Would a rain radar have made a difference?
The flash floods the swept through central Nelson on Monday have reignited the debate about the need for a rain radar in the top of the south.
An intense deluge of rain on Monday morning – combined with high tide and drains blocked by autumn leaves – caused torrents of water to wash down central Nelson streets, blowing out manhole covers as the stormwater system was overwhelmed, and flooding businesses and homes.
According to the MetService, 46.6mm of rain fell between 7am and 12pm on Monday, including 18mm from 9am to 10am, and a further 13.6mm between 10am and 11am.
Anything over 6mm an hour is considered heavy.
Mayor Nick Smith said the event reinforced the need for a rain radar.
“The weather events this week are a reminder of how our region would be better prepared if we did have a rain radar, and I made that point to the ministers that were visiting during the deluge yesterday.”
Both Smith and Tasman mayor Tim King have campaigned for a radar to help improve emergency response in the region, particularly after the August 2022 weather event caused significant damage.
Last year, The Nelson Mail revealed that MetService chief executive Stephen Hunt had referred to Nelson-Tasman as a “vulnerable region”, in emails released under the Official Information Act.
Following Monday’s weather event, Smith said more accurate forecasting with a local radar would have helped, as the region relies on the Wellington radar for forecasting.
“The issue with the Wellington radar is that it is quite limited in the shadow behind the Richmond Ranges and Marlborough Sounds and, as a consequence, we can have very intense rainfall coming and not knowing.”
While there still would have been surface flooding, there would have been an opportunity to warn affected areas, he said.
“The orange warning for [Monday’s] event was principally in the Western Ranges, rather than in Nelson City, and so the weather radar would have enabled us to be able to give better warnings to the central city area that was hit by the very intense rainfall.”
A MetService spokesman said the forecast for Nelson City on Monday was for heavy rain clearing the city during the morning.
“Since the previous Thursday the forecast had been highlighting heavy rain in the city. The heaviest rain was, however, forecast in the west of the region and this is where the focus of the Severe Weather Warnings was, over the ranges to the northwest of Motueka.
“Using the latest available forecast guidance, our team of meteorologists determined the risk of thunderstorms and downpours to be low but actively monitored the situation.
“The relatively brief, localised downpours … formed and dispersed quickly, leaving little time for a warning to be issued once radar and rainfall observations had been assessed.”
Asked if a local radar would have made a difference, they said radar was useful for a live picture of what is happening in a place, and therefore what is likely to happen in the next hour or two.
“For effective response to severe weather events, much longer lead times are needed for warnings – preferably at least a day in advance. Radar pictures are in real-time so do not give this information.
“Once weather is occurring in a region, the radar becomes valuable for real-time monitoring and is then the basis for Severe Thunderstorm Warnings that are issued once storms are identified in radar imagery and can be tracked.”
The 2023 upgrade to the Wellington radar replaced 30-year-old technology had improved performance for Nelson-Tasman, the MetService spokesperson said.
“However, due to the curvature of the earth and radar return shielding from mountains, there are still gaps in coverage in the lowest parts of the atmosphere. While this compromises data quality in the lowest layers of the atmosphere, the existing radar network still provides valuable data and information in the Nelson/Tasman region for real-time monitoring for forecasting and warnings.”
MetService was continuing to replace radars in the national network, with the next replacement planned for Auckland in early 2027.
A new radar for Nelson-Tasman would cost $3 million and needed Government investment to expand the radar network, they said.
Radar was just one tool for monitoring weather, they said.
“Rain gauges in land-based automatic weather stations also provide additional real time information to build a more complete picture of current rainfall in the region.
“We have a number in the area and also access to the council’s own gauges to help provide this detail. In the last two years we’ve gained access to an additional five rain gauges through third party partnerships.”
By Warren Gamble, Nelson Mail
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