In reply to FesteringSore:
I lived in Gruinard for 3 years. The first April was cold and clear, with lots of ice and snow and less wind and rain than usual. The second April was so dry that the riverbed at Dundonnell was almost completely dry and by May that year the mountains were like tinder (you remember the mountain fires in the Highlands in May 2011?) The third April it rained almost constantly, as it did everywhere else in the UK, everything was awash and there were lots of temporary new waterfalls throughout Fisherfield and Letterewe.
Does that give you any clues? - no! Go prepared for everything. Remember that it's likely to be wet underfoot and river crossings may be tricky; but take your ice axe, crampons and suncream too!