Dash 8-100
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 7000; Flight Crew Type 6210
Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control; Flight Crew Took Evasive Action
Weather
It had been a day of unusual weather while a very strong (992mb) low pressure system was moving through the area. In this 5 leg day we experienced light and occasional moderate turbulence; light and moderate icing; strong upper level southwesterly winds; and gusty surface winds you would expect during frontal passage. When we departed we climbed above the low stratocumulus that is typical after a low pressure center clears the area and at our cruising altitude of 16;000 feet; we were finally in very clear; smooth air for once that day. We gave the flight attendant the signal that it's safe to get up; and the flight was very smooth for about 30-40 minutes; but I decided to leave the Fasten Seat Belt sign illuminated. About 25 miles east of ZZZZZ Intersection; Center gave us a clearance to cross 5 miles east of ZZZZZ Intersection at 11;000 feet. My First Officer (FO); the pilot flying; began the descent. We noticed the before we would reach 11;000 we would probably enter the tops of the clouds below us; probably around 12;000-13;000 feet. The mainly flat stratus and occasional stratocumulus cloud deck looked rather benign - especially compared to what we had been encountering the whole day leading up to this flight - and appeared capable of light chop; at most. As we passed through about 13;500 feet; and before entering the cloud tops; the airplane was very abruptly jolted; as if we had just met some invisible solid obstruction in our way. There followed several strong rolling moments and strong turbulence. I told my FO 'disconnect and hand fly; and power to idle; let's slow down'. ATC gave us another instruction to which I started to answer but as we entered the cloud deck the turbulence increased in intensity and another rapid jolt knocked my hand away from the Push To Talk and almost completely took my headset off my head mid-sentence - which gives an idea of how strong the chop was.I told my FO then to not be concerned with the 180 knots 'rough air speed' for the airplane and to slow to below maneuvering speed; aim for about 160; and to descend as fast as possible while I tell ATC we need a lower altitude than 11;000 right away. I told ATC that we were 'encountering severe turbulence and we need a lower altitude than 11;000 (which we were getting close to by this point;) immediately; let's start with 7;000.' She cleared us to continue to 7;000; and reaching about 10;500 feet; the turbulence abruptly decreased to light; and we emerged from the (again; very benign looking) flat bases of the stratus. All anti-icing equipment was turned on prior to entering the clouds; but after the turbulence subsided; we both looked out our side windows and noticed that we had collected [a] large amount of ice; considering the very short (1-2 minute) time we were in-cloud; enough to cause prop vibration that lasted until the current prop heat cycle completed.I told my FO that if the autopilot hasn't been shaken into submission he can go ahead and re-engage it; and that I would be off frequency to check on the Flight Attendant (FA) and make an announcement to the passengers. Our FA said that fortunately; she happened to be seated through the whole event; and all of the passengers were buckled in; and that everyone was fine. I made an announcement to the passengers explaining that sometimes turbulence like this can be quite unexpected; but no cause for alarm; etc. etc. Since there were no injuries (or airsickness to speak of) and the airplane seemed to have experienced no ill-effects whatsoever; we decided to continue to our destination in the now much smoother air. I then called our dispatcher via SATCOM to tell him about the encounter and that we would be making an appropriate maintenance write-up once we arrived. Interestingly; about 5 minutes after the event; ATC issued a SIGMET on center frequency and read the body of it which included the area where we just were with the comment that several aircraft were now reporting severe turbulence.
DHC-8-100 Captain reported entering stratus clouds but experiencing severe turbulence. Ice also quickly accumulated while in the cloud.