Saturn’s great white storms play an important role in its atmosphere. In 2010, such a storm occurred in the northern hemisphere and encircled the whole planet with its clouds (Fischer et al., 2011; Sanchez-Lavega et al., 2011; Fletcher et al., 2011). An interesting phenomenon is that the storm-related bright clouds expanded to the south but not to the north. so that the bright clouds passed over the storm head on the northern side when it encountered the storm tail (Sanchez-Lavega et al., 2011, 2012; Sayanagi et al., 2013; Garcia-Melendo et al., 2013). Based on the wind and temperature fields retrieved from the Cassini ISS and CIRS observations (Sayanagi et al., 2013; Achterberg et al., 2014), we explore the mechanism behind this unsymmetrical expansion of bright clouds. Our analyses suggest that the large meridional gradient of the quasi-geostrophic potential vorticity, which can serve as a barrier of cloud mixing, can help explain the unsymmetrical expansion.