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{{Short description|Experimental numerical weather prediction model}}
{{unreferenced|date=February 2015}}
The '''Flow-following, finite-volume Icosahedral Model''' (FIM) is an experimental [[numerical weather prediction]] model that was developed at the [[Earth System Research
The FIM was 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 but never did so. The model 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 method|"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 [[Goldberg polyhedron]] with [[icosahedral symmetry]], with twelve evenly spaced pentagons (including two at the poles) anchoring a grid of hexagons.
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