If you desire for it to be disastrous, then I fear the provisions discussed previously aren’t quite sufficient, particularly for such a difficult hurdle to reach. perhaps 10Ap per scene would suffice.
But also something could be gained from weathering such a storm, much like rainwater when it rains?
What is the timeframe within which the parameters have to be met for world-magic to occur? 24, 48, or 6 hours? A week?
The longer the time or the lower the threshold the less impact and less the inhabitants stand to lose or gain.
Just some thoughts.