Cyclone Tam: upper North Island impacts

Cyclone Tam impacted the upper North Island with severe gales and widespread flooding across Northland, Auckland and the Coromandel Peninsula. Here is a meteorological summary timeline of how the storm unfolded.

13 April - Tropical low 98P commenced being tracked by JTWC. Water vapour satellite image showed deep convection starting to circulate over the Vanuatu archipelago.

Water vapour image 6:50pm, 13 April

14 April - Tropical low 98P consolidated its centre around 500km NE of Port Vila. Sea surface temps were 30°c with a low wind shear environment. Improved outflow suggested the low was strengthening with IR satellite image showing a hot tower reaching into the stratosphere.

IR satellite image 7pm 14 April

15 April - Sunrise showed that tropical low 98P had become cyclone 30P named by JTWC late on 14 April over the top of the Vanuatu archipelago. Pressure was down to 996 hPa, winds 40 kts moving South at 13 knots. Large plumes of deep convection continued overnight.

JTWC forecast track map for tropical cyclone 30P

Later in the day Fiji Met Service officially named Tropical cyclone Tam as a category 1 system.

Fiji Met 1st track map for tropical cyclone Tam

IR satellite of category 1 tropical cyclone Tam near Vanuatu

16 April - Cyclone Tam began the convective subtropical transition phase. Energy being sourced from the temperature differences between the surface and the cooling upper troposphere.

Water vapour satellite image with weather feature

Later in the day Cape Reinga starting gusting over 130km/h as ex-tropical Cyclone Tam starting developing a nocturnal low-level jet stream as its became an extratropical cyclone, 250km NW of Northland over the Tasman.

Radar with weather features 7:20pm 16 April

17 April - MetService issued their first cyclone Tam forecast track map as

Cyclone Tam had developed into a deep and strong extratropical cyclone with barometric pressure in the low 970 hPa. Cape Reinga had a wind gust early in the morning of 151 km/h.

Satellite image of Cyclone Tam having become a fully mature extratropical cyclone over the Tasman overnight

Cyclone Tam storm surge during high into the Waitemata harbour at the southern end of the Hauraki Gulf.

News media summary of days events

18 April - Cyclone Tam had weaken sightly and had become stationary off shore over the Tasman. This meant that outer spiral rainbands with embedded thunderstorms would impact the upper North Island over the next 24 hours.

A beached boat in Devonport, Auckland which broke its mooring 20km away in Maraetai the night before.

Euro model of the MSLP map at lunchtime on 18th April

A convergence zone from the outer spiral bands developed from Northland to Auckland around 10pm. This would produce an intense cluster of thunderstorms that would produce severe flash flooding from Auckland's Northshore to South Auckland. MetService did not release a severe thunderstorm watch until after midnight and most surprisingly did not issue a severe thunderstorm warning for at all for the Auckland area. Lessons from the 2023 Anniversary floods seemed not have been learnt. Rainfall totals from the intense storms 106mm Mt Roskill, 103mm Sandringham, 93mm Westmere, 85mm Birkenhead.

News media summary of the Auckland flash floods.

19-20 April - Further intense downpours moved through most areas of the upper North Island as ex-tropical cyclone Tam dissolved over the Tasman.

Notable weather observations of the event from16 to 20 April for the upper North Island. sources Council, private, MetService and NZ Coastguard.

Northland:

302mm Whakapara

274mm Waitangi

99mm Whangarei

155km/h (84 knots) Cape Reigna

Coromandel Peninsula: 

530mm Pinnacles

420mm Waitekauri River - Golden Cross

274mm Coromandel

86mm Thames

88km/h (47 knots) Port Charles

Auckland:

214mm Dome Forest

152mm Mt Roskill

142mm mt Albert

18mm Auckland city 86mm

102km/h (55 knots) North Head

Gulf Islands:

58mm Great Barrier

32mm Waiheke

139km/h (75 knots) Tiritiri Matangi