GOES-R IFR Probability Fields computed from GOES-13 on 17 February 2015, times as indicated (Click to enlarge)

Different kinds of fields are used in the computation of IFR Probability fields: fields that can change quickly with time (Satellite observations of brightness temperature difference, satellite observations of cloud type, model output showing low-level moisture); fields that can change slowly with time (Sea surface temperature and surface emissivity fields), and fields that don’t change (Topography). An IFR Probability field that is relatively constant with time, then, as over northern Alabama in the animation above, is showing the effects of terrain (color shaded in the image below). In this case, higher terrain is on either side of the Tennessee River Valley in northern Alabama is apparent in the IFR Probability fields (Lowered ceilings are more likely over the higher terrain than over the adjacent, lower river valley). Transitory patterns associated with propagating weather features are also apparent.

Topography over northern Alabama and surrounding states (Click to enlarge)