A PC12 pilot inadvertently entered a building thunderstorm despite the use of weather radar and a digital weather display system. Loss of control and extended altitude deviations ensued.

Narrative

Enroute on a reposition flight the aircraft I was piloting flew into a building cumulonimbus. Weather radar was in use as was nexrad weather. Weather was building to the east of my position; and it was visible from the cockpit. The plan to avoid the weather was to continue on a direct course and then deviate west between cells to avoid the weather that was building; as the cell tops were clearly visible from the cockpit. The cells formed a solid line; and continued to build below the aircraft; which I could not see. After entering what I thought was a thin layer; it soon became apparent I was in the top of the building cell. I had been cleared to deviate left and right of course as needed. The aircraft immediately climbed almost 1;800 FT. In an effort to maintain positive flight attitude; I disengaged the autopilot during the turbulence. The aircraft banked hard right; and descended almost 3;400 FT. During the encounter; the aircraft continued a turn to the right until I eventually found a hole to exit the storm. I believe there was a gain of 1;800 FT above assigned altitude and a loss of nearly 1;600 FT below assigned altitude. At one point during the encounter; I was cleared to maintain whatever altitude I could. After exiting the weather; the flight continued to the destination airport without further problems.