Westerly flow to dominate NZ until August, winter is being bypassed


WeatherWatch.co.nz says a blast of westerlies over New Zealand will continue off and on for the rest of July, helping push temperatures up above normal.

While not every day has a westerly (we have some nor'easters and southerlies in the mix) the dominant theme is for westerlies. The westerly flow is being caused by huge highs which seem stuck over Australia, sending a narrow ridge just north of NZ while lows continue in the Southern Ocean - this set up places New Zealand in the 'squash zone' between the northern highs and southern lows and this encourages windier westerlies.

Westerlies bring more rain to western NZ, especially the West Coast where the Southern Alps block the rain clouds and lift totals. With cold air also in the mix further south this rain will fall as heavy snow higher up in the mountains. WeatherWatch.co.nz says another metre of snow is possible at higher elevations, which increases the avalanche risks especially into next week.

Cold days are in the mix, but overall WeatherWatch.co.nz says more above normal temperatures are coming thanks to the westerly flow which will continue for the rest of this month - although some this weekend will have southerlies as a low crosses the country. This westerly flow is warmer and brings in weather that sometimes feels more like spring or autumn, or a feeling that true winter is being bypassed. Some colder nights next week may see frosts returning but even in Hamilton, which is one of the coldest northern centres in the country, has lows of +3 or +4. Christchurch looks even milder by night with lows around +6 to +8.

As for winter, regardless of when you think it starts and ends the coldest air is usually peaking from now and across the next four weeks. If we're being dominated by westerlies for the next two weeks that reduces the chances of major cold weather for NZ. Once we get to mid August the longer days are noticed by many and temperatures often start to tick upwards by a degree or two, even despite the risks of big snow storms and frosts which can occur right up to October. So has winter peaked already? Time will tell - but time is running out.

- Image / Next Wednesday shows more of the same for NZ with westerlies dominating most regions.

- By head forecaster Philip Duncan, WeatherWatch.co.nz