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Winter is well and truly coming.
A cold snap is expected to bring severe weather and possibly the first significant snowfall of the year.
MetService meteorologist Angus Hines said both islands would be in for rain today, and temperatures were expected to drop towards the end of the week.
"On Thursday evening, a wind change brings much cooler temperatures to the far south, but the main temperature drop is coming on Friday," he said.
Hines said heavy falls, thunderstorms and hail were expected southwest of the country, and "maybe, just maybe, a small tornado near the coast".
"We are expecting very heavy swell to develop as this week comes to a close," said Hines.
"The western coastline first hit by large waves on Friday ... with 6-8m waves forecast for South Island coasts."
MetService is reporting there were 849 lightning strikes in New Zealand between 5am-7am today.
Temperatures at Aoraki/Mt Cook are set to dip to -4C overnight on Friday and Saturday, and a road snowfall warning is in place for Crown Range Road and Milford Rd from this evening.
MetService predicts Queenstown will plunge to -1C this weekend, 0C in Christchurch, 5C in Dunedin, and 7C in Wellington - but Auckland will stay relatively balmy with a low of 11C on Friday.
Orange heavy rain warnings are in place for the Bay of Plenty ranges and parts of Westland.
Heavy rain watches are in place for Northland, Buller and the headwaters of the Canterbury and Otago lakes.
Strong wind watches have been issued for the Canterbury high country, Wairarapa, Wellington and the Marlborough Sounds.
"We could be paving the way for the first significant snowfall of the year as we round out the week," Hines said.
Snow levels are forecast to drop in the south between late Friday and early Saturday.
MetService is predicting snow is possible to 400 metres in Southland and Otago, 600m in the Southern Lakes and 800m in the Canterbury high country.
"It's definitely one to watch out for, especially in the high country of the lower South Island."
South Island skifields are gearing up for the possible snowfall.
The Remarkables ski area manager Ross Lawrence said incoming cold weather could mean 10cm to 20cm of snow on the mountain, "which is very welcome at this time of year".
Coronet Peak ski area manager Nigel Kerr said if temperatures allowed, the team there would be firing up the snowmaking guns and getting stuck into crew training for both Coronet Peak and the Remarkables staff.
On Monday, the crew moved about 20 snowmaking guns around the hill by helicopter, and Kerr said they hoped this weekend would be a good opportunity to iron out any kinks and check how that gun arrangement would work.
Meanwhile, boaties near the Foveaux Strait are being warned of high swells, poor visibility and "very rough" seas between Thursday and Saturday, before easing on Sunday.
"Large swell waves and dangerous coastal conditions are expected in the western marine areas of both the North and South Island from Thursday," MetService said.
"Combined waves are forecast to reach 6m for the western South Island later on Friday."