MD-83
Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Diverted; General Physical Injury / Incapacitation
Weather
Passengers and Flight Attendants (FAs) arrived a little late from inbound flight; once everyone was deplaned the Captain gave the Lead FA a briefing on the flight including the weather so we would leave them in their seats longer than normal.  After everyone was boarded and we pushed back; weather conditions made it impossible for taxi or takeoff.  Once finally in the air and clear of the weather which was approximately 19;000 feet MSL we chimed the FAs allowing them to start their service.  Climbing out there were the typical small bumps; but nothing out of the normal.  Once reaching our cruise altitude of 33;000 feet everything smoothed out even more; however we were still in IMC.  You could tell we were near the tops on how white and bright the clouds were.  ATC advised of convective activity in the next 30-40 miles 10-12 o'clock.  We saw some green on our radar and asked for a diversion to the right of course.  ATC then cleared us direct ZZZ when able; however we stayed on the heading to clear what we saw on the radar.  Almost right after this clearance by ATC; we encountered the severe turbulence.  It lasted for approximately 20 seconds and during that time we had a [plus or minus] 200 feet.  The autopilot disengaged and I proceeded to hand fly keeping the airplane attitude level and regain our altitude.  The Captain advised ATC of severe turbulence and when we were clear of the turbulence he reengaged the autopilot for me.  After that I took radios and flying while he assessed the situation in the back of the aircraft.  Informed me that a FA was hurt and we needed to divert; and I asked ATC for a heading [to a suitable alternate] while the Captain coordinated with dispatch and medical personnel to be ready for us when on the ground.  We made a safe approach and landing.  Once at gate area had the aft stairs lowered to assist the FA that was injured.  1) If radar is not showing any storms; however ATC says there are rain showers in area; let ATC know you do not show any showers and ask for them to point you in a good direction2) In IMC while rain showers has been advised by ATC ask FAs to sit down3) Have FAs not use galley carts.  Yes this would take more time; however if there would have been no galley cart; our FA would not have gotten injured as badly
A MD-83 First Officer reported diverting to an alternate after a flight attendant was injured in unexpected severe turbulence.