One of only two passengers on a Beech 99, early morning flight. The crew left the curtain unzipped, so I could see we were closing on a crescent-shaped echo on the weather radar. Throttles well forward. When we hit the leading edge of the echo, things got interesting for a bit. Felt like a giant had struck the aircraft from below. I had placed my briefcase on an empty seat across the aisle and secured it by fastening the seatbelt through the handle. At one point the case was straight up in the air, restrained by the belt. We landed (obviously) and I asked the F/O when he went back to open the door "what is the maneuvering speed on this airplane?" Answer "we thought that was ground return". OK. Let me off this airplane, please.