In squares 2C and 3C the freezing level in the warm sector is above 7,000ft and in the cold sector it is below 7,000ft.   At FL40 the aircraft would be very close to the cold sector freezing level and if it is slightly above the freezing level this would be the most likely area to encounter rain ice.   Thunderstorms are not usually associated with warm fronts.   There might be light to moderate turbulence on the frontal boundary but not ahead of the front.