Chronology of the weekend Storm

Mon, 19 July 2004

Chronology of the weekend Storm

Last weekend's deluge of rain over Bay of Plenty and wet conditions over eastern parts of the North Island were well picked by severe weather watches and warnings issued by MetService in advance.

Here is a chronology of the weekend storm: Thursday 15 July: The large high pressure system that had been giving a prolonged period of fine frosty weather to much of New Zealand during the school holidays shifted to the east of Chathams Island and then stopped moving again. A frontal rain band moved onto the North Island from the Tasman Sea but it was blocked by the high and so it stalled over the Bay of Plenty area. At 11:16am on Thursday MetService issued a 'Severe Weather Watch' for northeastern parts of the country, indicating that the ingredients for a heavy rain event were gathering. This was underlined with a news release explaining how the wet and windy weather was likely to affect travellers during the end of the school holidays.

Friday 16 July: The first 'Severe Weather Warning' for heavy rain for this event was issued at 10:48am on Friday. The north-south front over the Bay of Plenty linked with a low pressure system that formed between Norfolk Island and the Kermadecs. The northeasterly winds around this low helped to accentuate the rain into central parts of the Bay of Plenty.

Saturday 17 and Sunday 18 July: The subtropical low moved south and then southeast finally taking the rain clouds away. It was lucky for other parts of the country that the low did this side-way shuffle to the east, for this meant that they missed out on some of the forecast rain.

During the three days from Friday to Sunday the MetService rain gauge at Whakatane airport recorded 264mm (July long-term average is 130mm). The heaviest rain was well focused. Tauranga airport received 99mm and Gisborne 66mm. By comparison, the rain that fell in Wellington during the same period -including during the Bledisloe Cup match- was 52mm.

"There is nothing unusual in having a low pressure system moving onto New Zealand from the sub-tropics," commented MetService Weather Ambassador, Bob McDavitt. "However the amount of rain delivered by this low over the weekend raised the rivers to a level they haven't seen since 1998. An unusual feature of this event was that rain intensity over some parts of the lowlands was almost as much as it was in the hills. Normally the rain in the hills is three or four times heavier than the lowlands. This abnormal behaviour was also observed during the mid-February floods."

It has been sunny in Bay of Plenty on Monday. MetService is forecasting some scattered rain there during Tuesday night but otherwise dry weather for the rest of the week.

ENDS

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