Just futzing around with a simple weather table to give temperature, precipitation, lightning and wind all with a single d100 roll, coupled with a d4 roll for duration in days. The idea was to create periods of weather with minimal dice rolls allowing a GM to quickly plot out an entire month's worth or even year's worth of weather prior to play.
The results are weighted to give a normal seasonal temperature more often than not. Little to no wind and precipitation are more frequent than heavy winds and precipitation. Heavy precipitation has about a 50% chance of being accompanied by lightning in the spring and summer months.
The GM has to use judgment to interpret certain outcomes. Several days of heavy precipitation in the winter months will bring deep snow, several days of heavy precipitation in summer months may bring about flooding in low-lying areas, etc. Changes in weather can be made as gradual or abrupt as necessary for the GM to create a natural feel to the effects.