Clear skies and light winds would bring chilly overnight temperatures.
"You'll most likely see some light frosts in the area over the next couple of days," Parkes said.
"Overnight on Wednesday it will be 5C, and 3C overnight on Thursday.
"I think it'll be a bit cooler than that in sheltered places, though."
Parkes said Wednesday would reach a high of 17C, with similar temperatures expected for the rest of the week.
"The climate average for September is around 15.5C.
"It'll be 16C on Thursday, and then 17C through until the end of the weekend.
"Monday is looking like it'll get up to 18C."
Niwa principal forecasting scientist Chris Brandolino said it was likely Whanganui would have a warm spring.
"There will be cold snaps and chilly periods, but that won't define the period," Brandolino said.
"There is an equal chance that rainfall will be near normal or below normal."
Brandolino said "climate drivers" determined the conditions Whanganui would experience in the coming months.
"That's who is at the steering wheel of mother nature's car, and if you can identify that climate driver you have a fair idea of where things are going to go.
"At the moment, the main drivers are an emerging La Niña and our warmer-than-average ocean temperatures.
"Around Aotearoa New Zealand, ocean temperatures during winter have been anywhere from 0.6C to over 1C above average, which is significant."
There could be multiple, extended dry spells in the Whanganui region this spring, "more so than usual", Brandolino said.
"That will obviously have an influence on farmers and people who rely on regular rainfall for their way of life."
