I mean, the event doesn't happen--until after the surge occurs. Which is impossible to predict.
What I'm saying is, it should be possible to divine the result... but not until just before it happens. Do you understand? You can prepare all of the people necessary, however large a group, but at best you can only divine that the movements are based around waiting for something to happen, not even what they're waiting for--because they don't know.
Then it happens, and then they act. You can divine it then... but you don't have much time to do so at all. And maybe your scheduled diviner isn't doing the spell at that time. Maybe the next one is three hours from then, same as always?
That's a three hour window where no one sees a small army moving to invade a fortress, just as an example.
What Azel said basically, fogging divination that way goes all the way back to the invasion of Tyrosh