By Emma Blades, MetService Meteorologist

Out over the Tasman Sea, a warm and moist weather system is gradually making its way southeast towards New Zealand. Meanwhile cold weather is brewing in the Southern Ocean – preparing to blast the warm air away.

First the wet weather

A weather front bringing warm moist air is forecast to reach the South Island late Saturday, and then slowly make its way up the country over the next couple of days. The air behind the front has come across the relatively warm ocean, picking up plenty of water ready to cascade over New Zealand. As the saturated air is driven towards the South Island in a northwesterly flow and forced up over the mountain ranges, the western areas will receive their usual deluge, while the eastern areas can also expect periods of rain. Accompanying this rain will be some pretty hefty winds, with a possibility that they may reach severe gale in some areas. The severe weather outlook below highlights the possibilities of strong winds and heavy rainfall from Monday to Tuesday next week.

Severe Weather Outlook for Monday 27 and Tuesday 28 April 2015. Issued at 2:39pm Thursday 23 April 2015. Low confidence: a 20% likelihood (or 1 chance in 5) that the event will actually happen. Moderate confidence: a 40% likelihood (or 2 chances in 5) that the event will actually happen. High confidence: a 60% likelihood (or 3 chances in 5) that the event will actually happen. For the latest Severe Weather Outlook head to http://www.metservice.com/warnings/severe-weather-outlook Severe Weather Outlook for Monday 27 and Tuesday 28 April 2015. Issued at 2:39pm Thursday 23 April 2015. Low confidence: a 20% likelihood (or 1 chance in 5) that the event will actually happen. Moderate confidence: a 40% likelihood (or 2 chances in 5) that the event will actually happen. High confidence: a 60% likelihood (or 3 chances in 5) that the event will actually happen. For the latest Severe Weather Outlook head to http://www.metservice.com/warnings/severe-weather-outlook

The North Island doesn’t escape the wet weather, as the front is forecast to reach there during Sunday night. The moist flow turns more northerly as it reaches the North Island, so that areas in the north as well as the west are likely to receive a fair amount of rain. Heavy falls are possible for North Taranaki across to the central high country, the eastern Bay of Plenty, and the Tararuas. With northerly gales also likely in many areas of the North Island we might have to bring out our horizontal rain icon again.

MetService “sideways rain” weather icon produced for a previous Wellington forecast MetService “sideways rain” weather icon produced for a previous Wellington forecast

Then the cold weather

A cold front from the Southern Ocean then sweeps in to the far south on Sunday, gradually moving up the country during the first half of next week. As the cold air undercuts the warm, temperatures are likely to drop several degrees during the first half of next week. The forecast temperatures by the UK Met Office and the European Centre for Medium range Weather Forecasting (ECMWF) models are shown in the figure below. You can see that this far out, there are differences in the forecast temperatures, but both show the cold air approaching from the South.

At this stage, the cold southwesterly flow is likely to bring showers, falling as snow to about 700 meters in Fiordland and on the tops of the mountains to the north. There is still a lot of uncertainty in relation to how low the snow will fall and the amount. Winds will be strong and gusty, with gales likely in the east of the North Island on Monday and Tuesday.

Surface temperatures (degrees C) for midday on Monday 27th April 2015. The ECMWF forecast is on the left and the UK Met Office model is on the right. Surface temperatures (degrees C) for midday on Monday 27th April 2015. The ECMWF forecast is on the left and the UK Met Office model is on the right.

How accurate is this forecast?

At the moment, there is some disagreement between the models that we use to make the forecasts, particularly in relation to the amount and location of rainfall – but also how cold it will get. The figure below shows the rainfall accumulation predicted by the ECMWF and the UK Met Office models. There are differences in the amount and location of the heaviest falls. As the weather systems approach, the accuracy of our forecast will increase. You can keep up to date with the latest forecasts, severe weather warnings and watches at www.metservice.com.

Rainfall accumulation in mm for the 12 hours prior to midday Monday 27th April 2015. The ECMWF forecast is on the left and the UK Met Office model is on the right. Rainfall accumulation in mm for the 12 hours prior to midday Monday 27th April 2015. The ECMWF forecast is on the left and the UK Met Office model is on the right.