In other words if you suddelnly meet a strong upward airflow, this brings the relative wind flow over the wing to greater than the max AoA and hence the wing stalls?
I think more likely the other way round.
Let's say you are flying along at 100kt, and you meet a 100kt (+10,000fpm approx) vertical airflow. (Let's ignore for now the fact it will rip your wings off). Your effective airspeed has now risen from 100kt to the vector sum of the two i.e. 141kt. I don't think the wing will stall! If it did, then the wing loading (due to this airflow) would be very small and one could fly through CBs, at any speed, without concern.
A stalled wing has a low wing loading, which is where Va (basically) comes from - at Va, the wing will stall (and unload) at the design limit of 3.8g, thus (supposedly) protecting itself from failure.
I think the thing which might cause the stall warner to go off is downward airflow. But I think this is unlikely in reality; air doesn't usually flow vertically that fast. The momentary stall warner activations we all get come from plain simple gusts, or from wind shear i.e. variations in horizontal wind speed.