It may be worth noting that lightning doesn't uniformly flash from east to west. The same Greek word they somehow translated as "lightning" here is used to describe the shining of a candle over in Luke 11. It just means "light" in general. The light we're familiar with that goes from east to west is the sun. Instead of occurring instantaneously, it's going to happen gradually like the sun arising over a new day - only those awake early will see it at first, but before long it will enlighten everything and all will see. Compare with Malachi 4.