August 2022 Heavy Rain Event - South Island West Coast / Nelson Tasman

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16th August 2022

Situation

A high pressure system to the east of New Zealand is pushing a northerly flow onto the South Islands West Coast.
 
 
Between today and late Saturday evening, the high pressure is expected to intensify, and remain stationery. The system is associated with a band of heavy rain from the tropics, and this is expected to bring a significant amount of rain to Buller, and Westland, with well over a months rain set to fall within the next few days.
 
Metservice have issued a Red rainfall warning for these area, and an orange warning for Nelson and Marlborough. Watches and warnings in place are as follows:
 
For Buller and Westland, Metservice have issued the following red weather warning:
 
Note that for the most recent watches and warnings, you should always refer to New Zealand's official forecaster, Metservice.
 

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17th August 2022

Situation

 
The image below (Courtesy of Windy.com) provides a slightly wider view tonight, illustrating exactly where the moisture is coming from, and how this system is set up:
 
 
 
With the High pressure to the east of the country intensifying, it's essentially staying put - a blocking high for the low pressure coming in from the west. The air from the high pressure is pushed up into the tropics, heated and therefore can carry more moisture. It then hits the colder air associated with the low pressure - in the image above you can see there is a temperature difference of around 27C-18C = 9 degrees C. This causes the moisture to be dropped from the warm air, and it's directed down the back of the high pressure and forced onto the west coast of New Zealand.
 
Comparing this to last nights image the temperature difference was only about 5C, indicating that the rain-making engine has increased it's ability to generate moisture over the last 24 hours.
 
During the day, heavy rainfall has occured in the Nelson and Tasman regions. Many roads in Golden Bay are now closed due to slips.
A state of emergency was declared in Nelson Tasman, with over 200 homes requiring evacuation. 
 
The Maitai River, which is usually a fairly sedate river, has broken  it's banks.
                     
River flow measurements available on the Tasman District Council’s website showed the river went from flows of 3.31 cubic metres per second (cumecs) at 5am, to 446.87 cumecs at 4pm.
The flows increased steadily from 5am to noon, when there were 91.61 cumec flows, before sharply increasing from 1pm onwards.
  - Stuff.co.nz
                          
 
In Takaka (Golden Bay), more than a months rainfall occured within 15 hours.
 
Roads between Blenheim and Nelson have also been closed until at least Friday.
 
Further down the island, Buller and the West Coast have reported flooding, but so far not as bad as was expected. About 130 homes have been voluntarily evacuated. Rainfall has been heavy in river catchment areas, and for the Buller area, the Buller River is expected to peak over night tonight. This translates to flooding downstream, as was the case with the Maitai River, described above. Therefore it's likely the impact of this event on Buller will be better known from tomorrow.  
 
Moving the model forward it appears that there will still be significant rainfall to central New Zealand , and the West Coast until at least late on Saturday evening.
 
 

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18th August 2022

Situation

Rain continues to fall in Nelson, Golden Bay, and Buller. All three regions are now under red warnings from the Metservice. Heavy rainfall has also fallen today in Northland, and much of the North Island ins now under some sort of rain watch or warning.
 
Today's screen grab was taken at 9:00pm tonight:
 
The High pressure, largely stays put. As it's not moving, the Low pressure that was squeezing up behind the high pressure gets pushed south, and turns the tail of the rain band towards Marlborough and Wellington regions. Orange rainfall warnings are in place tonight for these areas.
 
Northland and Taranaki also get some significant rainfall as the rain band gets pushed further east. Another Low has developed to the North West of New Zealand. The temperature difference between the warm and colder side of the front is now slightly less at 6C. While this still creates stormy conditions (lightning) and heavy rainfall, it suggests that the rainfall generating area of this storm is weakening.
 
The map below shows the rather complicated setup at 6pm tonight:
 
 
The video below was captured at midday today (before the low pressure crossed the South Island). It captures a period of time using the predictive ECMWF model up to early Sunday morning when the main rainband moves off the country
 
 
You can see during this period that the high pressure weakens and moves north, allowing the part fo the rainband affecting the North Island, to move to the east of the country. However this takes a number of days, so at this point there is still alot of rain to come down from the tropics.
 

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19th August 2022

Situation
 
 

The anticyclone to the east of the country holds steadfast, although it's now showing signs of a slight weakening, having dropped 6mb of central pressure in the last 24 hours. The ECMWF model suggests a similar pattern over the next 24 hours. Rainfall wise, the slight weakening of the anticyclone and the splitting of the low pressure (central pressure around 1012 mb) has allowed the main rain band to move into a more northerly position.

This means that heavy rainfall is still (at 9pm tonight) occuring in the Nelson Tasman area. The good news is that the rain band will move rapidly across the North Island tonight, and is expected to lie to the east of the country at midday tomorrow. There will be some showers about but nothing as heavy as has been seen in previous days. 

A state of emergency has now also been declared in Marlborough today, following Nelson Tasman and Buller regions earlier in the week. Northland also has been hit hard with flooding. Wellington has also seen several additional slips today. Nelsons Mayor has said the damage done to the area this week will take years to recover from.

The video below indicates the ECMWF model prediction over the next 24 hours

 

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20th August 2022

Situation

 
Alot has changed since last night. The Anticyclone to the east of the country has again decreased it's intensity, allowing the main rain band to move off the country. There's still some showers about, but by and large it's time to dry out. A seperate system is now across Fiordland and as of midday this is the only area with a red warning in place.
 

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Related Media Subsequent to the above analysis

 

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