Okay, here we go:
Here’s the (slightly) bad news:
The snippet tuning files for weather seasonal forecasts are world-specific, meaning you’ll have to repeat the process below for each world where you want to change the weather.
But it’s a good idea to start with changing just the weather of one world at a time, so you can test your mod in game to see if there are any errors, before putting in a ton of work.
I’ll use Willow Creek as the example since it’s a base game world, and that way, you or anyone else can follow along regardless of whether you have additional packs besides Seasons.
(1) After creating a blank package, click Tools - Extract Tuning. This may take a minute or so, and then you'll see a window pop up that looks like this:
(2) Search "snippet/weatherSeasonalForecasts_"
If you scroll through, you'll see a bunch of snippet tuning files labeled by season and by world. For most worlds, there's an individual tuning file for each season (Batuu and the Magic Realm appear to be exceptions). Right now, we only care about Willow Creek, so select all the snippet tuning files that end in Willow_Creek (there should only be four, one for each season) and click Add to Current Package.
(3) If you toggle back to your package file, you should now see these files listed. Select the one that ends in Summer_WillowCreek and toggle over to the XML tab, and you'll see this:
The information we care about is located in Lines 5-6, 9-10, and 13-14. Each Key pertains to part of a season: early, middle, or late. The value in the line below each key determines the potential forecasts for that part of the season. Indeed, each line of green text is actually the name of yet another tuning file, i.e. the weather forecasts lists. You can search them in the Extract Tuning window to pull them up and add them to your package if you want to peek inside, but it's not necessary for the steps here. For example, depending on what the weather forecast list tuning files say, you might have thunderstorms all throughout the summer, but you have the highest chance of thunderstorms in late summer (likely Days 6 and 7).
The easiest way to accomplish winter all the time is to simply borrow one of the three tuning weatherforcastlist files that the game uses for winter in Willow Creek and use it to replace the weatherforcast lists for early summer, mid summer, and late summer (and the same for fall and spring). (Note: Weatherforcast is not a typo here, but how the file is named in-game).
(4) Select the tuning file in your package that ends in _Winter_WillowCreek.
Let's go with Mid for now. Copy the text that's in the red selection.
(5) Now toggle back to the _Summer_WillowCreek file. Replace the text in the three red selections with your copied text.
(6) Repeat Step 5 for the _Fall_WillowCreek and _Spring_WillowCreek files.
(7) Save your package, move it to your mods folder, delete your localthumbcache.package file under your Sims 4 folder, open up a new save, and test your mod by picking a non-winter season and moving your Sim into a house in Willow Creek.
(8) You can convert the rest of the worlds this way by adding the other snippet/weatherSeasonalForecasts_ files (that don't end in Willow Creek) to your package, and repeating Step 5 across the spring, summer, fall, and winter* files. Now, why do I include the winter tuning files as necessary? Because not all worlds have snow or even wintery weather during the winter season. Prime examples are Oasis Springs and Selvadorada. Carl 4 Sims Guide has a chart of Weather by World located
here that is a handy reference if you don't feel like poking through the snippet/weatherforcastlist_ files for every season of every world.
What I've provided above is the least labor-intensive method that I could think to accomplish what you're looking for. The outcome of the above, if repeated across all the relevant tuning files for all season and all worlds, make it Willow Creek Middle Winter weather all season, every season (except for winter in Willow Creek, which will still cycle through the early winter, middle winter, and late winter playlists, unless you also copy and paste that line of text into Lines 6 and 10 of the weatherSeasonalForecasts_Winter_WillowCreek tuning file).
Before replacing all the weatherForcastLists for every world, I would encourage you to take some time to play-test with your file to make sure you're happy with the forecasts. If you're not, you can check out at the weatherForcastList tuning files for each world (search snippet/weatherforcastlist_winter in Extract Tuning, select all, and add to your package) and look for ones where snow is weighted more heavily. If you open up these files, you'll get a better idea of what exactly the winter weather forecast options are. For example, remember how I said there's no snow during winter in Oasis Springs? Here's why:
Not a single snow-related forecast in sight. EDIT: I spoke too soon. There's one snow-related forecast in sight. But it's one possibility against so many non-snow options, many of which are weighted more heavily than it (i.e. are more likely to happen).
The values you see here are what you actually see in game if you pull up the weather panel.
There are other methods that you can use instead of the steps outlined above if you want greater variety across worlds (i.e. winter for every season, but each world has a unique form of winter), greater variety as time moves from Day 1 to Day 7 of the season (low chance of blizzards on Days 1 and 2, medium chance of blizzards on Days 3-5, highest chance of blizzards on Days 6 and 7), or if you want to further customize the chance that you'll have particular weather (i.e. you want blizzards and only blizzards every day, or you want snow, in some form, every day without fail, etc.). In order to keep this post a reasonable length, I haven't detailed them all here but I can explain if you're interested.
Let me know if you have any questions.
