Sheet lightning

For as the lightning comes from the east and shines as far as the west, so will be the coming of the Son of Man (Mt 24:27). 

The imagery may be describing sheet lightning. That phenomenon lights up the sky because lightning inside clouds illuminates the clouds, diffusing brilliant light over a wide expanse–like a light behind a screen. If clouds cover the night sky, a lightning flash embedded in clouds will instantly radiate out to illuminate the entire sky. In addition, some electrical storms have constant flashes of lightning, so that light and dark momentarily alternate in a continuous cycle until the storm passes over. 

The imagery of lightning emanating from the east may suggest an electrical storm approaching from the east. The eastern orientation has positive connotations because the sun rises in the east. Sunrise has positive connotations, whereas sunset and nighttime have naturally apprehensive connotations, especially for primitive people. But even in the modern age, nighttime is more dangerous than daytime.