There is a program SHweather , which allow weather randomization and other things..
http://www.subsim.com/radioroom/showthread.php?t=103202
but i did the changes manually.
these are the stock values of the campaign RND SCR LND layers
[Mission]
Fog=0
FogRand=0
Clouds=0
CloudsRand=0
Precip=0
PrecipRand=0
WindHeading=0
WindSpeed=5.000000
WindRand=0
WeatherRndInterval=5
SeaType=0
(You start in calm weather always)
This is the convoy academy attack weather,and i use it, curiosly almost every single mission has different weather setup and values, so you can try other if you want .
[Mission]
Fog=0
FogRand=0
Clouds=1
CloudsRand=1
Precip=0
PrecipRand=0
WindHeading=95
WindSpeed=2.000000
WindRand=1
WeatherRndInterval=2
SeaType=0
Some more information about what is every thing
*Fog=0 (0,1,2,3)
FogRand=0
*Clouds=1 (0,1,2)
CloudsRand=1
*Precip=0 (0,1,2)
PrecipRand=0
*WindHeading=95 (o to 360º)
*WindSpeed=2.000000 (0 to 15.000000 knts)
WindRand=1
WeatherRndInterval=2 (in hours, minimum 2 to 96 h)
SeaType=0
values in ( ) are different options for every thing , for example , clouds 0,1,2
none, partial, heavy
cloudsrand= no changes 0
small changes 1
heavy changes 2
precipRand = no changes 0
small changes 1
heavy changes 2
fogrand = no changes 0
small changes 1
heavy changes 2
winrand= no changes 0
small changes 1
heavy changes 2