Capt Claret, Ground Speed was 660 Kt against TAS 500 Kt, after the wind COMPONENT dropped by 80 Kt, the aircraft, due to momentum was initially still travelling at G/S 660 with 80 knots LESS Wind Component, thus a rise of TAS by 80 Kt, with commensurate rise in IAS and Mach No.
G-ANDY, Weather radar 'sees' precipitation, and cannot predict Clear Air Turbulence (We were well above the front). In our case we had plenty of warning from the SIGMET, and more particularly, the aircraft ahead which were having a bad time, hence our being at Turbulence Penetration Speed in advance. We, in turn, told the aircraft behind, didn't help the JAL guy much though!
Regards,
Old Smokey