Air carrier Captain reported encountering moderate turbulence on descent that resulted in large airspeed and altitude deviations.

Narrative

During the entire flight we had been cross referencing the aircraft radar; WSI; and FMS+ for ride conditions with nothing more than areas of light turbulence; maybe a moderate bump here or there. ATC descended us to FL330 just prior to WWODD on the LECTR3 arrival; I inquired about the rides upon being given the clearance. We were told nothing worse than light turbulence; but at that point I turned the seatbelt sign back on. While level at FL330 I looked visually out ahead and saw some clouds that looked like might give us some bumps; at the time I was only thinking maybe a moderate bump or two but I called the flight attendants (FA's) anyways and asked them to take their seats. The number 1 made an PA for all FAs to take their seats and maybe 30 seconds later was when the turbulence really started up. Initially we saw the airspeed rapidly increasing (+30 kts) so we extended the speed brakes and reduced the throttle and bugged speeds. The next thing I noticed was an altitude alert; we were anywhere from 150 to 200 feet low from our assigned altitude. We did have several strong jostles during this as well. The entire event couldn't have lasted for more than 30-60 seconds. Once we recovered the aircraft; I called ATC and advised them; then called the FAs to check on them. We never exceeded any limitations on the aircraft and the FAs never reported and injuries. Dispatch was notified during our arrival and I made a logbook entry at my earliest convenience.Cause: I think just rapidly changing weather caused this. None of the weather products including the onboard radar showed anything of consequence. It's hard to know how to mitigate fast changing weather when you think you are doing everything correctly.