GFS will continue to be run and maintained for several years afterward, much in the same way the GFS and its predecessor, the Nested Grid Model, ran concurrently for several years. The model currently produces similar results to the GFS, but runs slower on the NWS's operational computers. Its three-part name derives from its key features: "flow-following" indicates that its vertical coordinates are based on both terrain and potential temperature (isentropic sigma coordinates, previously used in the now-discontinued rapid update cycle model), and "finite-volume" describes the method used for calculating horizontal transport. The "icosahedral" portion describes the model's most uncommon feature: whereas most grid-based forecast models have historically used rectangular grid points (a less than ideal arrangement for a planet that is a slightly oblate spheroid), the FIM instead fits Earth to a truncated icosahedron, with twelve evenly spaced pentagons (including two at the poles) anchoring a grid of hexagons.
The FIM runs as a multiscale model, with a suffix number indicating the model's horizontal resolution. FIM7 operates at a spatial resolution of approximately 60 km, FIM8 at 30 km, FIM9 at 15km and FIM9.5 at 10km. The FIM7 and FIM8 both run twice daily (0z and 12z) with 6-hour temporal resolution out to 14 days. The FIM9 runs four times daily, also with 6-hour steps, out to 7 days. (FIM9.5 is not currently in operation.)