The Cold
This year for Christchurch, winter is colder than average. The coldest part of winter may be in two phases; the last week in June to mid July and mid August to mid September. The coldest of temperatures could reach -4 to -5C in mid June, the last week of June, mid July, mid August and the second week of September.
The Precipitation
Most winter precipitation is expected June through August, with heaviest falls in July, whereas June was last year’s wettest month. The heaviest downpours may arrive on or near 10 May, 1 and 22 June, 25-28 July and 1 September. Whereas in 2013 drier spells came in the second half of July and mid November with November the cloudiest month, this year September and October may be mostly dry with the cloudiest period between 18-30 July.
The King tides
As occurred last year high kingtides again arrive in July and August. The 2013 kingtides were at the end of June, July and August. In 2014 they are in the middle of July, August and September, plus this time are accompanied by stronger winds. This may provide both extra chill factor and increased water levels. It means electrical power and therefore electrical home heating may need to be suspended due to short circuiting hazards. Home fireplaces may have to be closed down where there is flooding, as surface water combining with hot coals producing steam may transport smoke and fumes throughout dwellings.
The Flooding
If rain is about, the kingtides (29-30 April, 15-17 and 28-29 May, 15 June, 14-16 July, 12-14 August, 10-12 September) will always bring flooding to Christchurch now that the land has dropped. The potential for more flooding in some parts of the city has been increasingly more serious since the increase over the past year of dropped ground levels due to the earthquakes. Compounding this will be the several chances of snow and ice due to subzero temperatures. In the 2014 winter, perigee (closer to earth) moons will be averagely closer to earth in the mid-winter months than occurred last year, when the coldest part of winter brought some snow around the end of June.
The Winds
Expect some of the strongest winds for winter to arrive around 22-23 June, and the 27th of both July and August. The three winter months may see around 23 days with below 999mb air pressures. Six of these may be in June, eight in July and nine in August. The lowest air pressure day of winter may be around 11 July, with the highest air pressure for the season only 4 days later. This rapid change in air pressures may signal some seismic risk.
Error factor
As always, weather is an inexact science and longrange estimations are opinions and not expressions of certainties. They refer to potentials for weather events and trends, as for all weather forecasting, be it short, extended or long-range. Warned-of weather may or may not occur, as that is the nature of potential. For example snow typically needs three ingredients - subzero temperatures, precipitation and a rising barometer. Without all three in position the potential may be there but snow may be delayed. If snowflakes cannot expand in slightly warming air there may instead be cold rain, hail or sleet.
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JUNE
June will be an unusually cool month for Canterbury and average to drier north of Christchurch, but wetter than average from Rakaia southwards.
In the second half of the month polar outbursts and snow to low levels may drop temperatures below the average.
1st
: first chance of light snowfalls for Christchurch. The diminishing high tide may bring mixing of floating ice with water.
5th
and 12th: more snow and with strong winds. Ice may block drains and prevent quick drainage. The 13th is full moon and southern declination, and the 15th brings the seventh closest perigee and the kingtide.
22nd- 26th
: the beginning of Christchurch's coldest spell of winter. Four days of snow may be followed by a week of frosts. Wind chill adds to cold. From 23-30 June may be the coldest week of the year.
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JULY
July will be a month with above average precipitation but average to less sunshine hours. The second week is the coolest
2nd - 4th
: chance of heavy snowfalls in the region. Fog may close the airport.
11th - 16th
: potentially very unsettled. Snow on 12th-13th. Seventh highest kingtide of the year (14th -16th) and fourth closest perigee (12th), also southern declination (11th). This bring flooding also an icing over of streets. Transport may be clogged. The 8th-15th may be the second coldest week of the year. Disruptively cold conditions may be even worse in Otago.
21st - 22nd
: Rain, fog and light snow are anticipated.
25th - 30th
: at least 7 days of continuous precipitation, starting with gusty conditions.
25th – 26th
: evening/overnight snowfalls, with possibly some heaviest falls of the season due to new moon in apogee.
28th - 29th
: Rain, some heavy, and more risk of flood. This ends an extended period of continuously subzero overnight lows. More snow is to come but may from now on be less regular and less heavy.
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AUGUST
August is a drier but cloudier than average month for most districts south of Blenheim, including Christchurch, and frequent southerly flows from depressions centred near the Chathams will remind that there is more winter to come.
5th
: cold southerlies bring moderate morning snowfalls
13th - 14th
: moderate snow likely just after closest perigee of the year (11 Aug) and fourth highest kingtide of the year between the 12- 14 August.
21st – 26th
: expect 5 days of rain that may include light to moderate snowfalls.
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SEPTEMBER
1st
: expect further snowfalls, some heavy, and the start of nearly two weeks of subzero minimums.
8th- 11th
: possibly the last snow blast, but exacerbated by further flooding due to the fifth closest perigee (8th) and fifth highest kingtide (11th).
After the second week in September, temperatures are due to rise, with a last and rather freakish chance of a light but relatively harmless snowfall around mid November.
Allow 1-2 days leeway in all forecasting.