Accurate models for the onset of nucleate boiling, density of active nucleation sites (N-a), bubble departure size (D-d), and departure frequency (f(d)) are essential to the success of computational fluid dynamics analysis of two-phase thermal-hydraulics involving subcooled flow boiling in nuclear reactor systems. This work presents an experimental study of subcooled flow boiling in a vertical upward narrow rectangular channel that mimics the flow passage in the plate fuel assembly of boiling water reactors. The experiments are conducted over a range of mass flux (G=122-657kg/m(2)s), inlet subcooling (T-sub=4.7-33.3C), and heat flux (q=1.7-28.9W/cm(2)). Based on the experimental data, empirical correlations are developed for the prediction of onset of nucleate boiling, N-a, D-d, and f(d) for given flow conditions. These correlations are valid in the nucleate boiling regime when the wall superheat is less than 12 degrees C and can be incorporated in the computational fluid dynamics codes to enable more precise simulation of subcooled flow boiling heat transfer and two-phase flow in nuclear energy applications.