Well, theoretically and off the top of my head, why couldn't we create a spell now that did this? The overlay graphic would be the rain or snow coming down, we could easily use the spell editor to lower the THAC0 (or whatever) and the duration would be set to last for a long time if desired. I'm not really familiar with the weather spells, so I'm not much more help than this.
Good ol' Wikipedia. It seems odd to me that only one GoldBox game would have weather in it's outdoor combats and that the game would not be Pools of Darkness (the last one made which did have some graphics improvements). Many of the changes for PoD made it into FRUA though, as it was still the engine used to create all of the GB (including Buck Rogers) games.
Maybe one of the FRUA folks knows...you might ask over in the FRUA community forums for a a more definitive answer.