  Chromospheric activities prior to solar flares provide important clues to
solar flare initiation, but are as yet poorly understood. We report a
significant and rapid H$\alpha$ line broadening before the solar flare
SOL2011-09-29T18:08, that was detected using the unprecedented high-resolution
H$\alpha$ imaging spectroscopy with the Fast Imaging Solar Spectrograph (FISS)
installed on the 1.6 m New Solar Telescope (NST) at Big Bear Solar
Observatory.The strong H$\alpha$ broadening extends as a blue excursion up to
-4.5 A and as a red excursion up to 2.0 A, which implies a mixture of
velocities in the range of -130 km/s to 38 km/s by applying the cloud model,
comparable to the highest chromospheric motions reported before. The H$\alpha$
blueshifted broadening lasts for about 4 minutes, and is temporally and
spatially correlated with the start of a rising filament, which is later
associated with the main phase of the flare as detected by the Atmosphere
Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). The potential
importance of this H$\alpha$ blueshifted broadening as a preflare chromospheric
activity is briefly discussed within the context of the two-step eruption
model.
