We present a high-precision mass model of the galaxy cluster MACSJ1149.6+2223, based on a strong-gravitational-lensing analysis of Hubble Space Telescope Frontier Fields (HFF) imaging data and spectroscopic follow-up with Gemini/GMOS and VLT/MUSE. Our model includes 12 new multiply imaged galaxies, bringing the total to 22, comprised of 65 individual lensed images. Unlike the first two HFF clusters, Abell 2744 and MACSJ0416.1-2403, MACSJ1149 does not reveal as many multiple images in the HFF data. Using the Lenstool software package and the new sets of multiple images, we model the cluster with several cluster-scale dark-matter halos and additional galaxy-scale halos for the cluster members. Consistent with previous analyses, we find the system to be complex, composed of five cluster-scale halos. Their spatial distribution and lower mass, however, makes MACSJ1149 a less powerful lens. Our best-fit model predicts image positions with an RMS of 0.91. We measure the total projected mass inside a 200~kpc aperture as ($1.840pm 0.006$)$times 10^{14}$M$_{odot}$, thus reaching again 1% precision, following our previous HFF analyses of MACSJ0416.1-2403 and Abell 2744. In light of the discovery of the first resolved quadruply lensed supernova, SN Refsdal, in one of the multiply imaged galaxies identified in MACSJ1149, we use our revised mass model to investigate the time delays and predict the rise of the next image between November 2015 and January 2016.
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