Glen Eden - Extended

Issued 01 May 2025

May 2025 Monthly Outlook

Description of April 2025 - A wetter month across most regions

It was a wet and warm April across much of the country, with soil moisture deficits largely disappearing, apart from parts of Otago and Southland. This was caused by an ever-present low pressure anomaly across the north Tasman Sea, with New Zealand sandwiched between this feature and a slow-moving high further east, leading to slow-moving systems with no exit strategy.

A northerly low at the start of April delivered large amounts of rainfall to parts of the North Island and upper South Island, with both Taranaki and Tasman seeing 400mm of rainfall. Kerikeri received 108mm on the 3rd, which is in stark contrast to the previously monthly total of 24.4mm, and was more than it normally sees in the first four months of the year combined. Meanwhile, Westport saw 120mm that day which broke the daily record, and Foehn winds brought anomalously warm temperatures to parts of Canterbury and eastern Otago.

Periods of thunderstorms followed, with a possible tornado in Levin on the 6th and localised heavy rain. Attention then turned to Tropical Cyclone Tam, which developed on the 16th and moved southwards across New Zealand. This was a very slow-moving system which brought a raft of weather impacts, and an accompanying tropical air mass was dragged over the country with temperatures well above average. This helped generate numerous thunderstorms across the Auckland region on the 19th resulting in surface flooding, and Auckland ended the month as the second wettest April on record.

Most of the North Island copped large rainfall totals in April, with Whangarei seeing 402.7mm (previous record 345.8mm in 1974), Kerikeri saw 573.2mm (previous record 429mm in 1999), and many stations in the top five. Further south, Nelson and Blenheim recorded their third wettest April (222.2mm and 160.2mm respectively), and Christchurch saw their record shattered with 80.2mm.

The month ended with a bang as another moisture-laden low brought significant wind and rain impacts to many regions, many of which are still ongoing.

Climate Drivers - ENSO Neutral, but La Niña-like patterns persist through autumn

The El Niño-Southern Oscillation (ENSO) index is now firmly at neutral levels as sea surface temperatures around the eastern Equatorial Pacific return to normal. However, atmospheric conditions in these regions still exhibit La Niña-like traits (e.g. more northerly lows), though this is forecast to gradually ease away as we head into winter.

Both the Tasman Sea Index (TSI) and Southern Annular Mode (SAM) are currently in negative territory given the ongoing low pressure system, but these are both forecast to trend much more positive (settled) from this weekend and persist through mid-month. The risk for tropical development also eases by mid-May.

Sea Surface Temperatures (SST) are much warmer than average, particularly surrounding the South Island, which may add fuel to the ongoing rain event.

May 2025 Outlook - A rough start, but becoming more autumnal mid-month

An intense low pressure system affects the country this week, delivering large amounts of rainfall to eastern regions with Christchurch and parts of Canterbury in the firing line, and the strongest southerly wind burst across the lower North Island since the June 2013 storm. With winter soon on its way, a rush of heavy snow is also expected for the Canterbury High Country, with snow already settling at higher stations. A State of Emergency has already been declared for the Selwyn District, and more rain is on the way before easing this evening. Expect gradually improving conditions through this weekend as the low moves away.

Next week sees high pressure build across the country with warming afternoons but chilly mornings, with largely dry weather. Fiordland may still cop the occasional passing Southern Ocean cold front with above normal rainfall, but any feature should significantly weaken as it spreads northwards. However, there are signals for another northerly low late in the week, so keep an eye to the sky for any further development.

High pressure moves east and weakens around mid-month, allowing for the return of a more seasonal progression of quick-moving fronts across the country. The North Island should trend near average or slightly drier-than-average, but conditions further south are looking close to normal with a passing front or two.

The last two weeks of autumn see more seasonable conditions (e.g. near-average rainfall) across most of the motu, but a strengthening westerly flow across the Southern Ocean hints at higher rainfall across Westland and Fiordland, while the odd northerly low will remain possible. With winter right around the corner, it will become increasingly important to be mindful that any southerly change accompanying a frontal passage would bring a short but sharp cold snap with potential for snow.

This product will stop on 30 June 2025.

Bottom Line

The first half of May sees the active weather continue, but with longer dry spells in between, and the month ends on a more seasonable but changeable note.