A deep upper-level trough of low pressure is moving over the South Island of New Zealand today, while an associated surface low lies to the southeast. The trough is dragging up cold air all the way from Antarctica in our first significant cold outbreak of the year that will continue to bring snow to parts of the South Island and high elevations of the North Island today and tomorrow. Thunderstorms with small hail are also possible. As the low-pressure system strengthens, it will also bring strong winds and high swells to coastal areas of especially the South Island.

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Snow warnings

Heavy snow warnings are currently in effect for Southland and southern parts of Fiordland and Otago, especially Clutha. From Monday morning through to Tuesday morning, the heaviest expected snowfall accumulation is 15 to 25cm forecast above 600m, although some parts of the South Island could see snow as low as 100m. Road snowfall warnings have also been issued for the higher passes of the South Island while snow is also forecast about the top of Desert Road tonight into early Tuesday morning.

Lows will be in the single digits across many parts of the country tonight, with Christchurch and Invercargill forecast for a low of 2oC and Queenstown -1oC. A ridge of high pressure will give the country a respite from the frigid weather mid- to late week, although wintry showers look to return going into the weekend.

You can view all the latest warnings at the MetService warnings page: http://metservice.com/warnings/home

The big picture – the weather-maker aloft

The following water vapour satellite image shows a deep trough over the Tasman Sea. Meteorologists like to use water vapour imagery as a tool to track the movement of mid- and upper-level systems and to tell at a glance how dry/moist the air might be.

Water vapour satellite image from 23Z on 12 April (11am local time, 13 April) Water vapour satellite image from 23Z on 12 April (11am local time, 13 April)

The water vapour channel of a satellite looks at the specific wavelength of energy emitted by water vapour, so that we can detect upper- and mid-level moisture in the atmosphere. Dry air is depicted as orange and red, while clouds and moist air are shown as white and greens/blues.

The trough looks like a big arch as it amplifies over the Tasman Sea, and the jet stream around the northern edge of the trough is visible as the boundary between the dry (orange) and moist (white) air. You can see the trough deepening in the following 12-hour loop of the water vapour imagery from about midnight to midday today.

Water vapour satellite loop from 11Z to 23Z on 12 April (11pm local time on 12 April to 11am on 13 April). Ex-Tropical Cyclone Solo is in the purple clouds north of New Zealand. Water vapour satellite loop from 11Z to 23Z on 12 April (11pm local time on 12 April to 11am on 13 April). Ex-Tropical Cyclone Solo is in the purple clouds north of New Zealand.

The following visible satellite image shows a frontal system moving over the North Island ahead of the cold air mass spreading across the South Island this morning. The honeycomb pattern of clouds south and west of the South Island are low-topped cumulus clouds that are common with this type of system.

False colour image at 22Z on 12 April (10am local time on 13 April), courtesy of NOAA False colour image at 22Z on 12 April (10am local time on 13 April), courtesy of NOAA

How do we get thunderstorms when it’s so cold?

One of the required ingredients for thunderstorm formation is instability. If the temperature in the atmosphere decreases rapidly with height, we call this an unstable atmosphere. Think for a moment of a hot-air balloon and how it will rise as long as the air inside the balloon is warmer than the air outside of the balloon. Similarly, when a “parcel” of moist air begins to rise, it can form a cloud if it continues to rise high enough, and there may also be the potential for a thunderstorm to form if other ingredients are in place as well.

We can get an unstable atmosphere by having warm air near the surface and cold air aloft. While this current weather setup is not favourable for warm air at the surface, the air aloft is cold enough to produce an unstable enough atmosphere for thunderstorms. Furthermore, since that air aloft is so cold, we also have the potential for small hail to form in the showers and storms.

You can see the latest thunderstorm outlook for New Zealand for today and tomorrow at: http://metservice.com/warnings/thunderstorm-outlook

Heavy southwest swells building

5m swells will be spreading up the west coast of New Zealand by tonight, then easing to about 3m tomorrow. Along the east coast, southerly swells are rising to 4m south of Banks Peninsula today and to 5m up the east coast of the North Island by tomorrow night. The following image from the ECMWF shows the forecast combined wave heights (in metres) at midnight tonight. Combined wave heights is a combination of swell as well as sea state. More information on sea state and swell can be found here: http://blog.metservice.com/2015/03/sea-state-and-swell/

Combined wave height (in metres) from the ECMWF model, valid at 12Z on 13 April (midnight local time) Combined wave height (in metres) from the ECMWF model, valid at 12Z on 13 April (midnight local time)

For all the latest forecasts and warnings, please continue to monitor our website at metservice.com