MetService introduces a red warning for life-threatening weather

Wednesday, 15 May 2019

MetService meteorologist Georgina Griffiths talking about 'SAM' - the Southern Annular Mode or polar vortex, which plays an increasingly important role in determining our weather.

The very worst weather nature can throw at New Zealanders will be the subject of new 'red warnings' issued by MetService.

The state-owned enterprise is moving to a two-tier severe warning system from May 21.

An orange warning will be issued for weather with the potential to affect people, animals and property, with a red warning reserved for extreme disruption and when 'immediate action' is needed to protect life and livelihood.

Watch out for one of these - the MetService's new red warning will be reserved for the absolute worst, most dangerous weather which can affect life and livelihood.

Severe weather watches – when bad weather is possible but not certain or about to hit – will be coloured yellow.

**READ MORE:

Heavy snow affects stock in paddocks along State Highway 7 to Hanmer Springs in June 2012. This kind of weather would probably require an orange warning and possibly a red one.

* Weather forecasters challenged by recent bad weather

* Polar vortex helping forecasters predict New Zealand weather

The new orange warning signifies approaching bad weather is likely to be bad enough for people to prepare for disruption and could pose a potential risk to them, animals and property.

* Power still out in parts of Auckland as MetService issues new wind warning after rain and gales

* Icy blast grips much of New Zealand**

A yellow watch is the lowest tier of the MetService's new system, signifying when severe weather is possible but not imminent or certain.

MetService says red warnings would probably have been issued for the extremely heavy rainfall in Westland in late March, for former tropical cyclone Gita in February 2018 and the Auckland windstorm in April 2018.

Somewhat ironically, the number of severe weather events affecting New Zealand has fallen during the past 12 years.

MetService senior communications meteorologist Lisa Murray said that was due to natural variability and the ability of today's computer forecasting models to more closely distinguish between whether an event reached the New Zealand coast or stayed just offshore, and therefore was not counted.

Since 2007, the number of severe events each year had been 72, 70, 69, 69, 66, 58, 60, 69, 63, 62, 51 and 56 last year, she said.

The amount of actual warnings issued for those events varied depending on how long the system bringing the bad weather stuck around.

Heavy snow falling at Christchurch Airport in June 2012, causing major disruptions to thousands of people's travel plans.

'So in that last 12 years there's been an average of 63 severe weather events each year. But we're not quite comparing apples with apples because some of the criteria have changed in that time.'

Regional councils could request specific changes to criteria for weather their residents were especially vulnerable to.

Auckland had a lower wind threshold for a severe weather warning in northeasterlies and the heavy rainfall criteria for North Otago and Dunedin were also lower than for elsewhere, Murray said.

Another example of how much better the computer guidance now was for forecasters was that in 2006 there were false alarm ratios of 29 per cent for heavy rain and 23 per cent for severe gales.

But in 2017, the corresponding figures were 9 per cent and 16 per cent, she said.

MetService issues warnings when forecasters expect that within the following 24 hours there will be widespread:

- Rainfall of greater than 50mm within six hours or 100mm within 24 hours.

- Snowfall below 1000 metres in the North Island, South Canterbury or Otago, or below 500m elsewhere across the South Island with a snow depth of 10cm in six hours or 25cm in 24 hours.

- Severe gales with a minimum mean wind speed of 90 km/h or frequent gusts exceeding 110kmh.

MetService's new warning system is similar to those of the UK Met Office and Met Éireann, the Irish Meteorological Service.