Longrange forecasting can look as far ahead as is desired. Because the orbits and daily positions of Sun, Moon and planets can be calculated in advance, we can determine interpretations of these combinations with respect to weekly and sometimes daily weather conditions.
The first of the main influences in the weather for November 2012 will be the spring full moon over the northern hemisphere, which creates warm moist northerlies that send rain to the north of the North Island but drier weather to the South Island.
This time, for the whole South Island, most of the first half of November should be mainly dry. For the North Island the upper half should get below average rain, and the lower half overall can expect above average rain amounts.
Hawkes Bay and parts of South Canterbury to coastal Otago may be significantly drier-than-average, with parts of the west and south of the South Island getting above average rain.
The second influence in weather could be the close perigee (perigee means the day the moon is closest to earth for the month), this month the third closest earth-moon distance for 2012, with the moon at this time sitting over the southern hemisphere. It means that rain should affect the lower half of the North Island over at least 8 days in the second half of November.
Altogether for the North Island, November can expect to deliver four rain phases. The first wet system is due on the full moon/northern declination combination around 3 November.
Looking at specific conditions, in the first ten days of the month, anticyclones east and west of New Zealand should bring at least a week of settled weather to the South Island, while a depression brings unsettled easterly conditions to northern regions of the North Island.
Over the four weeks following the 10 November, which also means slightly into the start of December, unsettled weather with frequent gales and excessive high rainfall should come to central, western and high country regions.
In contrast, there may be rather dry and sunny conditions in the north and east. Stormy conditions may produce high rainfall, with the chance of rivers flooding low-lying areas in Buller, Wairarapa and Wanganui districts.
The weather pattern for these four weeks may be caused by deep depressions that pass to the south of the country with anticyclones tracking over oceans to the north of New Zealand. These may produce a strong westerly flow over New Zealand with unusually strong winds. Gales may take their toll in central regions with winds lifting roofs in the Wellington region.
In the 4-week period following the 10th, three major storms may occur within the dominant westerly flows, producing high rainfalls throughout the high country and central and western regions of both islands. But over the same period, significantly dry conditions may prevail in Northland, Auckland, Bay of Plenty, Gisborne, Hawke’s Bay, Marlborough and coastal Canterbury, especially for Hawke’s Bay. Kaikoura too can expect low rainfall in this period.
Back to the weeks now, and in the second week of November, variable conditions should prevail when a weak ridge of high pressure forms over the North Island and wet troughs affect southern New Zealand. There is potential for high temperatures in Gisborne around the 9th. After the 11th southerlies should kick in, followed by anticyclonic conditions.
The closer perigee/new moon combination occurs around the middle of the month. This usually portends extreme weather immediately afterwards, also kingtides (15th) and heightened risk of earthquakes in prone areas. The middle of the month ushers in a period of downpours. These may affect the central, west and south of the North island, and most of the South Island.
The third week is therefore to be avoided for outdoor events, boat trips, and building projects.
Disturbed northwesterlies kick the third week off, bringing the chance of widespread heavy rain associated with significant electrical activity, with squalls and thunderstorms affecting many central New Zealand locations and much rain for the Tararua and Ruahine Ranges.
Flooding at this time may affect parts of Wellington, and rivers may burst banks in Wanganui, southern Wairarapa, Buller and Fiordland. Three or four days of strong westerlies may reach gale force in central and eastern regions, followed by widespread heavy rain.
In November’s fourth week, heavy rain should become widespread. There is the chance of a storm with some flooding in southern Wairarapa, also substantial rain and possible local flooding in the western high country.
In the last few days of November we can expect brief southwesterly outbreaks between the westerlies.
As far as sunshine goes, it may be a cloudier-than-average month in the North island, except for a sunnier-than-average Napier, and above average sun in the South Island except for parts of the West Coast.
Skies may also be sunnier-than-average in Canterbury, Northland, Bay of Plenty, Gisborne, Hawke’s Bay, Marlborough and Nelson. On the other hand, cloudy weather may prevail in the west from Taranaki to Fiordland, where daily sunshine amounts could be 25% below average.
In terms of temperatures, some eastern North island locations as well as Canterbury may be warmer-than- average. The warmest parts of the country may be Hawke’s Bay, Gisborne, Kaikoura coast and coastal Canterbury.
Both Taupo and Rotorua districts may receive low air temperatures for the season, particularly Waiouru. In contrast, some parts of South Canterbury may have unusually high temperatures.
Cooler conditions may prevail elsewhere, particularly on the West Coast and Fiordland. Southerly systems in November may be 8th-9th, 15th-16th, and 25th-28th.
November to December may see a dramatic reversal in weather trends, with a very much drier-than-average December for all regions. Apart from the West Coast of the South Island, over the whole of the last month of 2012 most districts can expect less than a handful of significant rain days.
Allow 1-2 days error in all forecasting, and about 50-80kms geographic radius.
Ken Ring of www.predictweather.com is author of the Weather Almanac for NZ for 2013 (publisher Random House)