An aircraft experienced turbulence near Guam due to a thunderstorm radar observation. The aircraft entered a severe turbulence area, resulting in accelerations of 1.8g to -0.4g, with no radar returns. The aircraft was flying in cirrus clouds with no lightning. The pilot initiated a diversion to Sydney after the turbulence, despite the aircraft appearing structurally and mechanically sound. The announcement prior to departure emphasized seatbelt fastening.An aircraft experienced turbulence near Guam due to a thunderstorm radar observation. The aircraft entered a severe turbulence area, resulting in accelerations of 1.8g to -0.4g, with no radar returns. The aircraft was flying in cirrus clouds with no lightning. The pilot initiated a diversion to Sydney after the turbulence, despite the aircraft appearing structurally and mechanically sound. The announcement prior to departure emphasized seatbelt fastening.