Recently I was trying to help someone out that did not want it to ever snow in Solstheim due to his computer not being able to handle the snow. I thought to myself "OK, this sounds easy enough." So I went and made a little mod that changed the weather settings for the following regions (numbers are before/after):

Felsaad Coast Region
Clear 30/40
Cloudy 10/15
Foggy 15/20
Overcast 15/25
Snow 20/0
Blizzard 10/0

Isinfier Plains
Clear 10/30
Cloudy 10/20
Overcast 30/50
Snow 30/0
Blizzard 20/0

Moesring Mountains
Clear 10/25
Cloudy 10/25
Overcast 20/50
Snow 20/0
Blizzard 40/0

Sadly that attempt failed. Yes it was the very last thing in the load order, and yet I still had snowstorms all over the place. No clue why this one is not working correctly. So for the second attempt I went the scripting route and made the script be a startscript:

Begin Solstheim_No_Snowstormsmodregion "Brodir Grove Region" 20, 25, 35, 20, 0, 0, 0, 0, 0, 0modregion "Felsaad Coast Region" 40, 20, 20, 20, 0, 0, 0, 0, 0, 0modregion "Hirstaang Forest Region" 20, 40, 0, 40, 0, 0, 0, 0, 0, 0modregion "Isinfier Plains Region" 30, 20, 0, 50, 0, 0, 0, 0, 0, 0modregion "Moesring Mountains Region" 25, 25, 50, 0, 0, 0, 0, 0, 0, 0modregion "Thirsk Region" 20, 25, 35, 20, 0, 0, 0, 0, 0, 0stopscript Solstheim_No_SnowstormsEnd Solstheim_No_Snowstorms

Interestingly this one works perfectly (I hit all the regions in this one because in my game I had snow in Thirsk so I figured may as well pre-empt whatever other mod messes with weather settings or it would defeat the purpose of the mod).

https://www.dropbox.com/s/ffftt2j1gq7jp38/Solstheim%20No%20SnowStorms.7z for any who are interested.

Had you already visited those regions in your current game before plugging in the mod? If so, it is likely the save already has the default weather and your mod's weather settings are ignored. I suspect if you cleaned your save of those records you mod would work fine. Of course, you can test it with a new game to know if it works.

The reason your scripted solution works (and may be the best solution in any case) is that it modifies the existing weather settings whether they are the default values or assigned by a mod.