By 18 UTC September 21, Vicki was a typical tropical cyclone that had strong horizontal convergence in the boundary layer and horizontal divergence near 150 hPa level, moving northward. At 00 UTC September 22, horizontal divergence spread to the north of Vicki in the mid-troposphere, although the dynamic tropopause was not lowered above it. Since there was an upper-level trough (PV maxima) west of Japan, southwesterly flow downstream of it and the upper- and middle-level warm core of the typhoon yielded horizontal warm advection and mid-tropospheric frontogenesis to the north of Vicki. In the same area, horizontal convergence in the lower layer was also increased in magnitude. It may have contributed to acceleration of movement of Vicki and decay of axisymmetrical structure. The upper-level trough was associated with dry air in the mid-troposphere. It also led to sudden dissipation of the warm core and extratropical transition.
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