"However, on Friday this southerly is going to get into the northern Tasman Seam where there's a small area of low pressure," Mr Duncan said.
The low, located roughly between Northland and Queensland, should greatly alter the system dynamics.
"This southerly is going to inject a whole lot of energy into it. So this low is going to rapidly deepen on Friday and Saturday and track over Auckland and northern New Zealand," Mr Duncan said.
The rapid drop in pressure meant WeatherWatch had alerted Civil Defence authorities in Auckland to possible flooding, gales and thunderstorm risks, especially for Saturday.
"It's not a weather bomb, but it's close to it."
Mr Duncan said a weather bomb happened when air pressure fell 24 hectopascals in as many hours. He said the looming system was expected to cause a 20 hectopascal drop in 24 hours.
Later this week, Wellington was expected to be dry - at least by local standards - but cold.
"Tomorrow's a bit of a cloudy, showery day, and warm. But Friday you've got the cold southerly arriving. So, your high on Friday's just 12 degrees."
He said the start of the weekend would be mostly dry in the capital.
A blast of wintry weather was expected for much of the South Island this weekend.
"This week we have already seen some chilly temperatures in central parts of the South Island - for example, Pukaki dropped to minus 3C overnight," MetService Meteorologist John Law said.
"On Sunday, another cold southerly blasts its way up the country, dropping the temperatures and bringing snow which could fall to low levels in some areas," he added.
MetService expected the biggest snowfalls to be in high-lying parts of Southland, Otago and Canterbury.
"However, with such cold air flooding onto southern New Zealand on Sunday, snow is likely to fall to even lower levels, with the potential to affect many roads and accompanied by strong winds and significant wind chill," MetService said.
The forecasters advised people to monitor the latest forecasts and check on current highway conditions on the New Zealand Transport Authority website.
Tomorrow's forecast:
Whangarei: Partly cloudy. Rain from evening. Northwesterlies. Low 10C High 18C.
Auckland: Sunny areas, increasing cloud, late showers then overnight rain. Northwesterlies. Low 11C High 18C.
Hamilton: Mostly cloudy, afternoon showers clearing around midnight. Northwesterlies. Low 5C High 17C.
Tauranga: Cloudy morning, evening showers clearing overnight. Northwesterlies changing to southwesterlies. Low 8C High 18C.
Wellington: Cloudy with showers around noon, clearing late afternoon. Brisk northwesterlies. Low 13C High 17C.
Christchurch: Early rain clearing, mostly sunny. Evening southwesterlies. Low 4C High 20C
(Source: WeatherWatch.co.nz)
