Why is New Zealand getting such a big cold blast? It's all to do with a high (+6 Maps) | WeatherWatch.co.nz

Why is New Zealand getting such a big cold blast? It's all to do with a high (+6 Maps)


This week is a very busy week weatherwise in the New Zealand area with a major southerly blast being the main feature but another tropical cyclone also getting into the mix.

Tropical Cyclone Keni has formed around Vanuatu according to the Fiji Met Service and will affect Vanuatu and Fiji in the coming days with wind and rain and big seas. It should miss Tonga to the south but remains one to watch for them.

Later in the week what was Cyclone Keni will merge with a large low pressure system just to the east of the North Island making for a much bigger area of low pressure that has tropical air and sub-Antarctic air being mixed up in it. Keni will not make a direct hit to NZ based on modelling that we trust.

The southerly change is the main feature this week for New Zealand bringing a dramatic cut back in temperatures for most areas with highs going from the low 20s to the mid single digits for some parts of Southland and Otago this week and in the eastern North Island going from the low 30s last week to daytime highs in the low to mid teens this week.

Snow is expected to settle down to 300m in the South Island and down to 500m in the North Island affecting alpine highways. The southerly blast will also run into moisture over central New Zealand encouraging cold rain lower down and heavy snow higher up in the mountains. Rain could be quite heavy in North Canterbury with heavy snow up in the mountains.

Strong winds may also be a feature in some wind tunnels with gales expected in various parts of both islands over the next 48 hours, mostly though western/central parts of the country.

The southerly blast is being caused by a long, tall, high pressure system in the Tasman Sea. "Most highs we get stretch sideways from west to east but every now and then you get one shaped like this and the air circulating around the high can have great reach. In other words, a long tall high like this can scoop up air from Antarctica and dredge it up and over New Zealand" says WeatherWatch.co.nz head forecaster Philip Duncan. "If this was happening in winter we'd be talking about snow to sea level up to Wellington, like we saw in August 2011".

WeatherWatch.co.nz says an incoming high will ease conditions in the South Island by mid to late week with frosts in the south and inland but the New Zealand weather pattern may be a bit messy this week with a number of highs and lows in the region right up until the weekend itself.

Warmer northerlies return to most places by Sunday or Monday.


GALES - Noon Tuesday


TROPICAL CYCLONE KENI - AS OF MONDAY MORNING (FIJI MET SERVICE)


NOON TUESDAY SET UP:


MIDNIGHT WEDNESDAY COMPARED TO MIDNIGHT THURSDAY (Showing Cyclone Keni merging with low pressure east of NZ)

12am Wednesday:

12am THURSDAY:

- WeatherWatch.co.nz