MetService Issues Narrative of the Big Storm

Narrative of the storm and its background.

Flooding hit Wellington, Wairarapa, Manawatu and Rangitikei areas hard early this week. This narrative article from MetService Weather Ambassador Bob McDavitt gives a sequential account of the main events leading to flooding.

Fri 6 - Sat 7 Feb: Waitangi Day marked the last period of mainly dry weather over New Zealand from an anticyclone or high pressure system. As this high moved away a change in pattern occurred towards unsettled weather.

Sun 8 - Tue 10 Feb: A vigorous front moved onto the South Island from the Tasman Sea. MetService issued heavy rain warnings for the Southern Alps and severe wind warnings for Wellington and eastern areas from southern Wairarapa to Southland.

Wed 11 Feb - The front stalled over central New Zealand. MetService issued a heavy rain warning for the Tararua and Ruahine ranges, and extended this to cover the headwaters of the Whanganui River and Mount Taranaki.

Thu 12 Feb - Computer models started to agree in highlighting the possibilities of a low pressure system deepening over the North Island during the coming weekend. Because of changes in the upper-air flow New Zealand was becoming an area where low pressure system were likely to deepen. Models did not have the same detail, but there is sufficient data for the severe weather outlook chart,updated daily on the MetService website, to go for areas of heavy rain and severe gales around the North Island.

Fri 13 Feb - A weak high crossed the North Island. To the south of Tasmania and near the ice shelf a low and a high combined to induce a southerly flow off the ice shelf. This southerly flow became strongest at the 8 to 10km height and moved towards the Tasman Sea.

Sat 14 Feb - Cold air from the south started to move into the Tasman Sea. The low that followed Friday's high across the North Island deepened rapidly from around 1000 to 990 hectoPascals and zipped along at about 40 knots. This brought thundery gales in the morning mainly to Taranaki Waikato and Bay of Plenty. These cleared in time for the Starlight symphony in the Auckland domain. This low then slowed down and started to do a clock-wise loop on the dateline. On Saturday evening the first lot of rain moved onto Wellington in time for the David Bowie concert. Meanwhile a small low started to move southwards from the region in the tropics between New Caledonia and Fiji. This low started out with a central pressure of about 1005 hectoPascals, and brought with it a bundle of tropically moist air slowly southwards into the area southeast of the Kermadecs.

Sun 15 Feb - Satellite imagery started to confirm what the computer models had been picking - that the moist air from the tropics was being drawn into the now multi-centred low deepening over the North Island. This mixing of moist air with cold air is the perfect recipe for rain-making. The low only deepened slowly but the weather prognosis was that its southerly winds now had the potential to deliver over 100 mm of rain in the next 24 hours onto the south end of the North Island. Accordingly MetService meteorologists issued a heavy rain warning soon after 8am. This warning mentioned amounts to 180 mm in 28 hours on the Wairarapa hills with intensities reaching 25 mm/hour.

After a full map discussion it was decided at noon on Sunday to issue a severe weather watch for Hawke's Bay south of Napier. These are issued to alert people that the ingredients for a severe weather event are possibly getting together in their area. Severe weather watch statements are available on the MetService Internet site www.metservice.co.nz or via an email service.

During the morning a southerly storm developed in Cook Strait and at 3:30pm the severe weather warning was reissued along with the mention of severe winds continuing until noon on Monday.

During the afternoon and evening steady rain started falling over the entire southern half of the North Island. At 8:34pm, in line with changing expectations, the Wanganui hill country, Mount Taranaki, Taihape and Hawke's Bay south of Napier were added to the heavy rain warning.

Mon 16 Feb: Overnight a small but intense low moved in close to the Wairarapa coast and accentuated the wind and rain. The rain was so heavy and so spread out that it hit places from southern Hawke's Bay to Manawatu that do not normally get hit. Orongoronga, Tararua and Rimutuka ranges received over 200mm of rain. Lower Hutt got around 168 mm in the 24 hours ending at noon Monday. A peak gust of 167 km/hr was measured at Beacon Hill early in the morning. Sustained winds over 100 km/hr affected Wanganui and Castlepoint, and gusts to around 100 km/hr hit places as far away as Auckland and Kaikoura. The low responsible for all this wind and rain moved away in the afternoon.

Tue 17 Feb: A new low deepened as it crossed central New Zealand. Thundery downpours were most intense in isolated spots of Marlborough. Waikawa ( near Picton) got 133 mm in two hours. The heavy rain was not widespread enough to warrant a warning, but a severe weather watch was issued for a while for the Wellington and Wairarapa areas.

More unsettled weather is likely later this week. A large low is forming in the southern ocean and this is expected to bring a vigorous westerly low to New Zealand. Conditions may well be at their most intense on Thursday and Saturday with periods of heavy rain possible on the Southern Alps and western North Island ranges, and with severe gales for Wellington and in the east for southern Hawke's Bay to Otago.

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