• Mefariah has put great effort into implementing the summer season in Skyrim's snowy areas through their Seasonal Landscapes - Unfrozen mod. They have also provided a large number of patches to make this work with other mods, but the list is far from complete. I did some work to extend that list at my own patch page, Seasonal Landscapes - Unfrozen - More Patches, but there are still a lot of mods needing patching. This patch tool allows you to create your own patches for Unfrozen.
  

What this mod does:
Used correctly, this tool will remove snow shaders and alternate snow textures from all Statics, Movable Statics, Furniture, and Activators in your mod list during summer. This only covers objects placed in an exterior worldspace covered by Seasons, and excludes objects already covered by the _SUM.ini files in the Seasons folder. This means most Vanilla objects will not be form swapped, since they are already covered by the original Unfrozen mod, and your output will mostly contain assets from mods. Similarly, objects covered by other patches for Unfrozen will be ignored.

The mod also searches for meshes on the format filenameSUM.nif suffix, and will use those for summer in case the original object uses the mesh filename.nif. This is limited to objects that should have been form swapped anyway, but nevertheless allows users to create custom meshes for summer.

Moreover, the scripts will search your mod list for appropriate form swaps, and only create a new object in case none exists. This minimises the size of the created .esp file. In some cases, no new .esp file is even needed.

What this mod does not:
It will not remove snow textures applied directly to the nif files, unless you provide a custom mesh for it (see above) and it either has a snow material property, or an alternate snow texture set in the .esp file. The mod is also limited to the above classes, and thus does not cover Flora, Trees, Grass, and Land Textures (those usually require manual work anyway).

The mod also only does the form swaps, which while being a good starting step, often is not enough. Seasonal Landscapes - Unfrozen also does a lot of landscape edits to get rid of grass from places which should not have it, so many mods require patches to account for this (either to resolve landscape conflicts or to remove grass from areas added by the mod). You may also need to provide blacklist files in winter to get rid of snow in areas which should not have it (such as covered by roofs).

Installation instructions:

• Drop the content of the Seasons folder into the Data/Seasons folder in your Skyrim installation path and the script Generate unfrozen records.pas in the Edit Scripts folder in your SSEEdit installation directory.
• If you want to include more worldspaces than the ones included in Missing_worldspaces_SUM.ini (for instance if your mods add custom worldspaces), add those to all files named Missing_worldspaces_xxx.ini. These will also be covered by the Seasons framework in game, and thus need to be taken care of in DynDOLOD etc.
• You need to have Python installed for this step! Enter the Seasons folder and run the Python script unfrozen_finder.py. The easiest way to do this is probably to open a terminal window in the folder and type: python3 unfrozen_finder.py
•  Now you should have one file named unfrozen_data.txt and another one named sum_meshes.txt in the Seasons folder. Make sure that this is indeed the case.
• Open SSEEdit and load the mods you want to patch, or if desired, your entire mod list. Also make sure to have Seasonal Landscapes - Unfrozen.esp loaded, as well as Unfrozen - Extra resources.esp in case you have it.
• Right-click anywhere in the mod list, and select Apply script from the drop-down menu. Choose the script Generate unfrozen records.
• Here you have three configuration options, which you need to set before running the script:
  • processSingleFile: Leave this at false if you want to patch all currently loaded mods, set to true if you only want to patch objects from the last file loaded. The latter is useful if you want to create a patch for a single mod, but keep in mind that you may miss objects from other files in your load order if those are not already covered by another patch.
  • filepath: Here you need to set the full path to your Seasons folder in the Skyrim data directory. It is very important that this is set correctly.
  • outfileName: The name of your ini file, without the _SUM.ini suffix. You can leave this at the default value if patching your entire modlist (the initial '0' makes sure that it loads early), but make sure to change it if creating multiple patches - otherwise it will get overwritten.
  • patchAllRecords: By default, the script will check whether any references to the object are placed in the game world to reduce the output. However, this is not desired if you want to patch a plugin that uses Base Object Swapper to place snowy objects. In that case, you should set this to true. It is not recommended to do so for your entire mod list, since that may produce a very large output.
    
• Press OK, save the file and choose an appropriate name for the .esp file. Let the script run until it finishes (may take anything from a few seconds to several minutes, depending on your file list).
• Review the output file. Delete the record StockadeScaffoldStairs01_NoSnow if present (this will show up due to a Vanilla bug that has now been fixed by USSEP; also search for the corresponding form swap in the created .ini file and remove it). If the file now is empty, do not save it (this means the script has found appropriate form swaps for all your objects among the loaded files). Otherwise, make sure to set the ESL flag before saving, since I was not able to figure out how to do this automatically.
• Activate the .esp file and test in game.
  
  

Updating existing patches:

• Delete the generated .esp file you want to replace and the corresponding .ini file.
• Repeat steps 2-10 above.
  

I am done, what next?
First, review the output. You may need to do manual tweaks to e.g. get rid of snow added to the base meshes, or add more appropriate alternate textures to objects where those got removed. If you want to replace objects of classes not covered by this tool, you will have to do that manually.

Next, resolve any landscape conflicts in the Creation Kit, and replace the textures in places that should not have grass in summer with something else than Snow01 or Snow02 (I can provide hints on how to do this efficiently if requested).

Finally, go through your mods in winter and blacklist objects that should not have snow (This is work in progress also for me. I will at some point publish a framework that will help doing this efficiently. No promises about when though).

Mods not requiring any extra steps (save from blacklisting):
These are the mods I am using where running the script is enough, as far as I can tell (at least for the summer season):

• Bells of Skyrim
• Dlizzio's Mesh Fixes
• JK's Candlehearth Hall
• JK's New Gnisis Cornerclub
• JK's Septimus Signus's Outpost
• Skyrim Sewers
• Static Mesh Improvement Mod - SMIM
• Stretched Snow Begone Definitive Edition - Stretched Snow On Walls and Buildings Fix
• The Old Ways - Nordic Religion
• Weathered Road Signs Fixed
  

Recommended mods:
I recommend using my complementary files to the main Unfrozen mod, found here: Seasonal Landscape Additions. Especially the resource file is useful to load along with the mods you want to patch - otherwise you may get a large number of Vanilla objects in the output. If loading any extra worldspaces, make sure to overwrite the files included on this page (or edit them manually).

Not recommended:
Do not get any files from Seasonal Landscapes - Unfrozen Assorted Patches. This mod is making almost all of those patches completely redundant (maybe excluding Ultimate College of Winterhold and Farmhouse Chimneys, but I would still rather do manual work on those), and generates more well-implemented files that are consistent with the original Unfrozen mod. If you want to know more about the issues with those patches, check my own patch page.

Acknowledgements:
I would like to thank Mefariah for Seasonal Landscapes - Unfrozen and powerofthree for the entire Seasons framework.