An active front is expected to move onto the lower South Island Tuesday afternoon and evening, then move north over the remainder of the South Island during Wednesday and weaken. A strong, moist northwest flow ahead of this front should bring a period of heavy thundery rain to the west of the South Island. About Fiordland, 150-200mm may accumulate during Tuesday, while for Westland, 250 to 300mm may accumulate, especially about the ranges, during Tuesday and into early Wednesday morning.