  We study the coherent propagation of light whose dynamics is governed by the
effective Schr\"{o}dinger equation derived in a magneto-optically-manipulated
atomic ensemble with a four-level tripod configuration for electromagnetically
induced transparency (EIT). The small transverse deflection of an optical beam,
which is ultra-sensitive to the EIT effect, could be drastically amplified via
a weak measurement with an appropriate preselection and postselection of the
polarization state. The physical mechanism is explained as the effect of
wavepacket reshaping, which results in an enlarged group velocity in the
transverse direction.
