  Hydrodynamic atmospheric escape is considered an important process that
shapes the evolution of sub-Jovian exoplanets, particularly those with short
orbital periods. The metastable He line in the near-infrared at $1.083$ $\mu$m
is a reliable tracer of atmospheric escape in hot exoplanets, with the
advantage of being observable from the ground. However, observing escaping He
in sub-Jovian planets has remained challenging due to the systematic effects
and telluric contamination present in ground-based data. With the successful
launch and operations of JWST, we now have access to extremely stable
high-precision near-infrared spectrographs in space. Here we predict the
observability of metastable He with JWST in two representative and previously
well-studied warm Neptunes, GJ 436 b ($T_{\rm eq} = 687~{\rm K}$, $R_{\rm p} =
0.37~{\rm R_J}$) and GJ 1214 b ($T_{\rm eq} = 588~{\rm K}$, $R_{\rm p} =
0.25~{\rm R_J}$). Our simulated JWST observations for GJ 436 b demonstrate that
a single transit with NIRSpec/G140H is sensitive to mass loss rates that are
two orders of magnitude lower than what is detectable from the ground. Our
exercise for GJ 1214 b show that the best configuration to observe the
relatively weak outflows of warm Neptunes with JWST is with NIRSpec/G140H, and
that NIRSpec/G140M and NIRISS/SOSS are less optimal. Since none of these
instrument configurations can spectrally resolve the planetary absorption, we
conclude that the 1D isothermal Parker-wind approximation may not be sufficient
for interpreting such observations. More sophisticated models are critical for
breaking the degeneracy between outflow temperature and mass-loss rate for JWST
measurements of metastable He.
