when in Chamonix I advise you go through La maison de la Montagne ( or similar name ) which is in the same building than the Guides Office
they have an office open to public where they advise about the current conditions in Mont Blanc Massif from the feed-back of other people, of guides, of PGHM / CRS ( the rescue teams ), etc ...
it is free of charge and you can also have access to many topo's and see some nice 3D scaled models of Mont Blanc area
also, the latest weather forecasts are punched on the wall by the door
it is in the center of Chamonix, not far from tourism office and from the church