How bad can the weather be before we have to turn around on summit day?

Strong winds, above around 50km/hr, can make the upper mountain a dangerous place due to the cold, getting blown around and the loss of a helicopter rescue option. Poor visibility in thick cloud means crevasses and steep snow slopes can not be negotiated efficiently. Fresh snow can present an avalanche risk and rain will elevate the stone fall risk in the Grand Couloir. Alpine electrical storms present a very real danger to the mountaineer. Any one of these weather factors are likely to result in being forced to retreat, or not leaving the hut at all, on summit day. Poor weather can be dealt with to a greater extent lower on the mountain, approaching the huts and for the training phase but good weather is required to climb Mont Blanc!