(RJ Lambert)

RJ Lambert created a random weather generator for his campaign in Clyster. The concept is fairly simple: roll d20 to find out what the weather will be based on a predetermined chart, which has roll modifiers depending on geographical location and season.

Here's the base table:

-1	Heavy Snow
0	Snow
1	Snow
2	Frigid extended rain
3	Heavy extended rain
4	Heavy extended rain
5	Light extended rain
6	Scattered showers
7	Frosty
8	Chilly
9	Nippy
10	Breezy
11	Hailstorm (roll d6, occurs on 1-3, otherwise fine)
12	Pleasant
13	Warm
14	Warm
15	Balmy
16	Humid heat
17	Dry heat
18	Searing heat
19	Scorching heat
20	Tropical downpour
21	Heatwave
22	Heatwave

The modifiers then for seasons, location and the previous day's weather:

Clyster global modifier -3
(ie colder geographical location, tends to colder end of scale)

(for your own campaign base you could, of course, have a different global
(ie permanent) modifier)

Summer +4
Spring +2
Winter -4
Autumn -2

Of course you could tinker with the seasonal modifiers to suit the
geographical location, or even mess about within the season so early Spring is
0, midSpring +1 and Spring 3rd month +2, morphing to Summer month 1
+3 etc.

Prev. day >19 +2
Prev. day >11 +1
Prev. day <9 -1
Prev day<2 -2

It must be remembered that these are for general GM inspiration and not
hard and fast. The Previous Day modifier I introduced to prevent wild swings
in weather variation (ie beyond reason). Again, the GM can tinker but it
would be unbelievable, for instance, to go from Snow (1) to Tropical
downpour (20) in one day, a -2 for the extreme weather on the previous day
would pull that back to Searing Heat but, after all, it was a 20. Deep in winter
such swings to considerable heat would be nigh on impossible.

Who knows, perhaps I have worked out the way of predicting real weather?!
(in any case, more predictable than the weathermen!)