Private forecaster WeatherWatch is also predicting a possible “winter blast” from about Tuesday.
Forecaster Philip Duncan said a blast of cold air was expected to move up from the Southern Ocean, but it was uncertain whether it would hit New Zealand.
Although the cold blast couldn’t be locked in, it was important to give farmers advanced warning, especially as they moved into lambing season.
MetService forecaster Mmathapelo Makgabutlane said next week would be a stark contrast to the warm coming days.
“Temperatures are forecast to be higher than average for this time of year, especially in the South Island, during the next two days,” she said.
“But next week we could see cooler temperatures and possible snowfall in those inland areas, especially in the central North Island.”
She said the polar blast would be caused by a low-pressure system drawing colder air from the south.
In its severe weather outlook MetService said the cold southerlies could bring snow to relatively low levels over the South Island, possibly affecting many roads.
Dry today, with warmer weather coming
Much of the country will have woken to a wintry chill this morning following a dry, clear night, with temperatures still well below zero in the central South Island.
But over the course of today, those clear, sunny skies should warm things up - except in the west where cloud could bring some rain to coastal areas, according to MetService.
Early showers were expected in Auckland but that would turn to “better, brighter weather” once the cloud cleared away in the afternoon, with a high of about 16C today and a low of 11C, MetService meteorologist John Law said.
Wellington could expect a high of 13C and a low of 8C, while Nelson, Marlborough and Canterbury would all get plenty of sunshine. Highs in those South Island regions were expected to hit 15C while lows would be around 1-2C.
As the week progresses a northwesterly flow associated with a high-pressure system from Australia is expected to bring warmer air to the country, boosting temperatures into the high teens during the daytime and cutting the chance of frosts.
In the South Island, temperatures could be well above average for this time of year by Friday, with potential for an added 7C in the far South, according to MetService.
