The weather in Scotland is unpredictable year-round. It can be cloudy and windy at any time; the sun may shine; rain, hail or sleet may fall – and all of these may happen within half an hour. Be prepared for anything, and often you will get a pleasant surprise. Visit the Met Office for a weather forecast before deciding what to carry with you each day.
It is vital not only to carry a waterproof top layer, but also to know that you can rely on it. Long before any multi-day expedition, test your waterproofs so you have time to reproof or replace. You are unlikely to find a suitable shop once you have set off. Be sure to take a waterproof cover for your rucksack.
The Mountain Weather Information Service (MWIS) provides specific forecasts (including wind strength, cloud level and rain/snow predictions) for the south-eastern Highlands here. Note that wintry conditions can occur at almost any time of year.