For a printable pdf of this newsletter: http://metsoc.rsnz.org/Newsletter/autumn2009b.pdf

Those members who have chosen the postal option will receive a hard copy of this in their “June” newsletter

NOTABLE WEATHER IN NZ - AUTUMN 2009

2. Strong west to northwest flow 14th-17th

This period was considerably warmer over the country, but unsettled weather affected western areas, with heavy falls about and west of the Southern Alps. This resulted in flooding of major rivers originating from the Alps, with flooding causing problems in the Clutha Valley and South Canterbury (including closures of State Highway One and the railway line where it crosses the Rangitata River) Mt Cook recorded 321mm on the 16th (highest ever recorded one day total in May), and Arthurs Pass received 214mm on the same day. Heavy rain also fell in Southland on the 14th, with flooding and disruption to roads.

In the east northwesterly gales lashed the Cook Strait area on the 15th (power cut to Karori and Makara), and areas from inland Canterbury to Wairarapa on the 17th, with more damage.  A small tornado caused some damage in Opunaki on the 17th.

However, temperatures rose to be considerably above normal during this period, with Kaikoura reaching 23C on the 16th (and 22C the next day) and many other eastern places and northern North Island recording 20-22C maximums the same day.

Mean sea-level analyses for midday NZST 14th May to midday NZST 17th May in 12 hour steps are shown here.

3. Prolonged very cold southerly outbreak 18th-26th

The warm spell ended on the 18th, as a trough and cold front crossed the country, followed by colder southwesterlies over the South Island with some snow on the high country. More fronts moved onto NZ that night, and a low moved to the east of the South Island the next day, with an even colder southerly flow developing over the island. Snow lowered to about 300m in Central Otago, with Queenstown dropping to 2C by 3pm, after a maximum of only 6C.

On the 20th, the low moved only slowly away to the east, allowing very cold air from near Antarctica to flow up over the South Island, bringing snow showers to low levels in eastern areas, with flurries even in some Christchurch suburbs. (the city only reached a 5C maximum) This southerly spread onto the North Island during the day, with snow levels lowering in central areas, including Taranaki. (as low as Stratford) Further north, hail affected many areas, and waterspouts were again reported offshore from the western Bay of Plenty.  in In Rotorua, heavy hail in the early evening was followed by snow on the surrounding hills, leaving a spectacular white blanket visible the next morning.

The flow tended southwesterly over NZ during the 21st, with conditions easing over the country for a while. However, low pressure persisted to the east and, with the help of further frontal disturbances, deepened during the 22nd. This allowed southerlies to strengthen again over southern and central areas, with cold, wet weather in the east and south and snow on the high country. A trough  brought thunder and hail to some northern North Island areas, especially the Waikato, where it was also unusually cold. (Hamilton recorded its lowest ever May maximum of 7C, and was on 5C at 3pm)

Instead of moving away, low pressure remained near the Chatham Islands on the 23rd. This, combined with an intense high in the southern Tasman Sea, enabled the southerly flow to strengthen even further with gales lashing many exposed eastern coastal areas; severe about Cook Strait, resulting in disruptions in and around Wellington and cancellations of Inter-islander ferries. Rain was heavy in some eastern areas from Canterbury to Wairarapa. Meanwhile, heavy snow on the central North Island high country closed the Desert Road, and made adjacent highways hazardous.

On the 24th, the low moved further away, and the high pushed into the area immediately south of the South Island, where it remained slow moving until the 26th. This resulted in the flow tending southeasterly over NZ, with heavy rain spreading north to affect Hawkes bay and Gisborne (welcome after a long dry spell, but temperatures were now too cold for grass growth), but easing further south with freezing levels rising so snow was now confined to alpine areas. South Taranaki also experienced a brief spell of heavy rain, thanks a local convergence zone, and a frontal system brought some hail to areas just north of Auckland.

During the 26th, the southern anticyclone extended onto the South Island and the southeasterly flow eased over the North Island, along with the rain in the northeast. The frontal system (linked to another low pressure system well to the north) in the north delivered a period of heavy rain in Northland.

While eastern areas were plagued by a prolonged period of cold, wet weather during this period, the West Coast of the South Island enjoyed fine weather from the 22nd. While there were southeasterly gales in some valleys on the 22nd and 23rd,  daytime temperatures soon became significantly warmer than normal, with maximums of 19C at Milford Sound (23rd) and Westport (25th) and 18C in Greymouth. (25th)

Mean sea-level analyses for midday NZST 18th May to midday NZST 26th May in 12 hour steps are shown here.

30 May to 1 June - Very cold southerly outbreak

A much colder than normal May ended with a southerly blast that was even colder than the ones before, though shorter lived and with generally lighter snowfalls.

A shallow low pressure system moved onto the North Island during the 30th, with rain spreading to many areas. Meanwhile, a weak south to southwest flow over the South Island strengthened in the far south later in the day, with snow showers to low levels at night.

During the 31st, the low system moved away to the east, allowing a strong, disturbed, and very cold southerly flow to spread over the whole country by later in the day. Snow showers again fell to very low levels in the east of the South Island, even as far north as Blenheim and Picton. Falls were mostly light, though up to 10cm fell in some inland parts of Mid and North Canterbury. Hanmer recorded an icy 2C maximum, while Arthurs Pass barely managed 0C.

In the lower North Island, snow fell to levels which would be unusual in mid winter, including flurries in downtown Wellington (first time since 1995), Napier, Hastings, Wairoa, and Palmerston North. Maximum temperatures were in single figures, with several places recording their lowest May maximums on record (including 1C in Waiouru and 5C in Dannevirke).  Areas north of the Central Plateau were sheltered from most of the precipitation, though trampers reported (non-settling) snow flurries on Mt Te Aroha. The snowfalls were, however mostly light, with roads generally staying open. The exception was the high country inland from Gisborne, where snow up to 20cm blanketed the Matawai area and closed the Waioeka Gorge highway overnight 31st/1st June. The Desert Road was closed by ice for a time on the 1st.

During the 1st June, the southerly flow gradually eased over the country. But it remained cold - a chilly start to the winter proper, after a May which already had much wintry weather.

Mean sea-level analyses for midday NZST 30th May to midday NZST 1st June in 12 hour steps are shown here.