Date: 201409
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Second reporter function: Pilot Flying; Captain
Anomaly: Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; General Physical Injury / Incapacitation; General Declared Emergency
Contributing factors: Weather
Primary problem: Weather

Narrative: To begin with; the flight was in cruise on the arrival. Sky conditions at the time were SKC. We heard another aircraft asking ATC if they had to make all the crossing restrictions on the arrival. They said they wanted to stay above the weather. ATC told the other aircraft that they must cross [next waypoint] at 10000. After hearing this I turned on my weather radar. At the time; I didn't see anything on my radar since we were way above it. As we continued on the arrival; we were able to see visually what the other aircraft was talking about. Then; the Captain turned on the seat-belt sign and called the FAs to asked them if they were still doing service. They said yes and the Captain told them to hurry up and finish because in 5 minutes we would be hitting turbulence. We began our descent to cross [next waypoint] and 10000. At this time the Captain started to slow the aircraft to from 290 to 270 knots. He told me to inform ATC of our speed. I told ATC we were slowing for turbulence. ATC asked how bad the turbulence was. I informed ATC that as of right now we're in smooth air; but we're anticipating hitting turbulence ahead on the arrival. ATC accepts our new speed and about 15 seconds later tells us that an aircraft ahead of us on the arrival reported moderate turbulence. Immediately after receiving this report the captain calls the FAs. He said that there was a report of moderate turbulence right in front of us and that they need to be seated right away. Looking at the radar the buildup didn't look that bad. I was showing light precipitation. However; it was so big you couldn't go around it. The top of the buildup was about 14000 feet. We entered the buildup at 11000ft approximately one minute after the Captain warned the FAs. Upon entering the cloud it was light turbulence follow by a upward moderated jolt. This jolt lifted me about and inch of my seat. After the jolt; it was light turbulence in rest of the buildup. About 5 seconds after the moderated jolt of turbulence; we get a call from the cabin. FA B is telling us that FA A has a broken leg with bone protruding out of her skin. We know instantly this is true because we can hear FA A screaming in pain in the background. The captain then tells me to declared and medical emergency. We're just passing ZZZZZ waypoint and I declare the emergency. ATC gives us direct to the airport and speed at our discretion.  I contact dispatch and ops while the captain handles the radio and fly's the airplane. I make an announcement of our situation to the passengers and ask the them to please remain seated to allow the paramedics to assist the FA. We land about 10-15 minutes after declaring the emergency. Paramedics met us at the gate.  Suggestions:  This situation is really hard to determine a way that it could've been prevented.  I believe we followed the correct flight procedure; but yet it still happen. I don't know what the FAs get trained in about turbulence; but if they hear moderate it should be a word that causes instant action. Also; sometimes we call FAs about turbulence and it's a false alarm. Maybe; these false alarms(crying wolf) are causing complacency and a lack of motivation to follow through with the procedure.


Second narrative: Once the paramedics were on board and were with FA A in the aft galley I allowed passengers to deplane.


Synopsis: When their ERJ-170 struck turbulence on descent a flight attendant suffered a severe leg injury.  An emergency was declared and their arrival to their destination was expedited by ATC.