Complex sentence: A few dozen climbers from both sides made it the top in mid-May before the weather turned again; however, by late May the classic window before the monsoon season hits was predicted to open and the bulk of the expeditions began their summit bid.
Split into simpler sentences: A few dozen climbers from both sides made it the top in mid-May before the weather turned again. In late May the classic window before the monsoon season hits was predicted to open and the bulk of the expeditions began their summit bid.