A ridge of high pressure has held sway over New Zealand since early November, and has set the tone for hot and dry weather.

In the last week of November conditions were favourable for inland thunderstorms driven by afternoon heating, but these eased off as we moved into December and stable weather resumed.

This week however, has seen an increase in the ability of fronts to make progress into the ridge, although they have weakened as they do so. Following the fronts southerly winds have allowed for cooler air masses, particularly in the south.

On Wednesday a southerly moved up the country, bringing rain and thunderstorms especially to eastern parts, and temperatures at Dunedin airport reached a daily maximum of only 14 degrees. Two days later, northerlies were back and the airport recorded a max of 28 degrees.

A front is working up the South Island today, and a Severe Weather Watch for heavy rain is in force for Fiordland. Following the front southerlies set in, and the watch extends to Dunedin and North Otago where heavy rain and thunderstorms could bring up to 40mm of rain in 12 hours. On Monday this front moves over the North Island, bringing a brief spell of rain.

Unfortunately, the front is fast moving, so total rainfall amounts are not expected to be large except in localised spots affected by thunderstorms. For this reason, a watch for heavy rain has been issued for ranges of Gisborne and eastern Bay of Plenty during Monday afternoon and evening.

Thunderstorms also make a return today and next week, with a watch for severe thunderstorms issued for the central North Island this afternoon and evening. The South Island could also see a few strikes, both in the west as the front passes and in the east in the following southerly. On Monday the risk of thunderstorms eases in the South Island, but persists for much of the north.

Looking ahead more fronts find their way through the ridge, bringing much of their rainfall to the southwest. Temperatures will yoyo, with many places expecting top temperatures around 10 degrees cooler tomorrow, and seesawing through the week. Christchurch for example has maximum temperatures of 30 today, 19 on Monday, 26 on Tuesday, 19 on Wednesday and 29 on Thursday.

'As fronts make inroads onto New Zealand we can expect more variable weather conditions,” says meteorologist Tom Adams.

'The biggest fluctuations between cool and hot, wet and dry, will be in the south. Further north the fronts will have weakened, but we still expect the coming weather to be more changeable than during the last few weeks.

'With the holidays approaching and people starting to make outdoor plans, it is worth remembering the weather does change, and it always pays to keep an eye on the latest forecasts. Don't take fine weather for granted!”