The white butterfly migration to the east is only attributed to change in climatic conditions. Butterflies normally migrate when the conditions are good as they prepare for the upcoming season.
Most of the time when the white butterfly migration is observed, it is followed by drastic changes in climatic conditions. Can be drought or heavy rains.
Though it looks exciting seeing white butterflies in masses in your area, they lay eggs which when at the caterpillar (larva) stage destroy plants.