A smidgen of seasonal solstice science

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This year’s winter solstice may start mild, but by the end of the shortest day of the year on Wednesday there will be rain, wind and even some snow.

NIWA meteorologist Ben Noll says a storm is brewing that will direct subtropical winds towards New Zealand in the second half of the week.

The June Solstice is the time at which the sun’s zenith reaches its northernmost point and the North Pole tilts directly toward the sun, giving 24 hours of daylight or “Midnight Sun.” Conversely, there is no sunlight at all south of the Antarctic Circle.

“In New Zealand, the length of daylight on the shortest day of the year varies: Auckland has 9 hours and 37 minutes, Wellington 9 hours and 11 minutes, but Invercargill 8 hours and 35 minutes (or about 7 hours less daylight than on the longest day of the year, December 22nd),” Mr Noll said.

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Shortest daytime in NZ centres 2017 [Image: NIWA}

Many Northern Hemisphere countries consider the June Solstice the official start of summer. But that doesn’t mean that it marks the official start of winter in the Southern Hemisphere. In New Zealand, Australia and South Africa, winter begins on June 1 and ends on August 31, otherwise known as “meteorological winter” combining the months with the most similar weather together, which makes record-keeping easier.

Just one month from the solstice, Invercargill will have about 30 minutes more daylight, Wellington 25 minutes, and Auckland 21 minutes.

Rain, wind and warmth

After a mostly tranquil start to June featuring higher pressure than normal across much of the country, a potent subtropical storm will dive south with wide-ranging impacts.

Driven by an atmospheric river of moisture originating near Vanuatu, rain may become heavy across Northland on Wednesday night, before moving south into Auckland early on Thursday,

Accompanying the rain wil be strong, gusty northeast winds that will linger into Thursday for much of the top of the North Island.

Rainfall from the storm across Auckland and Northland will most likely range from 25 to 75mm. Some of the storm's heaviest rain is expected to fall from the eastern Coromandel Peninsula, through Bay of Plenty, and to East Cape from Thursday through Friday—these areas may be the most prone to loaclised flooding and slips as total rainfall may exceed 150mm in spots.

Rainfall, some heavy, from the storm is expected to impact the remainder of the North Island and mid and upper South Island from Thursday through Friday. On Friday morning across most of the North Island, temperatures will be very mild with even a tinge of humdity in the north and east.

Snow

Good news for the recently opened South Island ski fields is the likelihood of snow from midweek. Accumulating snow is expected to target Canterbury and Otago, peaking later on Thursday but lingering through Friday. Snowfall is expected to accumulate several centimetres above 300m across much of Otago, with the highest totals likely across interior Canterbury.

While the weekend may offer periodic rain and snow for the South Island, there is likely to be some dry periods for those keen to shred some fresh powder.

Winter Solstice in New Zealand

Contact

Ben Noll, NIWA meteorologist
Tel 09 375 6334

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