Gulf Harbour - Extended

Issued 03 June 2025

June 2025 Monthly Outlook (Final edition of this product)

Description of May 2025

The overall pressure anomaly suggests high pressure dominating New Zealand throughout May – but this masks the steady migration of highs and intermittent fronts spreading across about once per week. Many locations saw lengthy dry spells punctuated by punchy wet periods, some of which produced very well.

The first of these wet spells was at the start of the month, with a very deep low sitting east of the country and led to a State of Emergency being declared in Selwyn District, and a red wind warning for the Wellington region. Some parts of Canterbury received almost their entire month of rainfall on the 1st day of the month alone.

The next fronts passed over New Zealand on the 8th-10th, with another from 17th-19th. In contrast to the start of May, these systems delivered a more typical rain to northern and western regions.

A final system late in the month brought widespread rain and some thunderstorms to the country, before ushering a significant pattern change to Southern Ocean-sourced southwesterlies. The month of May ended with most of the country seeing near-to-above average rainfall, with fluctuating temperatures averaging out near normal.

Climate Drivers

The El Niño-Southern Oscillation (ENSO) index continues firmly at neutral levels as sea surface temperatures around the eastern Equatorial Pacific sit near normal. However, atmospheric conditions still hold onto some La Niña-like traits, and this may allow northerly lows to continue to affect us in the near future.

Both the Tasman Sea Index (TSI) and Southern Annular Mode (SAM) are currently in negative territory, associated with an ongoing Southern Ocean outbreak. They are likely to remain negative for much of this week before recovering. At least one more dip into negative territory is possible throughout June.

Sea Surface Temperatures (SST) remain generally warmer than average. This helped boost rainfall totals during May, and could provide some moderation for any cold spells in June.

June 2025 Outlook

Expect some dynamic weather for the first week of June as a low from the north brings wet, windy and mild conditions for most of the country, then a dramatic shift later in the week as a surge of Antarctic air floods onto the country. This cold air hangs around throughout the weekend – ski fields will get a great boost from this system, and we may even see snow falling close to sea level in the South Island.

Next week starts with high pressure moving in and may remain stuck in place for several days, trapping the cold air and bringing frigid days and nights to inland regions. There’s a good shot at the full gamut of classic winter scenes – snow, freezing fog, black ice and possibly hoar frost. If you’re going to be out in the paddocks, now would be a good time to remember where you stashed your warmest socks.

Week two ends on a milder note, with a Tasman Sea low bringing bursts of rain and warming temperatures to much of the country, though the South Island basins will hang onto the cold air as long as they can. However, the cold will struggle to win out in the long run.

The second half of the month likely brings a north or northwest dominated pattern, with milder conditions and rain focussing on northern and western parts of the country, either from Tasman Sea fronts or lows from the north. Trailing behind these systems we could still see a few Southern Ocean cold snaps with tail-end snow events, especially for the South Island.

While the upcoming cold spell may live in the memory and the Camera Roll, it will probably struggle to put a serious dent in the overall temperature anomaly for the month: expected to come in near average for the South Island, and more likely on the milder side overall for the North Island.

We have been producing the Monthly Outlook for a decade and appreciate your readership, however, the decision has been made to discontinue it with this edition being the last.

Thank you also to those who took the time to answer the survey in the last email, highlighting their particular areas of interest. We are using this feedback to help us inform what could be developed for our website.

If you require long range forecast information for your operations please contact nzsales@metservice.com.

Bottom Line

A low pressure system this week heralds a major shift into some genuinely wintry weather, which should last several days. This may be the best cold of the month though, as milder systems are more likely to dominate in the second half of June, continuing the overall wetter trend of recent months.