Wait until next winter and reshoot?
The clone and the heal tool are what LR has to fill in areas of an image with other areas of an image.
Is the problem that you can’t cover over the snow without dark areas showing through, or that there isn’t enough snow to overlay the non-snow areas without the duplication of areas being obvious?
<Wait until next winter and reshoot?>
This is the first time in 15 years this place has been accessible with snow on teh ground! Waiting longer would be ... disappointing.
<...there isn’t enough snow to overlay the non-snow areas without the duplication of areas being obvious?>
As Trevor might have been suggesting, about the only thing to do would be to create a larger image as a composite, using some other tool, from your image and one from the internet with alot of similar-looking snow, then you can import that composite into LR and use the other image as the source for the snow.
Something you could try if you are reasonably skilled with masks and brush control is to make templates that follow the contors of the terrain, and in a new layer, paint black for shadows and white for highlights with a fully soft brush. Then turn that layer off, and paint in the snow in a layer between the BG and B&W layer, with an off-white brush (About 240, 240, 240). Now turn the B&W layer back on and set its blend mode to Overlay.
Thank you, ssprengel and Trevor Dennis.
You gave me some good ideas. I might actually try a combination - find or shoot an all-snow photo, use it as layer and use a mask to paint in from this new layer selectively.