PCS:Weather changes rapidly when menus are activated
Oblivion's weather changes rapidly when menus are activated (i.e., when talking to NPCs, going into your inventory, etc.).
Causes and Solutions
Cause 1 - Fast-Changing Weather Bug
This was a problem with the original Natural Weather mod.
Solution #1: If you want to stick with NW, grab the fixed version, RD Natural Weather Unofficial Test Fix.