New Zealand: Adverse weather forecast over much of South Island and parts of North Island through at least May 20 /update 2

Event

Severe weather is forecast across portions of South Island and North Island through at least May 20. Heavy rainfall is expected along the west coast of South Island and over northeastern and northwestern areas of North Island May 18-19 and strong winds are likely over large parts of South Island and southern areas of North Island late May 18-May 19. Snowfall is also possible over higher ground in western parts of South Island. The New Zealand National Meteorological Service (MetService) has warned of a high risk of thunderstorms with possible small tornadoes over western parts of South Island May 18-19. There is also a moderate risk of thunderstorms across northern parts of South Island and southwestern parts of North Island May 19 and a low risk of thunderstorms in northwestern parts of North Island May 18-19.

As of late May 18, the MetService has issued the following weather warnings across the affected region:

• Orange (middle level on a three-tier scale) Heavy Rain Warning: Ranges of Bay Of Plenty east of Opotiki; Westland about and south of Otira; and Fiordland.

• Heavy Rain Watch: Northland; Buller, Westland north of Otira; the headwaters of Canterbury lakes and rivers about and south of Arthur's Pass; and the headwaters of Otago lakes and rivers.

• Strong Wind Watch: Canterbury High Country; Central Otago, Southern Lakes, Clutha, Southland, and Stewart Island; Wairarapa, Wellington, and Marlborough Sounds; and Marlborough excluding the Sounds.

• Road Snowfall Warnings: For Crown Range Road and Milford Road (SH94) in southern South Island.

Authorities will likely issue new alerts or update/rescind existing advisories as weather conditions change over the coming days.

Hazardous Conditions
The storms could produce rounds of heavy precipitation (including rain and snow), strong winds, and isolated thunderstorm activity in affected areas. Where precipitation falls as rain, flash and areal flooding is possible. Such flooding is possible in low-lying communities near watercourses and other large bodies of water, as well as in urban areas with easily overwhelmed stormwater drainage systems. Sites located downstream of large reservoirs may be subject to flash flooding after relatively short periods of intense rainfall.

Precipitation could fall as snow in the higher elevations over the coming days. Wind gusts could cause blowing and drifting snow; decreased visibility is likely in mountainous areas. Rain-induced landslides cannot be discounted in areas of elevated terrain; there is also the possibility of avalanches in mountainous areas where the snowpack has become unstable due to heavy snowfall. Power outages could occur throughout the affected area.

Transport
Floodwaters and related debris may render some bridges, rail networks, or roadways impassable, impacting overland travel in and around the affected area. Flooding in urban areas could also result in significant traffic congestion. Heavy snow will likely make driving hazardous in some areas; authorities could implement temporary road closures or detours in such locations. Mountain passes and tunnels could be closed as a precautionary measure during periods of intense snowfall.

The disruptive weather may cause some delays and cancellations at airports in the worst affected areas. Authorities may temporarily suspend port operations if strong winds trigger hazardous sea conditions, impacting freight and passenger maritime traffic. Flooding or snow could block regional rail lines; freight and passenger train delays and cancellations are possible in areas that see heavy rainfall and potential track blockages.

Disruptions triggered by inclement weather and resultant hazards, such as flooding, landslides, or avalanches, could persist well after conditions have improved - it could take days before any floodwaters recede and/or officials clear debris. If there is severe damage to infrastructure, repair or reconstruction efforts may result in residual disruptions.

Advice

Monitor local media for weather-related updates and advisories. Confirm all transport reservations and business arrangements before traveling in affected areas. Seek updated information on road conditions before driving or routing shipments through areas where severe weather is forecast; plan for possible supply chain disruptions throughout the affected areas. Stay away from elevated streams, creeks, and other watercourses that are prone to flash flooding. Do not attempt to navigate flooded roadways. Exercise caution in elevated terrain due to the threat of landslides, as well as mountainous regions where avalanches pose a threat. Charge battery-powered devices in the case of prolonged electricity outages.

Resources

New Zealand National Meteorological Service