  We are on the verge of characterizing the atmospheres of terrestrial
exoplanets in the habitable zones of M dwarf stars. Due to their large
planet-to-star radius ratios and higher frequency of transits, terrestrial
exoplanets orbiting M dwarf stars are favorable for transmission spectroscopy.
In this work, we quantify the effect that water clouds have on the amplitude of
water vapor transmission spectral features of terrestrial exoplanets orbiting M
dwarf stars. To do so, we make synthetic transmission spectra from general
circulation model (GCM) experiments of tidally locked planets. We improve upon
previous work by considering how varying a broad range of planetary parameters
affects transmission spectra. We find that clouds lead to a 10-100 times
increase in the number of transits required to detect water features with the
James Webb Space Telescope (JWST) with varying rotation period, incident
stellar flux, surface pressure, planetary radius, and surface gravity. We also
find that there is a strong increase in the dayside cloud coverage in our GCM
simulations with rotation periods $\gtrsim 12 \ \mathrm{days}$ for planets with
Earth's radius. This increase in cloud coverage leads to even stronger muting
of spectral features for slowly rotating exoplanets orbiting M dwarf stars. We
predict that it will be extremely challenging to detect water transmission
features in the atmospheres of terrestrial exoplanets in the habitable zone of
M dwarf stars with JWST. However, species that are well-mixed above the cloud
deck (e.g., CO$_2$ and CH$_4$) may still be detectable on these planets with
JWST.
