Not sure what weather forecast you’re looking at but checking the one I mostly use (yr.no) it doesn’t look too bad at all: https://www.yr.no/nb/v%C3%A6rvarsel/daglig-tabell/1-272810/Norge/Nordland/Moskenes/Moskenes

I’d say, bring raingear but do fly over and take it as it comes. You’ll most likely be able to find some decent weather either in the North or the South of Norway, so worst case scenario either bike south instead of North or skip the flight up to Lofoten completely and do a tour in the South of Norway. There’ll be people here able to help with last minute change of plans 🙂