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.