  We report on a strong outburst of the W49N H2O maser observed with VERA.
Single-dish monitoring with VERA 20 m telescopes detected a strong outburst of
the maser feature at V_LSR = -30.7 km/s in 2003 October. The outburst had a
duration of ~100 days and a peak intensity of 7.9 x 10^4 Jy, being one of the
strongest outbursts in W49N observed so far. VLBI observations with the VERA
array were also carried out near to the maximum phase of the outburst, and the
outburst spot was identified in the VLBI map. While the map was in good
agreement with previous studies, showing three major concentrations of maser
spots, we found a newly formed arc-like structure in the central maser
concentration, which may be a shock front powered by a forming star or a star
cluster. The outburst spot was found to be located on the arc-like structure,
indicating a possible connection of the present outburst to a shock phenomenon.
