Turbo Commander 690 Series
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 100; Flight Crew Total 4700; Flight Crew Type 200
Situational Awareness; Training / Qualification; Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Human Factors; Weather
While in cruise at FL240; Radar showed a line of precipitation as scattered.  Center did not report any significant weather ahead. The flight had included route deviations for thunderstorms here and there.  We entered clouds at FL240 and in about 30 seconds sustained significant altitude changes and airspeed fluctuations.  I also temporarily lost radio communications with Center.  I made a 180 degree heading change and descended to FL210.  The aircraft had accumulated approximately 1/4 inch of ice.  I was able to re-contact  Center who provided vectors and altitudes.  It is my belief that we entered an embedded;  rapidly developing thunderstorm that was not painted on our radar and not shown on the weather radar.  I took necessary actions to protect the crew and aircraft.
AC690 Captain reports entering a developing thunderstorm while IMC at FL240 with no indications on Radar or Nexrad.  ATC communications are lost at the same time and the reporter elects to turn 180 degrees and descend to FL210.