If you'rer rolling for random encounters, technically weather is not one of them. There's a table in the DMG (bah, no one uses 3E anymore so I won't even list the page number) which you roll on for the day's weather. Encounters are unexpected confrontations with danger or risk of some type, not another reason to complain about mucky Waterdhavian roads. It's not like the PCs will be free of meeting anything except for that 10% of the time. There are always rabbits, and birds, and travelers passing by. What an encounter, whether randomly generated or not, entails, is something which presents a problem of some sort that the PCs need to solve to earn XP, and entails some sort of reward/punishment factor. Sometimes that challenge is merely a group of peasants out for a hunt, who can be milked for information with a good skill check. Other times, it's a hungry bulette. Me, I don't roll randomly for encounter type, but I do for having the encounter happen in the first place. Then, I open the DMG and look at the monster-by-terrain charts, and then hand-pick a CR-appropriate and climate-appropriate monster from it.