Which condition makes FLEX takeoff prohibited?

The condition that makes FLEX takeoff prohibited is when warnings of windshear or downdrafts have been forecast. This is because FLEX thrust is a reduced thrust setting that allows for a smoother takeoff while still providing necessary power. However, it is crucial to maintain full power capability when facing the potential hazards of windshear and downdrafts, which can create sudden changes in aircraft performance.

Using FLEX thrust in such conditions could compromise the aircraft's ability to respond effectively to sudden wind changes, putting the safety of the flight at risk. Therefore, when weather conditions indicate potential windshear or downdrafts, pilots are required to use full thrust to ensure adequate performance and safety during takeoff.