MWIS give a short narrative near the top of each forecast of what area is covered by each forecast:
Northwest Highlands:
MWIS wrote:Areas north from Knoydart in the west, and the Great Glen towards the east (NB. Does not include Mull and areas west of Loch Linnhe, these are found in the West Highlands forecast.)
Western Highlands:
MWIS wrote:Western Highlands accessible from, and south of, Glenfinnan (Road to the Isles) and Glen Spean (includes Creag Meagaidh). This area include Ben Nevis and the mountains around Glencoe. In the east, includes Ben Alder south to Loch Lomond and Trossachs NP. Includes Arran and Mull.
Worth checking Mountain forecast too:
https://www.mountain-forecast.com/maps/scotland