A coroner has questioned why MetService publishes hourly rainfall forecasts at all, after finding they may have played a part in a school's decision to go ahead with a caving trip on which a student died.
The findings, reported by 1News on Tuesday, came as MetService had severe thunderstorm watches in place for Auckland, Great Barrier Island, the Coromandel Peninsula and Northland, and heavy rain watches from Northland to the central North Island.
What did the coroner find?
A 15-year-old Whangārei Boys' High School student died on a school trip to Abbey Caves in Whangārei in 2023. An orange heavy rain warning was in place for Northland at the time.
Coroner Alexander Ho criticised the school's decision to go ahead with the trip, 1News reports. He also focused on the hourly rainfall numbers shown on MetService's website and app.
These hourly figures are generated automatically by MetService's computer weather model and are not checked by its forecasters before they are published. The worded forecast for Northland that day was for rain with squally thunderstorms and downpours. The hourly figures the morning of the trip showed 1 to 4mm of rain an hour between 7am and 5pm, according to the coroner.
"This raises the obvious question of why MetService publishes hourly numbers at all," Ho wrote.
"Not receiving the expected forecast rainfall for one's lemon tree is inconvenient. Being caught outside in a thunderstorm during a high-risk outdoor activity is dangerous."
He also found that many people misread the hourly figures. The coroner said a rainfall figure shown in the "9 am" column is for the hour leading up to 9am, not the hour after it.
He recommended MetService make its definitions and disclaimers more prominent, and make the wording the same on its website and app.
MetService chief meteorologist Chris Noble said watches and warnings "should always take precedence over other forecast information", and that MetService had already been making changes. Its website now says the hourly model data can differ from the written forecast and icons, which are produced by meteorologists, and that "if they differ significantly, please use the text forecasts."
What do the alert levels mean?
MetService is New Zealand's only authorised provider of severe weather warnings. It uses a colour system:
- Watch (yellow): severe weather is likely but not certain. Stay alert and check your local forecast for updates. A watch usually comes two to three days ahead and includes a "chance of upgrade" to a warning.
- Orange warning: the weather is expected to meet MetService's severe weather criteria. Be prepared and take action, as there may be disruption and risk to people, animals and property. Most warnings are orange.
- Red warning: reserved for the most extreme events. MetService's message is "act now", and follow the advice of official authorities and emergency services.
Thunderstorms are handled slightly differently because they develop quickly. A thunderstorm watch means there is a higher risk of severe thunderstorms, so be ready for sudden strong wind, heavy rain and hail. A thunderstorm warning is always red and is issued when a severe thunderstorm has actually been detected. MetService says to take immediate action.
The chance of upgrade is given as minimal (under 10 percent), low (10 to 30 percent), moderate (30 to 60 percent) or high (over 60 percent).
What does that mean for this week?
Tuesday's thunderstorm watches warned of possible downpours of 25 to 40mm an hour. MetService says rain that heavy can cause surface and flash flooding, especially around streams, rivers and narrow valleys, and may lead to slips. Driving can be hazardous, with surface flooding and poor visibility.
The heavy rain watches for Auckland, Northland, Coromandel and other areas had a moderate chance of being upgraded to a warning when they were issued on Monday night.
Which forecast should I trust?
Go by the official watch or warning first, then the written forecast for your area. Treat the hourly numbers as a rough guide only, and remember each figure covers the hour before the time shown.
This matters most if you are planning something where weather can turn dangerous quickly, such as fishing off rocks, tramping, river crossings or caving. Our rock fishing safety guide has more on checking conditions before you go. If heavy rain does damage your home or car, our earlier story looks at what rising weather claims mean for household insurance.
