New experimental observations on the trigger mechanisms for the departure from nucleate boiling in subcooled flow boiling in a vertical, rectangular channel with an inner diameter of 40 mm are presented. A critical review of available mechanistic models and trigger mechanisms is given and a comparison to the experimental results is presented. Experimental results are derived using a matrix of complementary measuring techniques to review the validity of the proposed mechanisms for flow boiling of Novec 649. Mass fluxes are in the range of 500-2000 kg m(-2) s(-1) and subcoolings range from 27 to 9 K. A conceptualization for a refined phenomenological modeling approach is presented, and possible future research discussed. (C) 2015 Elsevier Ltd. All rights reserved.