Apparatus and method for balancing the mass flow rate of molten glass flowing over a forming body to form a glass ribbon. The apparatus comprises upper and lower transition members that surround the glass ribbon as the glass ribbon is drawn from the forming body. The upper and lower transition members are independently moveable, and separated by a gap less than 8 cm across. The narrow gap minimizes disruption to the thermal environment contained within the upper and lower transition members, and is positioned a sufficient distance above the setting zone of the ribbon to minimize any influenceon the dimensional consistency of the ribbon.