Narrative:

Significant thunderstorms had moved through the area [of our flight] most of the afternoon. Enroute we were talking with dispatch about weather that was still pushing clear of [our destination]. The dispatcher made reference that the area of weather depicted as an arc to the south side of [our destination]. His recommendation was to slow down as much as feasible to allow additional time for the storm to push through the area. I was the pilot not flying and we made the decision to slow to 180 knots. We were cruising at 6;000 ft and I became aware that the airspeed was bleeding off and the aircraft was pitching up with no change in torque. I mentioned the airspeed; which was dropping below 150 KTS; to the pilot flying. It took a power setting of nearly 80% and itt temps of 840 to get us to accelerate back to 180 KTS. Light continuous to occasional moderate turbulence began shortly thereafter. We accelerated to the rough air penetration speed of 190 KTS. While talking with both center and dispatch; they saw no significant weather between us and destination; other than moderate rain. When we checked on with [new] center they informed us that we were between 10 and 20 miles from entering an area of intense precipitation extending south for 100 miles. Suddenly the turbulence increased to continuous moderate. Dispatch suggested we head west to additionally delay our arrival; and thereby approach from the west. After being handed back over to [the next] center; they and dispatch both stated that they did not see 'significant weather' between us and destination; and the turbulence had decreased to continuous light to occasional moderate. We know attempted to turn back towards destination; and we were still at 6;000 ft. As we approached between 20 and 30 miles from destination we checked on with center and he advised us of the same weather; now at our 12 o'clock; 10 miles; extending for 100 miles. The turbulence then increased significantly becoming continuous moderate; and occasional severe. I do not recall the airspeed fluctuations but there were uncommanded altitude changes between 100 and 200 ft and uncommanded roll of up to 50 to 55 degrees of bank. I am not sure at what point the autopilot was disconnected; but we received at least two separate 'bank angle' call outs. We immediately departed the area to the west and then advised center and dispatch that were diverting to our planned alternate. Upon returning to [our alternate] the aircraft was written up for severe turbulence. Upon landing the flight attendant advised us that a passenger had been hanging on so tightly he had broken a piece of plastic on his armrest.

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Original NASA ASRS Text

Title: An SF-340 First Officer reported a severe turbulence encounter.

Narrative: Significant thunderstorms had moved through the area [of our flight] most of the afternoon. Enroute we were talking with Dispatch about weather that was still pushing clear of [our destination]. The Dispatcher made reference that the area of weather depicted as an arc to the south side of [our destination]. His recommendation was to slow down as much as feasible to allow additional time for the storm to push through the area. I was the pilot not flying and we made the decision to slow to 180 knots. We were cruising at 6;000 FT and I became aware that the airspeed was bleeding off and the aircraft was pitching up with no change in torque. I mentioned the airspeed; which was dropping below 150 KTS; to the pilot flying. It took a power setting of nearly 80% and ITT temps of 840 to get us to accelerate back to 180 KTS. Light continuous to occasional moderate turbulence began shortly thereafter. We accelerated to the rough air penetration speed of 190 KTS. While talking with both Center and Dispatch; they saw no significant weather between us and destination; other than moderate rain. When we checked on with [new] Center they informed us that we were between 10 and 20 miles from entering an area of intense precipitation extending south for 100 miles. Suddenly the turbulence increased to continuous moderate. Dispatch suggested we head west to additionally delay our arrival; and thereby approach from the west. After being handed back over to [the next] Center; they and Dispatch both stated that they did not see 'significant weather' between us and destination; and the turbulence had decreased to continuous light to occasional moderate. We know attempted to turn back towards destination; and we were still at 6;000 FT. As we approached between 20 and 30 miles from destination we checked on with Center and he advised us of the same weather; now at our 12 o'clock; 10 miles; extending for 100 miles. The turbulence then increased significantly becoming continuous moderate; and occasional severe. I do not recall the airspeed fluctuations but there were uncommanded altitude changes between 100 and 200 FT and uncommanded roll of up to 50 to 55 degrees of bank. I am not sure at what point the autopilot was disconnected; but we received at least two separate 'bank angle' call outs. We immediately departed the area to the west and then advised Center and Dispatch that were diverting to our planned alternate. Upon returning to [our alternate] the aircraft was written up for severe turbulence. Upon landing the Flight Attendant advised us that a passenger had been hanging on so tightly he had broken a piece of plastic on his armrest.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.