Rain + about to drop + no shelter = ARGH

[PoisonPen]

If you need to stop for the night while it's raining and you miscalculate the length of time to build a shelter, it becomes an extremely irritating process of repeatedly collapsing unconscious, then waking a few minutes later because of the rain, then trying to set up your shelter, then collapsing again, then waking up again, over and over and over.  And you can't do anything about it at that point.  Perhaps having rain wake you at weary instead of exhausted would be a good compromise.  At least then it wouldn't take you a dozen or more cycles of build-collapse-wake-build.

[Silenia]

Alternatively, it could also work to take surroundings into account for calculating how soon a character is likely to wake up from the rain. If you're under the canopy of a large tree, it might not be as good as a proper shelter but still protects the character from the weather a bit more than trying to sleep in the rain under the open sky would.