You can pick weather to fit your campaign or roll on the Weather table to determine the weather for a given day, adjusting for the terrain and season as appropriate.

Temperature:
d20 = 3

Normal for the season

Wind:
d20 = 2

None

Precipitation:
d20 = 1

None

Temperature
d20Temperature
1-14Normal for the season
15-171d4 x 10 degrees Fahrenheit colder than normal
18-201d4 x 10 degrees Fahrenheit hotter than normal
Wind
d20Wind
1-12None
13-17Light
18-20Strong
Precipitation
d20Precipitation
1-12None
13-17Light rain or light snowfall
18-20Heavy rain or heavy snowfall