A heavy rain warning also applies to Taranaki Maunga from 7am to 7pm on Wednesday. MetService expects 180-220mm of rain about the upper slopes, with lesser amounts at lower elevations. Rainfall intensities of 25-40mm an hour and thunderstorms are possible, especially during the afternoon.
People in those areas are advised to clear drains and gutters, avoid low-lying areas and drive cautiously.
A northerly wind will accompany the wet and cloudy weather in Whanganui, continuing into Thursday, with the potential for it to become strong inland.
“Looking to Thursday, there’s still some rain early morning but it will be a more showery situation with gaps between the rain,” Makgabutlane said.
While the windy conditions continue into Thursday, they are expected to become more westerly.
“The winds will be something to look out for on Thursday.”
The warmer-than-average temperatures will also remain with a forecast 19C before “quite a change” with a drop to a maximum of 14C on Friday.
“Showery weather is still expected but it should be a bit cooler than the rest of the week,” Makgabutlane said.
The strong winds are expected to ease on Friday before becoming southerly in the evening.
The maximum temperature is forecast to drop to 12C and the minimum to 5C on Saturday and Sunday.
“It’s definitely looking like a different week to end off with, much cooler, showery weather with cold southerly winds.”
This week’s forecast is a fair overview of the winter season as a whole, with Niwa’s seasonal outlook predicting rain and warm temperatures most of the time and the occasional cold snap when the airflow turns southerly.
A warmer-than-average winter awaits Whanganui, and much of the country, while rainfall expectations change week to week as La Nina is expected to control the season.
“Our expectation as a whole is that temperatures will be warmer than average,” Niwa principal scientist Chris Brandolino said.
“Of course, there will be cold snaps and frosts but our expectation is there will be less of them than usual despite the fact that we’re getting one later this week.”
There were no “strong signals” that another significant cold snap would occur this month, he said.
There was a medium-high forecast confidence that the entire country would experience a warmer-than-average winter.
The La Nina system will result in northerly air flows for much of the season, still becoming southerly at times, but the flow’s changing tendency towards a latitudinal direction brings uncertainty to the rain forecast.
There is low-medium confidence that the lower half of the North Island will have average to above-average rainfall this winter.
Brandolino said whether the airflow tended more easterly or westerly would “tilt the odds” of wet weather, but which way the flow would move was unpredictable and could change quickly.
“Part of the reason there is some uncertainty with rainfall is because of this lingering La Nina-like weather pattern,” he said.
“It really depends on which side the northerly airflow appears on a certain week or period of time.”
If the airflow tends west, more rainfall can be expected, whereas an eastern tendency could result in drier conditions.
Olivia Reid is a multimedia journalist based in Whanganui.
