For much of New Zealand it was a warm Easter break with many places recording temperatures well into the twenties. On Easter Sunday, 5th April 2015, the warmest spot in the country was Kawerau in the Bay of Plenty when the thermometer reached 25.7°C. However, as we look ahead to this weekend the forecast looks very different, with cold air spreading across New Zealand from Sunday.

What’s the forecast?

Over the last few days the weather has been dominated by northwesterly winds. Over the weekend the winds are set to swing round to the south bringing a change in air mass to the country. A deep area of low pressure is forecast to move closer to New Zealand during Sunday and Monday bringing some wetter, windier and colder weather; giving us a real taste of winter.

Forecast for Monday 13th April 2015 This is the current forecast from the ECMWF, the European Centre for Medium range Weather Forecasting, for the early hours of Monday morning. The tightness of the isobars (white lines) is a good indication of the wind strength. Also shown on the charts are the model rainfall and snow fields, blue and white respectively. These show the wetter weather spreading north and some snow fall over the tops of the Southern Alps and the highest ground of the lower South Island.

The air that is forecast to reach Aotearoa on Monday has its origins over the Ross dependency in Antarctica.

Forecast trajectory The forecast movement of a parcel of air as it heads northwards, reaching Dunedin at midnight on Monday.

So is it going to be like the Antarctic?

Although the air is arriving from the deep south, fortunately there will be some modification of the air mass as it moves over progressively warmer seas between the ice and New Zealand. If you do want to find out what the weather is doing at New Zealand’s home on the ice, Scott Base, head to http://antarcticanz.govt.nz/weather/SBweather/sbweather.html

It will, however, still be a very cold start to next week for us in New Zealand as the first significant cold snap of the season arrives. Temperature forecasts for southern parts of the country will struggle to break out of single figures during the day and minimum temperatures for parts of Southland and Central Otago are likely to dip below freezing. Find out the latest forecast on the Towns and Cities pages of MetService.com

As well as the colder temperatures there will be some wintry precipitation for the lower half of the South Island. Check out the Severe Weather Outlook for the latest thoughts from the forecasting team. http://metservice.com/warnings/severe-weather-outlook

The effects of this deep area of low pressure aren’t just going to be felt over the land. Through the weekend the developing strong southerly is expected to bring heavy southwesterly swells towards the coast of New Zealand, especially to those south and western facing coasts. Check the latest marine forecasts before heading out on the water.

Wind Chill

Wind Chill In stronger winds heat is lost faster from our bodies compared to still conditions, this is called wind chill.

The colder temperatures combined with the stronger winds will lead to a significant wind chill. In stronger winds heat is lost faster from our bodies compared to still conditions. The wind- chill factor is one of the elements used to calculate the “feels like” temperature that we display on MetService.com. You can find out more about how this calculation works in this blog post http://blog.metservice.com/2010/01/feels-like/ or you can use this table to help work out the wind chill for different wind speeds and temperatures.

Wind chill table To calculate the wind chill find where the wind speed and the temperature meet.

How sure are you?

As with any forecasts, the further away from now the more uncertainties there are embedded within the forecast. To help reduce the uncertainties the duty meteorologists can look at several different models to compare how the situation evolves and also look at how the situation changes between computer model runs. The less variation between the models and runs can help provide more confidence in the forecast.

In this example there has been a clear signal for this colder outbreak on all the models run by the duty meteorologists, and for several model runs in succession. As we get closer and closer to Monday more information will become available to the meteorologists allowing them to fine tune and tweak their forecast, so stay up to date with these throughout the weekend.

Is that it for the warm temperatures?

Hopefully not! Although the temperatures are set to drop dramatically this week there looks to be some slightly milder temperatures returning for a time the following week. Heading through the next months, the temperatures will continue to fall but this looks to the first real taste of winter for many parts of New Zealand.