Air carrier pilot reported unable to maintain altitude for several minutes due to encountering downdrafts and turbulence.

Narrative

Upon reaching cruise flight (FL300); the CA and myself noticed we were entering an area of light radar returns. Above us (at FL340) and ahead of us (10-20 nm) an aircraft reported to ATC they were unable to maintain altitude; and were in a climb above FL340. Within seconds of that call; we entered an area of moderate turbulence and an updraft. We too; were unable to maintain altitude and began to climb. To the best of my recollection; we climbed to FL302 before we entered the subsequent downdraft. Once the altitude came back down through FL300; the autopilot attempted to maintain the altitude; but the airspeed degraded below green dot and all the way to Alpha Protection (a-prot). Once at a-prot; we began to sink; to the best of my recollection; at about 800-1000 FPM to FL290. The entire time we were in moderate turbulence. Somewhere before reaching FL290; I reported to ATC the turbulence as well as the inability to maintain altitude. I had my eye on the TCAS as well; and the aircraft above was the only one that was nearby. There were no lower aircraft we could see that would be an issue. ATC gave us a 30 degree left turn for traffic (assuming it was for the aircraft above also sinking ahead of us). However; at this point; the autopilot had been disconnected and we did not make the turn. Everything happened incredibly quickly; and we were able to return to FL300 before making the turn. I advised ATC that we were back at assigned altitude; and I think we were recleared direct to a fix. By my best estimate; we initially descended out of FL300 to FL290 and back to FL300 in less than 2 minutes.The main cause of this event was the large cell of weather we flew through. We did not have a good picture of it with the onboard weather radar; as there was nothing other than green that I remember. I was rotating through several radar apps; and saw nothing alarming about our route.If I was presented with the information we had again; I don't think I would have done much different to avoid the weather. We simply did not know the weather was there. Once the event began; however; I could have done a better job helping the PF with making sure we made the turn as instructed by ATC. I was definitively task saturated making sure the airplane was still flying (aviating); and did not do as good at navigating and communicating as I could have.