Though weather is mentioned a few times in various circumstances, there is no weather table. The closest I've found is the wind-speed table for ships, which is a step in the right direction, but doesn't tell me much about thunderstorms, tornadoes, blizzards, and other such weather phenomena.
That said, I've created a few tables to help out. Roll 1d20 for the appropriate season and location (Polar is arctic conditions, Tropics straddle the equator, and Temperate is between the two) to get precipitation:
Roll 1d20 to get temperature:
|Frigid||1||--||1||2-6||Thick ice on rivers and lakes; move 1/2|
|Cold||2-6||--||2-6||7-14||Thick ice on lakes, thin on rivers; movement 1/2|
|Hot||20||15-20||20||--||2x water consumption, movement 1/2|
For Polar, shift one step cooler (Warm becomes Cool); for Tropics, shift one step warmer (Warm becomes Hot).
Finally, check the chart below to see what weather occurs:
Windstorm/Blizzard: in desert or snow-covered, vision is reduced to 20' and archery is impossible. Movement is reduced by half. Tornado: along with weather from Very Strong Wind, there is a tornado, moving with the wind at 240'. Clear/Rain/Sleet/Snow: regular weather. Deluge: vision reduced to 20'; causes floods 10'/turn in valleys. Storm: vision reduced to 60'; chance of lightning strikes. Hailstorm: Anyone not under cover is hit with hailstones (as 2nd level fighter, 1d2 damage, every round).
Roll temperature once per week, wind once per day, and precipitation 4 times a day (morning, afternoon, evening, night).
This is just a rough draft... it seems to work ok, but it's an awful lot of rolling, and I'm not sure about some of the percentages. That, and it's possible to get sleet in the afternoon and an overnight tornado. And speaking of tornadoes, they, along with blizzards, storms, and frigid temperatures need fleshed out more - some way to handle the massive damage they dish out, but at random.