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Weather until November

TUESDAY SEPTEMBER 11, 2012

(pic, Mt Egmont-Taranaki)

The three main cycles of the moon; perigee, declination and phase, combine to bring known variations in weather. This varies for each locality, although still in a cyclic pattern.

New moons and perigees are still a couple of days apart, a situation bringing wet westerly conditions to the North Island. In November and December new moon and perigee occur within 24-hours of each other. Also, perigees are moving closer to earth in months ahead, with September’s perigee the ninth closest for 2012, October’s perigee the sixth closest, the November perigee the third closest and December’s perigee the second closest.

Perigees bring extreme weather, depending on the season. It is why we may see some unusual warmth from high pressure systems in the second half of December. As this effect progressively decreases in the subsequent months of summer, the beginning of summer may be amongst the best season’s weather for the country. We may have to wait until February for more warmth to arrive.

Over the next three weeks, disturbed westerlies and southwesterly airflows are expected, followed by deep depressions crossing the South Island. Low rainfall may continue in Gisborne/HB with less than half average for the time of year, and three-quarters average in eastern Northland.

Over the next four weeks, with wintry conditions returning through September and October,  Auckland to Manawatu and Marlborough may be about twice as wet as usual but Gisborne and Hawke’s Bay should receive less than half average rainfall for the period. Fiordland may be very cold with some lowest temperatures in two decades.

In about four weeks’ time depressions may bring substantial rainfall to Canterbury, Marlborough, and BoP . In Fiordland, temperatures may be well below average. Sunny conditions may prevail in the east from Gisborne to Canterbury, with sunshine totals more than average. However, the rest of NZ may get cloudy conditions, especially Waikato and the West Coast. Hamilton’s sunshine hours may be the lowest in two decades over the second half of September and the first half of October.

As for daily developments, the following is a reprint from our subscribers’ free newsletter, sent out on 21 August. The reader may like to monitor what is written here with what actually happens. Remember to allow a 24-hour error and about 50 miles radius of applicability.

“Between September 10th-11th a southerly system develops associated with the anticyclone in the north.

By 13th a frontal system brushes the North Island, then dissipates but polar blasts resume around the 15th.

About 16th-17th, squally fronts cross the country.  Downpours are expected in central North Island districts. Snow is likely to low levels in the south of the South Island, Southland and east coasts of the South Island in many places. By 17th, a slow moving high sits in the Tasman, and then dominates weather until 20th. Milder temperatures are expected.

Around the 18th snow to 400 meters may be likely and roads may be closed south of the South Island and the Desert Road.  Snowfalls may become widespread and many new born lambs in the far south may be lost during this cold snap.

About the 22nd, low pressure systems that have been building down the eastern seabord of Australia progress towards NZ, and at the same time temperatures are warmer.

Between 24th-27th low pressure dominates the country and cold fronts continue up from the deep south. Snow and ice may cause disruptions on a number of Southland and Otago highways as well as the Arthurs, Lewis, Porters and Lindis Passes, including Nelson’s Takaka Hill. After 28th the winds drop as anticyclones take over.

In the last days of September, lows prepare to move onto NZ. For NZ, the southerly systems in September may be: 3rd-4th, 10th-12th, 15th-16th, 18th-19th, 28th-30th. Potential dates for snow are: 11th-12th, 16th-18th, 21st, 28th, 30th

October starts with a low and a cold front, bringing much rain with the country dominated by low pressure for most of the first week.

At or near 8 October, a cold flow passes through most of the country. A high pressure system over the Tasman Sea starts to build, but NZ is still dominated by low pressure until about the 10th. Snow is likely in the east of the South Island.

From 11th-15th a high pressure system over Tasmania starts to move towards and then over NZ and should mostly affect the North Island. Meanwhile the South Island is dominated by fronts that are fed from below the country, and anticyclones that form in the north should keep the fronts confined to southern parts.

The 17th-19th may be fairly settled.

Around the 20th a passing cold front whisks by and within a week temperatures may drop sufficiently to bring a front with cold southerlies which may close the Desert Road. Downpours are likely in the north and west of the South Island, and in the east of the North Island.

By 24th snow may close the Desert Road as a front brings cold southerlies around 25th-26th. Ensuing ongoing frontal systems should continue to affect the country for the rest of the month. The southerly systems in October may be 8th-10th, 12th, 15th-16th, 22nd-26th and the potential for snow dates are: 1st-2nd, 9th, 24th”

The short-lived but quite intense anticyclone is currently coming over the north as forecast. In a couple of days it should be replaced by a low pressure system. Another high pressure system in the Pacific above NZ (weaker than the current one) follows, and should pass to the northeast of NZ by the 17th, beneath which a cold wintry front passes over us between 15th-17th. This current anticyclone is the strongest we can expect this month.


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