When the crew contacted the Milan ACC, 20 min before the occurrence, they were already engaged in storm cell avoidance manoeuvres. Other crews were also avoiding storm zones. This weather situation had been forecast. The meteorological file contained information about it.The Milan ACC controllers regularly asked the crew to call back when they were able to head to BORDI waypoint, which is the point of tacit transfer between the Milan ACC and Nice approach.The controllers were not able to see the meteorological information on their screens. They could not see the exact location of the storm zones that prevented the crew from heading to the requested waypoint for over 20 minutes.After circumnavigating what they thought to be the last storm cell, the crew stated that they were able to head to BORDI waypoint. Approval was granted by the air traffic controller. On turning, they suddenly saw on the weather radar and had in sight, storm cloud developments embedded in the cloud mass, immediately behind the cell they had just circumnavigated. It is possible that these storm clouds may have been hidden by the previous storm cell and that the onboard weather radar was unable to detect them earlier. It is also possible that they could not be detected by the onboard radar when the cell was in the radar?s detection beam. Lastly, it is possible that they may have been developing and appeared only shortly before on the aircraft?s path. The crew were unable to avoid this cell and expected turbulence. They alerted the passengers. The aircraft passed through a zone of strong windshear and turbulence due to the presence of convective uplifts in a predominantly very strong southwesterly air flow.Flight through this zone was short but very turbulent. Strong vertical and horizontal accelerations associated with windshear and variations in speed followed in succession in the space of around seven seconds.It is very probable that it was at the beginning of this phase that a cabin crew member, who was busy securing equipment in the aft galley, was thrown to the ground. His foot then became trapped under an escape slide housing and his ankle broke during the vertical accelerations that followed.