In reply to Michael MacArthur:
I usually use a combination of :
MWIS SD
http://www.mwis.org.uk/english-welsh-forecast/SD/
Met Office
http://www.metoffice.gov.uk/public/weather/forecast/gcmnn3630
and
Windyty
https://www.windyty.com/53.105/-3.911?53.105,-4.488,9
In particular if you choose Windyty, centre on Capel Curig, choose the 'rain' overlay and play the animation you can watch for wind and rain at all the potential alternative sites -
NW Anglesey for Holyhead / Gogarth;
Central North Coast for Ormes / Craig y Forwyn
Tremadoc
and Llangollen for Clwyd Limestone
Additionally, the rock at the Moelwyns is usually grippy enough to climb when wet without affecting grade.