  The James Webb Space Telescope (JWST) will measure exoplanet transmission and
eclipse spectroscopy at un-precedented precisions to better understand planet
structure, dynamics, chemistry and formation. These are essential tools on the
march towards biosignature searches on potentially habitable planets. We
explore a range of exoplanet atmospheric conditions and forecast the expected
results with JWST. We take realistic CHIMERA models that match existing Spitzer
and HST results and simulate the spectra achievable with the JWST MIRI + NIRCam
Guaranteed Time Observations (GTO) survey. We then retrieve atmospheric
parameters from these spectra to estimate the precision to which the planets'
atmospheric compositions can be measured. We find that emission spectra have
well-constrained unimodal solutions but transmission spectra near 10X solar
abundance and solar C/O ratios can suffer from bimodal solutions. Broad
wavelength coverage as well as higher precision data can resolve bimodal
solutions and provide dramatically better atmospheric parameter constraints. We
find that metallicities can be measured to within 20% to 170%, which approaches
the precisions on Solar System planets, and C/O ratios can be constrained to
~10% to 60%, assuming that observers can leverage short wavelength data to
select the correct solution from the bimodal posteriors. These compositional
precisions are sufficient to validate or refute predictions from disk formation
models on final atmospheric abundances as long as their history is not erased
by planet evolution processes. We also show the extent to which eclipse mapping
with JWST is possible on our brightest system HD 189733 b.
