"There will be a further decrease in temperature on Sunday when a high of 21C is forecast and it will be significantly cooler at night with a low of 13C."
Ferris said there was a possibility of surface flooding during the weekend.
"Heavy rainfall is expected in the hills north of Whanganui so there will be run-off from that.
"In conditions like these, water levels in rivers and lakes will rise quickly so there may be surface flooding and people should drive carefully.
There is no heavy rain watch in place for Whanganui at this stage but rainfall accumulations in Taranaki, Taumarunui, and Taihape may approach warning criteria.
"People might be disappointed that it won't be fine for the long weekend but Whanganui needs some rain," Ferris said.
"There has still only been 0.02mm recorded this year and that is barely worth a mention. The good news is that it will be quite short-lived and there should be some fine spells on Monday."
The Whanganui forecast for Waitangi Day on Monday is for a high of 23C with scattered showers. Southwesterly winds are expected to ease before an overnight low of 15C.