Randomly-generating weather for a sandbox campaign

I’m running a sandbox campaign and I’ve been looking for a method of randomly determining the weather for the day.

Features I’m looking for:

  1. Low-to-medium handling time during play. Big tables are fine if they’re easy to use. I’m fine if the system needs to be tuned or setup for my specific setting before play, but during play I want to quickly determine the results and move on.
  2. It has to give concrete results that I can immediately apply and don’t require further interpretation. (I’m not interested in systems that merely suggest a vague weather state and leave me to fill in the details.) For example, “a bit hot, heavy rain, no winds” is something I can immediately apply to the situation of “they’re in the badlands, so flash flooding is now a possible hazard”. A result of “extreme weather” requires too much interpretation – it could mean the same but could equally mean tornadoes or blistering heat or something else.
  3. It has to take into account the prevailing weather, including seasons and local climate.
  4. My game is analogue, pen-and-paper only during play, though if a weather system uses a computer for before-play setup (say, to generate custom per-area tables) that’s just fine.
  5. Ideally, it wouldn’t require pre-generating specific weather results. When we sit down for a session, I don’t have any idea whether play will encompass a few in-game hours or months of in-game time. (If it can pregenerate months of weather for multiple locations in a compact, printable form, that might work.)

My motives for wanting concrete results to work with are to a) eliminate my bias and avoid unnecessary DM fiat, and b) have weather trends emerge that even I don’t expect.*

I don’t just want “set dressing” for overland travel, I want weather because it’s important to a medieval-ish society for agriculture and warfare – and therefore politically too. I want to be able to roll and tell the players something like, “The drought continues, and the desperate villagers demand your magic-workers do something. They’re angry and superstitious…”, or “The pass to the north is still under lots of snow. You’re unlikely to get a lowlands army through there intact until the thaw.” I want those sorts of things to naturally arise without me deciding to run a “drought plot” or capriciously blocking the players’ plans.

I have the Wilderness Survival Guide† and I like the system in it conceptually (it hits features 2 and 3 easily), but its heavyweight implementation looks like it would be a pain to use quickly at the table. (I could be wrong. Telling me how you’ve make it work for your game would be a good answer.) There are lots of suggested systems floating around on the blog-o-tubes, but they’re either too simple and suited only to set dressing or don’t take into account existing weather. The WSG system’s results are my ideal, but I’m looking for something that’s better-designed to not have a ridiculous handling time.

I’m not looking for ways to make weather important since I’ve got the tools for that already, although if that’s integrated into the weather system that’s great.

Something you’ve made up, something posted online, or a specific print reference are all fine, though for print I’m not likely to accept an answer until I can find a copy so I can evaluate the system.

(This post is tagged dungeons-and-dragons because that’s the style of game I’m running, but weather systems are often game-system agnostic so it gets that tag too.)

* Also, I live in a rainforest climate so my sense of what is “normal” weather is less than useful.
† My first-ever RPG purchase.