‘All weather’ means ‘works well in most weather’ the same way ‘all terrain’ means ‘works well on most terrain’. An aircraft that’s described as being all weather will have a wider operating range than one that isn’t, but it will still have limits to what it can do.
Water absorbs IR wavelengths, so any device that uses that part of the electromagnetic spectrum will be degraded by its presence. Thermal imagers, IR missile seekers, laser rangefinders/designators, ‘nightvision’ devices(image intensifiers), and IR illuminators are all affected by clouds, rain, snow, etc. Using thermals to spot targets through clouds is going to still work better than using your eyes because your targets will be glowing, so you’ll notice them sooner. The FCR should be extremely resistant, if not nearly immune, to very bad weather, so consider using that over the TGP to conduct attacks when there’s low clouds over the target.