Saturday and Sunday are both expected to be cloudy with occasional showers, with more persistent rain forecast for Saturday afternoon.
Those lows were bringing in warm, moist air from the subtropics, which in turn meant temperatures would remain warm over the weekend, McInnes said.
"For Whanganui we're looking at a high of 24[C] for both Saturday and Sunday.
"With that warm, moist air, things don't cool down as much at night either.
"Normally in November you'd get around 11, but overnight Friday and Saturday are both sitting at 18.
"No duvets for sure."
McInnes said there was the chance of a bit of sun on Sunday, but winds would keep bringing showers through the Whanganui region.
As for the start of next week, conditions will remain very much the same.
"There'll be periods of cloud and showers, and you've got blustery westerlies to look out for as well.
"While not super strong, it can still be a bit uncomfortable."
Temperatures would start to drop off a bit from Monday, with a high of 22C expected, McInnes said.
"They'll go down to the 20 mark from Tuesday, which is a tad above average.
"Once we get through the weekend this northwesterly weather system moves away, and winds turning more westerly, it's pushing slightly colder air on to us.
"If you do like a duvet at this time of year, you might be back to it at the start of the week."