A rare set up with two high pressure systems side-by-side will create a line of heavy tropical-like rain across the upper North Island in the next few days, then funnel it down across the rest of the North Island.

The heaviest rain will be mainly in a straight line from Northland to Hawke’s Bay, with the heaviest most soaking rains affecting Northland, Auckland, Waikato and parts of Bay of Plenty. On the fringes, but still in some risk zone for heavy rain, are Gisborne and Hawke’s Bay.

HIGH PRESSURE GUIDES THIS RAIN
So what’s happening with these two highs? As of Thursday morning high number one will lie over the South Island and southern Tasman Sea while high number two will stretch east of Bay of Plenty and Northland.  The line of rain that will develop will be fed by sub-tropical air thanks to high number two, meanwhile the high over the South Island will basically ‘cradle’ this rain – stopping it from moving away and helping intensify it.

As the highs shift, change shape and merge we’ll see this band of heavy rain expand over the North Island. 

By Friday the high over the South Island will absorb the high to the north, creating a big barrier of high pressure over the South Island and lower North Island with a couple of centres – the main one being out over the Chatham Islands. This will then encourage a wet nor’easter which will feed down around Gisborne and Hawke’s Bay creating a few heavy falls there (mainly in the ranges).  The rain clouds will march southwards into Hawke’s Bay on Thursday and Friday then Wairarapa into Friday and Saturday.

High pressure on Saturday will mainly keep wet weather out of the South Island but a few drizzly showers are possible in Canterbury north of Christchurch mostly, and around Marlborough.

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NORTHLAND’S DROUGHT
Northland will see drizzly showers returning today, turning to rain in the Far North later today and tonight then across Thursday this narrow but significant band of rain will slide down Northland into Auckland, Waikato and Bay of Plenty – but may not leave the Northland region as it grows southwards. This band of rain, should it stay over the Northland region as currently forecast, may break the drought – or at least put a huge dent into it. 

Will it definitely end the drought? We cannot say – simply because no forecaster can guarantee the rain will fall as forecast. Recent rain events in Northland have been frustratingly hit and miss – some getting 50mm in recent weeks, others just 5mm. This event does look far more widespread and the totals do look fairly even across Northland (60 at the lower end and 120mm at the higher end over a three day period). The only thing that could go against soaking rains affecting everyone in Northland would be for this rainband to drift to the east or west a little (as the rainband almost perfectly matches the size of Northland – which doesn’t give us a lot of wiggle room for error, however the modelling and data has been more confident with each day of rain – so too are MetService who have some confidence of rain heavy enough to prompt a possible rain warning for the drought affected region.

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GOOD RAINFALL TOTALS POSSIBLE

Both Thursday and Friday look quite wet for the upper North Island while eastern, central and more southern parts of the North Island are exposed to briefer heavier falls on Friday and Saturday.

The tropical element to this rain is going to give it more oomph – and rainfall totals as far south as Wellinton may now reach 20mm around late Friday/early Saturday.

But the areas most exposed to heavy rain are in the upper half of the North Island from the Far North to Lake Taupo.

Latest rainfall totals via Wunderground are showing some great numbers – for many this would bring a huge amount of relief should it eventuate.  

Totals in Northland, Auckland, Waikato and western Bay of Plenty will be between 80 and 100mm at this stage (give or take 20mm). Keep up to date with any MetService warnings and watches too. Higher totals are possible, especially in the hills and ranges – but with the rain band so narrow we’ll be closely monitoring incase any dry areas look as though they may miss out on the soaking rains.

At this stage eastern Bay of Plenty and Gisborne may get around 30mm – but this is a changing forecast for north eastern areas. The heaviest rain still looks most likely in a straight line from Northland to Taupo, but the nor’east wind change around Friday may see heavier falls in the Gisborne area.

Extending on from that line from Northland to Taupo means you head into Hawke’s Bay – and Napier’s rainfall totals (at this early stage) suggests Napier may get more than what Gisborne gets, with 60mm or so.  Remember these totals are just a general guide – our mountains and ranges can really alter the figures. Keep an eye on our rain maps to better work out the chances of rain where you live, due to the volatile nature of this band of rain (and how narrow it is) there may still be more adjustments to totals and those most affected.

Please remember this rain band is narrow – so for those on the fringes drizzly showers that may not amount to much are to be expected. But for those many more people in the direct line of this rain band a soaking event with even the risk of slips and flooding is possible.

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– Thursday’s rainfall map shows the narrow but heavy band of rain from Northland to Taupo, with some spillover into Gisborne and Hawke’s Bay / Weathermap

– WeatherWatch.co.nz