IFR Probabilities give information about IFR Conditions that occur when terrain ascends up into the clouds, as happens sometimes in the Sierras. A stratus deck might exist over the Central Valley of California, but as that stratus extends to the east, the terrain rises up into the cloud, and IFR conditions result. The animation above shows IFR Probabilities during the night of 17-18 December 2014. Stations at elevation are not common, but Blue Canyon (KBLU) in Placer County is at 1600 meters above sea level. IFR Probabilities in/around north central Placer County (where KBLU is sited) are highest with the visibility is most restricted. Note in the animations that Truckee, CA, to the east of Blue Canyon, does not experience IFR Conditions, perhaps because it is east of the crest of the Sierras.
MODIS data occasionally gives very high-resolution information. The data below from 2100 UTC on 17 December and shows the IFR Probability banked up against the Sierras.