Large low to affect weather for entire week

The weather patterns, they are a changing. Following several weeks of dry, mostly settled, weather we're seeing Autumn shift up a gear despite warmer weather moving in. Since early March high pressure has dominated the Tasman Sea therefore controlling the weather moving in from the west. However last week the high finally vacated it's Autumn camping spot and gave way to two sub-tropical tourists who brought rain last Monday and gales on Saturday. The last sub-tropical low also helped give us an incredibly mild weekend. I doubt there are many New Zealanders who when asked to describe 'typical Anzac Day weather' would say "An 18 degree, mild, morning". And while it wasn't that warm in the South Island for dawn services northern New Zealand certainly did wake to almost summer-like conditions. The South Island had their turn too though. While most places in the North Island hovered around a very mild 19 to 20 degree mark on Sunday the South Island went up a notch to what could be described as 'hot' weather. Dunedin, Timaru, Alexandra and Kaikoura all made it to 27 degrees yesterday, Christchurch and Oamaru 26 and a string of other centres reached the mid 20s. No guessing which way the wind was blowing. The North Island was also hot for April - Palmerston North made it to 25 and Wanganui 24.

Northerlies will continue to dominate New Zealand for the start of this week giving us more frost-free mornings and t-shirt afternoons (if you don't mind a few showers here and there). So does that mean winter is going to be mild? Has global warming removed our typical Autumn? Will the flowers continue to bloom right up until Spring? It's pretty obvious I'm about to say no. We're being lulled into a false sense of security. While locally around New Zealand the winds are warm and temperatures are high the cold air is starting to build around Antarctica...quite significantly. The sun no longer rises and sets at McMurdo base in Antarctica...any "heat" that was around Antarctica is now seeping out of the atmosphere and into outer space.

What does that mean for New Zealand? Well it means it's only a matter of time before a storm comes along and kicks us into winter...gives the weather patterns the shove they need to get into the really cold stuff.

We're in a change in patterns now. It's unclear how long it will last but it looks as though we're in for another week of low air pressure in the Tasman Sea fuelling more rain or showers across the country. A very large low near Tasmania will slip south of New Zealand this week - the low is so large it will literally affect our weather for the entire week. Starting off with the warm, wet, northerlies...and changing to cold southerlies or south easterlies around Wednesday in the South Island then reaching the North Island as a cold south-easterly on Thursday. Snow will fall to relatively low levels for a time in the south and daytime highs from Invercargill to Timaru will struggle to climb much above 10 degrees from Thursday to Saturday. That means eastern places (of both islands) will feel the brunt of the cold change with western and northern areas warmer - and drier - towards Friday. Christchurch and Kaikoura will fair only a little better with highs around 12 and the cold southerly or south-easterly arrives in the Capital on Thursday with highs also around the 12 degree mark heading into the weekend.

Auckland and other regions, on the other hand, will only drop to the 16 or 17 degree mark on Friday and Saturday thanks to the east coast absorbing much of the cold air (and cloud cover).

A small high pressure system may nick the South Island towards the end of the week bringing a brief period of settled, possibly frosty, weather but already long range weather maps (or 'charts') are showing another belt of low air pressure moving in. This may all be indicating a change to wetter Autumn weather and a strong sign that we're shifting gear towards winter.

Philip Duncan

Photo / Paul Estcourt