Karen Overall describes that feline issues with the litterbox most frequently boil down to 4 categories:
- Preference for a specific litter
- Dislike of a specific litter
- Preference for a location
- Dislike of a location
If we take a look at these possibilities in light of your observations:
The litter you're using may normally be fine, but when it's cold or wet outside, the feel may change. Bear in mind that litter is absorbent and extra humidity or moisture in the air could cause a change in how it feels to the cat sufficient for it to not want to use it.
With respect to location, that too may be fine most of the time. However, you noted that the presence of guests and that may make the cat skittish and reluctant to leave your presence at night, resulting in elimination in your bedroom. In addition, with respect to bad weather, the box and the immediate location may be covered, but is the path? Cats don't like getting rained on if they can avoid it.
If you're unable, or unwilling, to provide an indoor location for the cat which would, I have to admit, probably eliminate the undesirable behaviour, then a couple of things to try are:
Change the litter to one that is less reactant to local climate conditions. You may need to get one that is less absorbent or is better suited to outdoor use. Check with your supplier store or online to see if there are some options that might help.
Look at a litter attractant. Basically, it's an additive to the litter that encourages the cat to use the box. We had a huge issue with inappropriate elimination for quite some time with a couple of our cats and the use of this has virtually eliminated it.
Try to socialize your cat with the guests as much as you can. If their presence is leaving your cat reluctant to leave the bedroom, then you may have to not let him in.
Provide a covered means to get the litter area on bad days. Like I noted, the cat is going to be very disinclined to wander into the rain to do his business, make it easier.
If none of that helps or works, the you could take him into see a behavioural specialist. My feeling, though, is that the outdoor litterbox is counterproductive to the desired behaviour of him using it and they'll probably tell you that.