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Source: MetService

Covering period of Sunday 3rd – Tuesday 5th March – This weekend was hardly a calm start to meteorological autumn, with a series of fronts bringing areas of rain with heavy falls to both islands, as well as strong winds and fluctuating temperatures. But heading into a new week, MetService is forecasting widespread severe weather as an intense trough of low pressure closes in from the west.

The trough reaches the South Island tonight, moving eastwards across Aotearoa New Zealand during Monday. Heavy rain is expected for the western South Island as the trough passes, as well as the already sodden central North Island. Orange Heavy Rain Warnings are in place for parts of Fiordland, the Otago headwaters, the Tararua Range and Mount Taranaki. Heavy Rain Watches are in place for the Westland ranges, the Headwaters of Canterbury lakes and rivers from Arthurs Pass southwards, central and western areas of the North Island and the ranges of easter Bay of Plenty and Tairawhiti.  

Widespread strong or gale northwesterly winds also accompany the trough, easing in the south on Monday morning as the trough moves away, but affecting central areas until Tuesday afternoon. A Strong Wind Warning is in effect for the Canterbury High Country with gusts expected to reach 120km/h in exposed places. Strong Wind Watches are in force for Fiordland, inland Southland and Otago, and central and southern parts of the North Island, gusting 100 km/h in exposed places.
MetService meteorologist Ngaire Wotherspoon advises, “Stay up to date with the latest forecasts on as the situation develops. More Warning areas could be added further north, and existing Watches could be upgraded as we gather more information.”

Strong, cold southwesterly winds follow the trough, spreading heavy showers with embedded thunderstorms over southern and central Aotearoa. This unseasonable cold change will be felt across the country at the beginning of the week, and snow could settle above 1000 metres in Fiordland and the ranges of Otago on Monday.
Wotherspoon notes, “Hot temperatures occurred in eastern South Island areas over the weekend, so this cold southerly change will be felt keenly over the next couple of days. Alexandra had a high of 29°C on Saturday, and by Tuesday they have a forecast high of 14°C, and 4°C overnight.”

The strong southwesterlies are likely to generate large swell along the south and west coasts later in the day on Monday, before easing on afternoon Tuesday as the trough moves away. A Heavy Swell Warning has been issued for Otaki to Cape Terawhiti with northwest combined waves rising to 3.5 – 4 meters from Tuesday 1am until 1pm.  “An end to the wild weather is on the horizon; a ridge of high pressure settles over most of the country on Wednesday, bringing calmer, clearer conditions,” says Wotherspoon.

Understanding MetService Severe Weather Warning System

Severe Thunderstorm Warnings (Localised Red Warning) – take cover now:

This warning is a red warning for a localised area.
When extremely severe weather is occurring or will do within the hour.
Severe thunderstorms have the ability to have significant impacts for an area indicated in the warning.
In the event of a Severe Thunderstorm Red Warning: Act now!

Red Warnings are about taking immediate action:

When extremely severe weather is imminent or is occurring
Issued when an event is expected to be among the worst that we get – it will have significant impact and it is possible that a lot of people will be affected
In the event of a Red Warning: Act now!

Orange Warnings are about taking action:

When severe weather is imminent or is occurring
Typically issued 1 – 3 days in advance of potential severe weather
In the event of an Orange Warning: Take action.

Thunderstorm Watch means thunderstorms are possible, be alert and consider action

Show the area that thunderstorms are most likely to occur during the validity period.
Although thunderstorms are often localised, the whole area is on watch as it is difficult to know exactly where the severe thunderstorm will occur within the mapped area.
During a thunderstorm Watch: Stay alert and take action if necessary.

Watches are about being alert:

When severe weather is possible, but not sufficiently imminent or certain for a warning to be issued
Typically issued 1 – 3 days in advance of potential severe weather.
During a Watch: Stay alert

Outlooks are about looking ahead:

To provide advanced information on possible future Watches and/or Warnings
Issued routinely once or twice a day
Recommendation: Plan