Predictability and a possible tropospheric precursor of a zonal-wavenumber (WN) 1 stratospheric sudden warming (SSW) event occurring in December 2001 are examined using all ensemble members of the 1-month forecasts carried out by the Japan Meteorological Agency (JMA). The SSW is predictable from at least 2 weeks in advance, and high sensitivity to the initial condition for the SSW prediction is observed during the onset period of the SSW. Dipole zonal wind anomaly pattern along 80¿N/60¿N in the upper troposphere during this period is significantly related to the subsequent warming in polar stratospheric regions through deflecting the WN 1 propagation poleward. The leading EOF of predicted zonal-mean zonal wind variations among ensemble members for the onset period also shows the similar profile. Thus, this coincidence would be responsible for the high sensitivity to the initial condition observed during the onset period of the SSW.