  We present a spectroscopic catalog of the 1,564 brightest (J<9) M dwarf
candidates in the northern sky, as selected from the SUPERBLINK proper motion
catalog. Observations confirm 1,408 of the candidates to be late-K and M dwarfs
with spectral subtypes K7-M6. From the low \mu>40 mas/yr proper motion limit
and high level of completeness of the SUPERBLINK catalog in that magnitude
range, we estimate that our spectroscopic census most likely includes >90% of
all existing, northern-sky M dwarfs with apparent magnitude J<9. Only 682 stars
in our sample are listed in the Third Catalog of Nearby Stars (CNS3); most
others are relative unknowns and have spectroscopic data presented here for the
first time. Spectral subtypes are assigned based on spectral index measurements
of CaH and TiO molecular bands. A comparison of spectra from the same stars
obtained at different observatories however reveals that spectral band index
measurements are dependent on spectral resolution, spectrophotometric
calibration, and other instrumental factors. After systematic corrections and a
recalibration of the subtype-index relationships, we find that we can
consistently and reliably classify all our stars to a half-subtype precision.
The use of corrected spectral indices further requires us to recalibrate the
\zeta parameter, from which we estimate metallicities +/-0.5dex accuracy in
dwarfs of subtypes M2 and earlier, and +/-0.2dex accuracy for later (M3-M5)
subtypes. Existing geometric parallax measurements are extracted from the
literature for 624 stars, and are used to determine spectroscopic and
photometric distances for all the other stars. Our catalog will be most useful
to guide the selection of the best M dwarf targets for exoplanet searches, in
particular those using high-precision radial velocity measurements.
