This paper uses a combination of numerical and theoretical techniques to evaluate dynamical threshold for cavitation in water triggered by weak tension. Within the test section of a once-through, nonrecirculating cavitation tunnel, a cylinder is installed, behind which cavitation occurs, and once cavitated, water cannot return to the monitoring section. The single-phase water flow has been simulated using an LES-based CFD solver to observe velocity distribution at the throat, pressure distributions before and after the throat, flow separation, and vortex formation downstream of the separation point. Bubble dynamics analysis using the Rayleigh-Plesset equation is conducted for conditions under which bubble nuclei supposed to exist in water grow according to CFD-based pressure histories and along streamlines, revealing that nuclei with O(1 μm)-radius suddenly start growing beyond the throat, largely depending on the streamlines. Finally, dynamical cavitation threshold theory is successfully applied to predict nuclei growth, resulting in rather good agreement with the bubble dynamics analysis.