Disclaimer: this mod was partly made with Gemini AI, i made it for myself but now i am posting it because i figured someone might be interested in it. 

This mod basically blocks the automatic chunks updates from removing snow, and implements new melting mechanics (snow remains on mountains and cooler zones, north slope shadow calculations). Grass with snow layers, loose rock, loose flint, loose sticks, loose boulders are also not updated by the game and managed by the mod's mechanics.
This comes at a small cost: unloaded chunks will not automatically lose their snow if you reload them again in spring/summer, but there are special measures to counter this, for example if the temperatures are high enough the snow in nearby chunks will be totally molten in around 100 seconds, at default settings.

The mod works with a "scanner" that performs calculations on blocks on the surface, in nearby chunks, to check their temperature, shadow level and melting chance.
There are 3 shadow levels: 30 degree shadow (existance of a solid block face facing north on the block just south of the examined block), 45 degrees shadow (the solid north block face needs to be located 2 block southward and 1 block upward, relative to examined block), 60 degrees shadow (the solid north block face need to be located 1 block southward and 1 upward, relative to examined block).

The mod is extremely configurable, just about any factor i used in the calculations can be changed. This of course includes time interval between scans, radius of scan, blocks checked per scan, every temperature threshold used by the mod, every melting chance used by the mod. I warn you that on default settings, snows in shade sticks around for long.

Now, this mod will lag a little bit when heavy melting happens during spring. It is not the scans that are the source of this, but more so the block updates. If you want to lower lag, in the config you can tweak certain parameters to lower lag (having less scans but melting more often, just to name one solution). Feel free to experiment and figure out what config work out best for you.

Compatibility: the mod shouldn't have problems with other modded blocks, but just to be safe i reccomend creating a backup of your world before using it. Also, if you have other mods that change snow behaviour, it might not load properly. Since this mod does not add any new blocks, uninstalling from a world in which you used it will simply cause the default vintage story system to take care of snow.