"Most of the rain is forecast for the southern and eastern part of the North Island though, so Whanganui may escape most of it as the front moves through."
There would be a high of 18C on Friday before the southerly hit, Glassey said.
That would cause temperatures to drop to 15C on Saturday.
Overnight lows would rise as the week progressed.
"We are going for 6C on Tuesday morning, 8C for Wednesday, 11C on Thursday and 12C on Friday morning.
"Then the southerly hits and we are back down to 7C on Saturday morning, Glassey said.