Ensemble forecasts with 101 members, including one ensemble mean, using ensemble Kalman filter analysis were performed to understand the atmospheric conditions favorable for the development of a meso-β-scale vortex (MBV) that caused shipwrecks as a result of sudden gusty winds in the southwestern part of the Sea of Japan on 1 September 2015. A composite analysis was conducted to reveal the differences in the structure of the MBV and atmospheric conditions around the MBV between the strongest 8 (STRG) and weakest 10 (WEAK) ensemble members. Two of the strongest ten members that developed the MBV much earlier than the other members were excluded from the analysis. The analysis revealed that the near-surface cyclonic horizontal shear to the northeast and the south of the MBV was stronger for STRG than for WEAK. In addition, larger low-level water vapor and its horizontal flux for STRG contribute to greater convective available potential energy to the southeast of the MBV, which results in stronger convection around the MBV. The results of the composite analysis are statistically supported by an ensemble-based sensitivity analysis. Differences in the near-surface horizontal shear were closely related to the structure of the extratropical cyclone in which the MBV was embedded. Although the strength of the extratropical cyclone for STRG was comparable with that for WEAK, the cyclonic horizontal shear of winds in the northeastern quadrant of the extratropical cyclone was greater for STRG than for WEAK.