I changed the crop area to difference the images using the previous 50th.
Just plotting the average level of the resulting images shows a consistent increase corresponding to the wave formation.
I include some of the key frames: 300, 325, 350, 375, 400, 425, 450, 475, 500.
I tried to reduce noise to signal ratio, repeating the process using more distant frames (50th, 49th, 48th, 47th, 46) and averaging the results.
Around frame 305 there is an anomaly that could indicate the exact time of the wave formation.