When the published MSA will not provide terrain clearance.
I know this is the ATC forum but I just realized another ramification of cold weather altimetry. On our approach charts (Jeppesen) we have published MSAs that provide terrain separation within 22NM/25SM of the stated reference point. The published MSAs are not valid when the aerodrome is significantly colder than ISA. If the MSA for a sector is 4,000’ the clearance above the underlying terrain will decrease as the temperature decreases if you fly the same indicated altitude.