He explained that thunderstorm notifications came in three stages: outlooks, watches and warnings.
“Outlooks are designed to give a heads-up that severe or impactful weather is coming at some point in the future (usually three to six days),” Noble said.
“As the event draws nearer and it becomes more certain we are likely to have weather that reaches the criteria to justify a warning, we’ll issue a watch; this is a heads-up we’re close to an event and it looks like it could be impactful.”
The watch would be upgraded to a warning once it became relatively certain that the storm was imminent.
Noble also explained the criteria MetService used to rule that a thunderstorm was “severe”.
“MetService classifies a thunderstorm severe if one or more of the following criteria are met: heavy rain from thunderstorms of 25 millimetres per hour, large hail, strong wind gusts from thunderstorms of 110 kilometres per hour or more, or damaging tornadoes with a wind speed more than 116 kilometres per hour.”
Weather warnings were assigned colours from yellow to red to indicate severity, and given the term “broad scale” if they covered an area of 1000sq km.
Noble said it was very rare for a weather event to be both on a broad scale and meet the criteria for a red warning.
The “broad scale red warning” label was only issued when there was an event with expected severe impacts and a potential threat to life over a significant area, and just 16 of these warnings had been issued since the colour-coded system was introduced in 2019.
Auckland’s anniversary weekend storm was the ninth of these, and Cyclone Gabrielle the 10th, Noble said.
He said when the colour-coded system was first established, MetService only expected one to three weather events on that scale each year but eight had occurred since the start of 2023.
Responding to questions about the pitfalls of weather forecasting, Noble said forecasting thunderstorms was like trying to predict which popcorn kernel would pop first.
“If you can imagine a pan sitting on your stove and you tip some kernels into the pan we can identify some of the necessary conditions for those kernels to pop, but it’s very difficult to understand exactly which of those kernels will pop first,” he explained.
“Thunderstorm forecasting is not too dissimilar, just on a much bigger scale.”
He said flaws inherent to the technology used by meteorologists limited their ability to provide truly accurate forecasting.
“He noted that the models used by MetService had underestimated the storms.
“Two of our local models that were available approximately six hours prior to the start of the event were indicating substantial rainfall but remained well short of the actual severity of the storm, reflecting inherent weaknesses in the model performance for this type of weather system,” he said.
Cyclone Gabrielle, on the other hand, could be seen several days in advance.
“The higher predictability on a large or broad scale for the presence and track of the cyclone allowed forecasts and warnings to be issued with a longer lead time than normal”, Noble said.
“For the most part, forecasts and warnings related to Cyclone Gabrielle performed well, but, as with the Auckland Anniversary storm, we were unable to predict extreme outcomes on very localised scales such as in the Esk Valley.”
The inquest will continue this week with further evidence from Niwa and emergency response agencies including Nema and Fire and Emergency NZ.
– RNZ
