Extreme rainfall event – Rare Red Warning for Heavy Rain issued for Canterbury region of South Island, New Zealand

The New Zealand MetService has issued a rare Red Warning for Heavy Rain for the Canterbury region of South Island for the weekend. Meteorologists warn this could be the most significant rainfall for the region in 10 years. Heavy rain is also expected over other parts of central New Zealand, and severe gales over the South Island's west coast.

• This is only the second Red Warning issued by MetService for the Canterbury region.
• Some areas are expected to receive 2 to 3 months' worth of rain in just 2 to 3 days!
• The rain is expected to cause dangerous river conditions and significant flooding.
• Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities.
• The atmospheric river headed for Canterbury this weekend can be linked back to the Solomon Islands and Fiji, some 4 000 km (2 500 miles) from New Zealand.
• If you live in the affected area, prepare now.

A complex low pressure system over the Tasman Sea is forecast to move slowly southeastwards across northern and central New Zealand over the weekend, May 29 – 30, 2021, before moving away to the east on Monday, May 31.

This system is expected to direct a strong and moist southeast flow across the South Island during this time, delivering an extended period of significant heavy rainfall to Canterbury, and severe gale southeasterlies along the South Island west coast.

In addition, an associated front is forecast to move eastwards across northern and central New Zealand on Saturday, delivering a relatively short burst of heavy rain to some areas.

This is expected to be a significant heavy rainfall event for Canterbury.

A Red Warning for Heavy Rain is in force for the region south of Amberley, and an Orange Warning for Heavy Rain is now in force for the region from Amberley to Kaikoura.

Dangerous river conditions and flooding are expected.

Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities.

The Strong Wind Watch for Fiordland and Westland south of Hokitika has also been upgraded to a Warning for severe southeasterly gales.

People are advised to keep up to date with the latest forecasts in case further areas are added, or any changes are made.

Credit: NZ Met Service

From 15:00 LT on Saturday, May 29 to 11:00 LT on Monday, May 31, Canterbury High County and foothills can expect 200 to 300 mm (7.9 – 11.8 inches) of rain, with 80 to 120 mm (3.1 – 4.7 inches) of rain farther east about the Plains and coast. Peak hourly rates of 10 to 20 mm (0.4 – 0.8 inches) are expected, especially from later Saturday and during Sunday.

Also note, the freezing level is forecast to be around 1 400 m (4 600 feet) on Saturday, then lifting to around 1 800 m (5 900 feet) on Sunday, before lowering to 1 400 m again on Monday.

Heavy Snow is expected above the freezing level throughout the event, but heavy snow may fall as low as 1 000 m (3 300 feet) in some places at times, especially on Saturday and on Monday.

This rain is expected to cause dangerous river conditions and significant flooding. Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities, MetService meteorologists warned.

From 07:00 to 20:00 LT on Saturday, Canterbury about and north of Amberley, and Marlborough about and south of Kaikoura, can expect 120 to 200 mm (4.7 – 7.9 inches) of rain and peak rates of 10 to 20 mm (0.4 – 0.8 inches) per hour on Saturday evening and during Sunday.

Heavy rain may cause streams and rivers to rise rapidly in this area. Surface flooding and slips are also possible and driving conditions may be hazardous.

From 13:00 LT on Saturday to 09:00 LT on Monday, Westland south of Hokitika and Fiordland can expect southeasterly gales, forecast to be severe at times, with gusts reaching 120 km/h (75 mph).

Winds of this strength could damage trees, powerlines and unsecured structures. Driving may be hazardous, especially for high-sided vehicles and motorcycles.

Featured image credit: NIWA