The present work investigates the extension twinning in rolled Mg alloy WE43 using a combination of scanning electron microscopy with digital image correlation (SEM-DIC) and crystal plasticity finite element (CPFE) simulation. Rolled Mg alloy WE43 was subjected to in-situ uniaxial compression along its rolling direction. Full-field displacement maps were gathered using SEM-DIC during load pauses, and twin variant maps were obtained from these displacements using post-processing analysis. CPFE was used to investigate the experimental results via a multi-scale twinning model developed for HCP polycrystals. In addition to stress-strain curves, crystal plasticity parameters were calibrated using the variation of twin area versus the applied strain to accurately capture the twinning parameters. A new SEM-DIC pipeline was also developed for the open-source PRISMS-Plasticity CPFE software that can read in the precise deformation map generated by SEM-DIC experiment as an input boundary condition for the finite element simulation and conduct the CPFE simulation. It is shown that CPFE can successfully capture the macroscopic response and model both strain and twin area fraction maps. However, the model cannot capture sharp strain localization and twinning bands, instead it smears certain areas of localizations.