Tropical Cyclone Lusi

Update as of midday Monday 17th March

Cyclone Lusi retreating to open waters

Cyclone Lusi crossed the upper South Island overnight and is now pulling away across the open waters of the southern Pacific Ocean.

Pressure analysis and satellite image from 5am  on the 17th March 2014

Pressure analysis and satellite image from 5am on the 17th March 2014

Lusi brought a period of heavy rain to the Tararua Range, as well as much of the hill country around Nelson and northern Marlborough. Gusts reached 140km/h about the hill tops of Wellington, and 110km/h downtown at the wharf.

Below are the rainfall accumulation maps from 9am Thursday until 9am Monday, in 24 hour steps.

Accumulated rainfall across New Zealand for the 14th to 16th March 2014.

Accumulated rainfall across New Zealand for the 14th to 16th March 2014.

It shows that some of the dry pastures from Waikato to Taihape did not receive much precipitation at all, with 4.5mm at Hamilton. New Plymouth recorded 2.8mm, whilst  Mount Taranaki managed over 150mm during the same time.

A summary of observation for the passage of Lusi

 

Northland:

Rainfall 60–80mm generally, but Kaikohe received 105mm and PuhiPuhi 101mm.

Winds gusting 139km/h at Cape Reinga, 95km/h at Hokianga and 115km/h at Marsden Point.

 

Auckland:

Rainfall mostly 25–40mm, but a few stations recorded over 60mm (no warning issued for rain). Winds gusting 113km/h at Whangaparaoa and 90km/h on the Harbour Bridge.

 

Coromandel and the Bay of Plenty:

Rainfall generally 50–80mm, but 230mm at the Pinnacles and 156mm at Waikura.

Winds gusting 96km/h at Golden Valley and 107km/h at the Mamaku radar.

 

Waikato, Waitomo and Taranaki:

Mostly 5–15mm, but 131mm at Te Aroha and 133mm at Paeroa as well as 147mm at North Egmont and 169mm at Dawson Falls. Winds were gusting 75km/h at Taupo and 105km/h at Hawera.

 

Taihape:

Rainfall mostly 20–50mm but 66mm at Ruapehu and 40mm at National Park.

Winds gusting 81km/h at Mangatepopo and 105km/h at Waiouru, but in excess of 180km/h on the Mt Ruapehu ski fields.

 

Gisborne:

Rainfall mostly 20–40mm, but 122mm at Harapara, 142mm at Arowhana and 145mm at Hikuwai.

 

Hawkes Bay:

Rainfall mostly 10–30mm, but 215mm at Parks Peak and 83mm at the Waipoapoa Bridge. Winds gusting 122km/h at Mahia and 125km/h at Castlepoint.

 

Wellington:

Rainfall 20–30mm, but between 150 and 245mm in the Tararua Range. Winds gusted 100km/h at Paraparaumu and 117km/h on the Rimutaka Hill Road. Both Kelburn and Aotea Wharf gusted 110km/h.

 

 South Island

Tasman:

Rainfall mostly 30–70mm, but between 150 and 300mm about the inland hills and ranges, and 134mm at Takaka.

 

Marlborough:

Mostly 20–50mm, but 100–140mm inland and 87mm at Kaikoura.

 

Canterbury:

Rainfall amount mostly 20–40mm , with 38mm at Darfield and 35mm at Christchurch.

However, Akaroa received 80mm and 51mm at Rangiora.

 

Otago:

Rainfall amounts 15–30mm about the eastern coastal hills, but a few areas received just in excess of 40mm.

 Warnings issued:

We issued Severe Weather Warnings for Northland, Auckland, Waikato, Coromandel Peninsula, Bay of Plenty/Rotorua, Gisborne, Hawkes Bay, Wairarapa, Wellington, Nelson, Marlborough, Buller, South Canterbury and North Otago. The first warning was issued on Friday morning, the first watch for the event was issued on Wednesday morning, and reference to significant weather first appeared in the Severe Weather Outlook on Sunday afternoon even before Lusi was named as a tropical cyclone.

 

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Update as of midday Sunday 16th March

The remains of Cyclone Lusi

polar sat 16 march

Here’s a satellite image showing ex-tropical cyclone Lusi lying west of Taranaki this morning.

 

temp map 16 march

The resulting tropical airmass it brought a warm start to the day for most over the North Island

Conditions continue to improve over the upper part of the North Island, with a few showers expected there for the remainder of the day.

rainfall 16 march

The image above shows the progression of the rain over the last two days and the expected rainfall over the country until tomorrow morning. Of note is that some data is missing from the middle panel, where rainfall exceeded 200mm in places over the Coromandel. A further 10-20mm is expected for the Christchurch region for the remainder of today, before starting to ease overnight tonight.

wind 16 march

As the low crosses over the central parts of New Zealand tonight, strong winds are also expected. Winds around Wellington should increase dramatically this evening as the flow turns from a sheltered and warm northeasterly to a more exposed and much more vigorous northwester. Rain may accumulate to warning amounts for South Canterbury and North Otago. At this stage we are not expecting Christchurch to exceed warnings amounts.  Keep up to date with the latest warnings at http://www.metservice.com/warnings/home

Where to from here?

pressure 16 march

Ex-cyclone Lusi, will continue to move away from New Zealand into the open waters to the southeast of Otago.

Keep up to date with the latest forecasts at www.metservice.com

 

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Update as of midday Saturday 15th March

Gales and rain spread southwards

Cyclone Lusi continued to push southwards overnight bringing with it strengthening winds and bursts of rainfall. Ahead of the worst of the weather a shield of high cloud spread across the North Island.15SATA1008Gusts of around 130 km/h have so far been recorded at Cape Reinga, closest to the centre of Lusi, but elsewhere across Northland and Auckland gust speeds have been creeping up through the morning, with gusts of almost 90 km/h recorded at sites in the city.

Surface pressure analysis from 7am 15th March 2014

At 7am this morning Cyclone Lusi was situated to the northwest of North Cape.

The strong winds and gales are forecast to continue through the day so please stay up to date with all the latest Severe Warnings and Watches

This snapshot shows the strongest gusts recorded in the hour before midday on the 15th

This snapshot shows the strongest gusts recorded in the hour before midday on the 15th

As well as the winds rain is still moving across the country with rainfall totals so far reaching 40 to 60 in many spots in the North of the country. The rain gauge at Paeroa has recorded over 80mm of rain in the 12 hours since midnight. As with the wind there is still some more rain to come and this rain is likely to head onto the South Island overnight and into Sunday.

Midday radar image

Midday radar image from the 15th March 2014

 

Where is Lusi going?

The forecast for the rest of the weekend is looking similar to that from yesterday with the centre of Lusi tracking southwards to the west of the country today before sliding the across the top of the South Island on Sunday. The change in direction is likely to bring gales from different directions, bringing the risk to places like Taranaki and Wellington on Sunday.

Here are the latest forecast positions into the start of next week. The thin black lines are isobars which represent lines of equal pressure on the map, like contour lines on a map, where these lines are closest the winds are strongest. Around centres of low pressure in the southern hemisphere winds travel in a clockwise direction, but remember that New Zealand’s rugged terrain plays a large part in local wind speeds and directions

Forecast charts for the next 36 hours

Here are the forecast charts for the next few days.

Please stay up to date with all the latest Severe Warnings and Watches

 

 

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Update as of midday Friday 14 March

The approach of Cyclone Lusi

Cyclone Lusi, now a category 2, has moved out of the tropics south of 25S, and is now approaching northern New Zealand.

he image above shows the analysis and satellite picture at 7am on the 14 March.

The image above shows the analysis and satellite picture at 7am on the 14 March.

RSMC Wellington has taken over the reins of Lusi from RSMC Fiji this morning, currently a Category 2 tropical cyclone, and at 5am is lying about 800km north of Cape Reinga. Lusi’s speed of movement is about 35 km/h, and on its current track is expected to pass just to the northwest of North Cape around midday Saturday.

Lusi is showing signs of becoming an extra-tropical system, with cloud shearing away from its centre, but it still contains a lot of energy and severe weather.

MetService has been monitoring this system for several days, and there are severe weather warnings and watches for a number of North and South Island places. As this low passes by the far North, a period of heavy rain and easterly gales is likely from Northland to Gisborne. The upper South Island is also likely to see substantial rainfall from late Saturday into Sunday. Rainfall in Christchurch will be from an easterly direction, and amounts will be modest by warning standards.

Ensemble model guidance produced by UKMO at midnight on the 13th March 2014 for the movement and position of ‘Lusi’.

Ensemble model guidance produced by UKMO at midnight on the 13th March 2014 for the movement and position of ‘Lusi’.

MetService will continue to update our blog daily as the storm approaches to highlight the risks to New Zealand. You can view the latest Severe Weather Video update here: http://metservice.com/tv/#severe

Please stay up to date with all the latest Severe Warnings and Watches

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Update as of midday Thursday 13 March

Cyclone Lusi now a Category 3 and moving southwards

Cyclone Lusi has intensified into a Category 3 cyclone and is beginning to move southwards toward northern New Zealand. It should pass well east of Norfolk Island.

ir sat picture of lusi

The image above shows the analysis and satellite picture at 5am on the 13th of March 2014. Tropical Cyclone Lusi is located near the centre of the image. The North Island of New Zealand is showing at the bottom of the image.

 

On the current track, Lusi is forecast to cross latitude 25 South around midday Friday New Zealand time, moving from the jurisdiction of RSMC (Regional Specialised Meteorological Centre) Fiji into RSMC Wellington’s area of responsibility. You can read more about the RSMC regions and cyclone categories in this blog post: http://blog.metservice.com/2013/10/tropical-cyclone-season-2013-14/

cyclone forecast track fiji met

The track map produced by the Fiji Meteorological Service shows Lusi taking a southerly track around 175E.

The expected track of Lusi is slowly becoming more defined, but there are still some subtle yet significant variations in speed and intensity. This can be seen most clearly on the comparison between global models valid for 1pm on Saturday.

march 13 global model runs 1pm saturday

 

The images above show the forecast position of Lusi for 1pm on Saturday (New Zealand time) according to five different computer models.

Ensemble guidance is showing the track to be around, or just to the north of, North Cape before re-curving across the upper South Island. There is increasing agreement between the various modelling centres.

Ensemble strike prob 12 marchEnsemble positions and track prob 12 march

Regardless of the eventual cyclone track, Lusi is expected to bring a period of heavy rain and gale to severe gale winds to many places from Northland to Canterbury. A watch has been issued for this event, and an increasing number of regions will be included as the finer details become clearer. Severe Weather Warnings will be issued on Friday.

13SWO

In the meantime, please keep up to date with Fiji Met  while Lusi remains in the tropics as well as the MetService Severe Weather Outlook

If necessary, warnings and watches for this event will be issued 24–36 hours in advance on our website. There is more information about severe weather warnings, watches and outlooks in this blog post http://blog.metservice.com/2013/10/warnings-watches-and-outlooks/

This has the hallmarks of being a significant weather event so it is important to read CDEMs information on ‘Get Ready, Get Thru.’

This blog will be updated regularly.

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Update as of midday Wednesday 12 March

Tropical Cyclone Lusi now a Category 2

Tropical Cyclone Lusi has been moving slowly south-southeast and is currently lying just east of Port Vila where it brought heavy rain and strong gales.

Pressure analysis and satellite image from 5am  on the 12th March 2014

The image above shows the analysis and satellite picture at 7am on the 12 March. Tropical Cyclone Lusi is located near the centre of the image, marked with a white T. A remnant of ex-tropical cyclone Hadi is denoted by a white L.

Lusi is expected to intensify to a Cat 3 (winds 119–157 km/h) cyclone Wednesday evening while moving southeast over the open waters of southern Vanuatu and Fiji.

TC Lusi Warning 22

The track map produced by the Fiji Meteorological Service shows Lusi taking a southerly track around 175E.

Computer models are still predicting Lusi to move out of the tropics (south of 25?South) on Friday as it moves in a southerly direction towards the upper North Island. At this stage, the cyclone is expected to make landfall somewhere between North Cape and western Bay of Plenty during Saturday or Sunday, before re-curving across New Zealand and moving swiftly out over open waters east of the South Island on Monday. The various computer models are still struggling to agree on its future location and speed of movement, but a track over the upper North Island followed by a path over the upper South Island is currently favoured.

UKMO Ensemble

The above images show the Ensemble model guidance produced by UK Met Office of the movement and position of Lusi. The image on the left shows a swathe of possible tracks with colours to distinguish the range of probabilities. The image on the right shows the average track of all the forecast tracks.

The above images show the Ensemble model guidance produced by UK Met Office of the movement and position of Lusi. The image on the left shows a swathe of possible tracks with colours to distinguish the range of probabilities. The image on the right shows the average track of all the forecast tracks.

As this system moves across New Zealand this weekend, it will bring with it the potential for a period of significant severe weather for most of the North Island and the northern half of the South Island. A burst of intense rain and gales are likely. A Severe Weather Watch has been issued for Northland, Auckland and the Coromandel Peninsula as a heads up. Further Severe Weather Watches and Warnings are likely in the coming days.

As the track of Lusi has the potential to  be erratic, the exact distribution of severe weather is likely to change between now and Lusi’s arrival over New Zealand.

In the meantime, please keep up to date with Fiji Met  while Lusi remains in the tropics as well as the MetService Severe Weather Outlook

If necessary, warnings and watches for this event will be issued 24–36 hours in advance on our website. There is more information about severe weather warnings, watches and outlooks in this blog post http://blog.metservice.com/2013/10/warnings-watches-and-outlooks/

This has the hallmarks of being a significant weather event so it is important to read CDEMs information on ‘Get Ready, Get Thru.’

This blog will be updated regularly.

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Update as of midday Tuesday 11th March 2014

Tropical depression 18F was upgraded and named Tropical Cyclone Lusi during the morning of Monday 10th March while lying over northern Vanuatu.

ressure analysis and satellite image from 7am  on the 11th March 2014

The analysis and satellite picture at 7am on the 11 March. Cyclone Hadi is on the left and Lusi near the top centre of the image.

Lusi is currently lying just east of Espiritu Santo and moving very slowly east-southeast. It is expected to intensify to Cat 3 (early hurricane intensity) on Wednesday midway between Vanuatu and Fiji (New Zealand can be seen at the bottom of the picture above). ‘Hadi’ is expected to move northeast towards the southern Solomon Islands.

Tropical cyclone track map

This track map was issued by the Fiji Meteorological service at 8:27am (NZ local time) on the 11th March 2014. For the latest information from Fiji head to www.met.gov.fj

The track map produced by the Fiji Meteorological Service shows Lusi taking a path with its most damaging effects expected to stay to the west of Fiji.

Currently, computer models are expecting Lusi to leave the tropics (by moving south of 25º South) on Friday before heading in a southerly direction towards the northern half of the North Island. At this stage, the cyclone is expected to landfall over New Zealand somewhere between North Cape and the Bay of Plenty during Saturday or Sunday, before re-curving and moving swiftly out over open waters to the east of the South Island on Monday.

Computer model guidance from the UKMO

The above images show the Ensemble model guidance produced by UK Met Office of the movement and position of Lusi. The image on the left shows a swathe of possible tracks with colours to distinguish the range of probabilities. The image on the right shows the average track of all the forecast tracks.

As the cyclone moves across New Zealand this weekend, it will bring with it the potential for significant severe weather from Northland to Canterbury including intense rain, severe gales, large waves and abnormally high tides in many places.

As with all weather systems, there is still a degree of uncertainty about Lusi’s track near New Zealand. This will obviously have a bearing on the exact distribution of severe weather over the country.

In the meantime, please keep up to date with Fiji Met  while Lusi remains in the tropics as well as the MetService Severe Weather Outlook

If necessary, warnings and watches for this event will be issued 24–36 hours in advance on our website. There is more information about severe weather warnings, watches and outlooks in this blog post http://blog.metservice.com/2013/10/warnings-watches-and-outlooks/

This has the hallmarks of being a significant weather event so it is important to read CDEMs information on ‘Get Ready, Get Thru.’

This blog will be updated regularly.

 

 

Christmas Weather Daily Update

Ex-Tropical Cyclone Evan is playing a part in New Zealand’s weather this Christmas.

Now that it has passed across the seas to the north of the North Island, the heaviest of the rain over northern New Zealand is over. This will be the last update of the “Evan”  blog.

Merry Christmas.

Update: Tuesday 25 December

Heavy rain in Northland has eased. It’s still raining on and off there, though, as it is over about the northern half of the North Island. A Severe Weather Watch covers the possibility of a few further bands of heavy precipitation rotating around Evan.

Today, afternoon and evening showers and thunderstorms – mostly inland – are part of the weather picture. The action is expected to be from Taranaki through to Wellington and Wairarapa, and in Buller, Nelson, Marlborough and north Canterbury. Keep an eye on the Severe Thunderstorm Outlook; Severe Thunderstorm Watches or Warnings may follow.

Also today, it’s fine (or becoming fine) and not windy over about the southern half of the South Island. It’s muggy, so please remember the Cancer Society’s message about being out in the sun.

On Boxing Day, Evan is expected to lie off to the northwest of Taranaki. At this time it will maintain a flow of very warm moist air over the North Island, and rain or showers are expected in many places. Heavy falls are possible in eastern Bay of Plenty and in thundery showers over high ground. See the Severe Weather Outlook for more detail.

Meanwhile, a southerly change with rain is expected across the South Island on Wednesday. This should cool things off a bit – and lower the humidity.

Where to find important forecast information

Severe Weather Warnings and Severe Weather Watches:

http://metservice.com/national/warnings/severe-weather-warnings

http://metservice.com/national/warnings/severe-weather-watch

Three day rain forecast: http://metservice.com/maps-radar/rain-forecast/rain-forecast-3-day

Rain radar: http://metservice.com/maps-radar/rain-radar/all-new-zealand

Tropical Cyclone Evan: Special Weather Bulletin from Fiji Meteorological Service

Now that Evan is south of Viti Levu and moving away, this will be the last blog post relaying Fiji Meteorological Service’s Special Weather Bulletin.


Special Weather Bulletin Number THIRTY FOUR FOR FIJI ON SEVERE TROPICAL CYCLONE EVAN ISSUED FROM RSMC NADI at 9:31am on Tuesday the 18th of December 2012 TROPICAL CYCLONE WARNING

A GALE WARNING REMAINS IN FORCE FOR KADAVU, BEQA, VATULELE AND NEARBY SMALLER ISLANDS .
A STRONG WIND WARNING REMAINS IN FORCE FOR THE REST OF FIJI.
A DAMAGING HEAVY SWELL WARNING REMAINS IN FORCE FOR FIJI.

SEVERE TROPICAL CYCLONE EVAN CENTRE [970HPA] CATEGORY 3 WAS LOCATED NEAR 19 DECIMAL 7 SOUTH 177 DECIMAL 2 EAST OR ABOUT 220 KM SOUTH OF NADI OR ABOUT 130 KM SOUTHWEST OF KADAVU OR ABOUT 220 KM SOUTHWEST OF SUVA AT 10:00AM TODAY. THE CYCLONE IS MOVING SOUTH AT ABOUT 14 KM/HR.
CLOSE TO ITS CENTRE THE CYCLONE IS EXPECTED TO HAVE AVERAGE WINDS UP TO 150 KM/HR WITH MOMENTARY GUSTS TO 210 KM/HR.

ON THIS TRACK, THE CYCLONE IS EXPECTED TO BE LOCATED ABOUT 380 KM SOUTH OF NADI OR ABOUT 260 KM SOUTH-SOUTHWEST OF KADAVU OR ABOUT 360 KM SOUTH-SOUTHWEST OF SUVA AT 10:00PM TODAY AND 375 KM SOUTH-SOUTHWEST OF KADAVU OR 475 KM SOUTH-SOUTHWEST OF SUVA AT 10:00AM TOMORROW.

DESTRUCTIVE WINDS MAY BEGIN SEVERAL HOURS BEFORE THE CYCLONE CENTRE PASSES OVERHEAD OR NEARBY.

FOR KADAVU, BEQA, VATULELE AND NEARBY SMALLER ISLANDS:
DAMAGING GALE FORCE WINDS WITH AVERAGE SPEEDS UP TO 85 KM/HR AND MOMENTARY GUSTS TO 110 KM/HR. PERIODS OF HEAVY RAIN WITH SQUALLY THUNDERSTORMS WITH FLOODING OF LOW LYING AREAS. DAMAGING HEAVY SWELLS WITH SEA FLOODING OF LOW LYING COASTAL AREAS.

FOR THE REST OF FIJI:
STRONG WINDS WITH AVERAGE SPEEDS TO 55 KM/HR AND MOMENTARY GUSTS TO 85 KM/HR. PERIODS OF RAIN, HEAVY AT TIMES AND SQUALLY THUNDERSTORMS.
FLOODING INCLUDING SEA FLOODING OF LOW LYING COASTAL AREAS.

The following information is provided especially for the mariners:

HURRICANE FORCE WINDS AND PHENOMENAL SEAS NEAR THE CYCLONE CENTRE. STORM FORCE WINDS WITHIN 30 NAUTICAL MILES OF CENTRE AND VERY HIGH SEAS. GALE FORCE WINDS AND HIGH SEAS TO ABOUT 100 NAUTICAL MILES FROM CENTRE. ELSEWHERE, STRONG WINDS AND ROUGH TO VERY ROUGH SEAS. HEAVY SWELLS.

The next Special Weather Bulletin for Fiji on Severe Tropical Cyclone Evan will be issued at or around 12:30PM today.

Hobsonville wind storm Thursday 6 December 2012

Update, Friday 14 December

This tragic event has been widely reported as a tornado. On Friday 7 December, as I wrote this blog, there certainly was an absence of evidence of a tornado. But absence of evidence is not evidence of absence. Over the last few days it has become increasingly clear — from inspecting aerial views of the damage swath, and from correspondence from people in the area at the time — that this was much more likely to have been a localised wind storm known as a microburst. Corresponding amendments have been made below, and post-event analysis will clarify this further.

Few weather events are as dramatic, dangerous or challenging to predict as localised wind storms, including tornadoes.

Around midday on Thursday 6 December an active trough line passed slowly through Auckland. A thunderstorm in this line produced a tornado localised wind storm that touched down near Hobsonville, tragically killing three people.

Could this tornado localised wind storm have been forecast?

No. Technology to forecast the very small small and short-lived wind storms like tornadoes or microbursts – as we see them in New Zealand - does not yet exist.

Expert meteorologists are able to identify areas where there is a significant risk of small-scale severe weather, or where small-scale severe weather is already occurring. MetService is able to forecast the conditions favourable for the formation of (severe) thunderstorms, and sometimes we can anticipate the likelihood of tornadoes that have nothing to do with severe thunderstorms. It all comes down to being able to represent phenomena at the time and space scales upon which they occur – which in this case is a few minutes and a few hundred metres respectively.

A little bit about tornadoes in New Zealand

Tornadoes in New Zealand are quite different from those that occur in the Midwest of the United States primarily in the warm part of the year. In New Zealand, tornado occurrence is primarily related to convection along strong cold fronts – and thus they are largely a “cold season” phenomenon. New Zealand tornadoes are also very small and short-lived in comparison to US tornadoes, and in most cases form and dissipate within minutes.

Some notable recent instances include the Waitara tornado of 15 August 2004, the Greymouth tornado of 10 March 2005 (very likely a cold season event, even though it occurred in March), the New Plymouth tornadoes of 3-4 July 2007, the Cambridge tornado of 17 October 2008, and the Avondale tornado of 11 September 2011. The Albany tornado of 3 May 2011 formed under different circumstances.

About the storm of Thursday 6 December

For the few days leading up to this event, it was clear that a large area of rain-bearing air would move from the northern Tasman Sea across the North Island on Thursday 6 December – and this was communicated in forecasts.

Late on Thursday morning, an active cold front approached Auckland from the northwest with a line of showers ahead of it. Ahead of this line the winds were moderate northeasterlies; behind it, they were moderate northwesterlies. Along the line, the winds converged – that is, pushed against each other.

The Severe Thunderstorm Outlooks issued on MetService websites on the evening of Wednesday 5 December, for Thursday 6 December, are below.

Severe Thunderstorm Outlook for the 12 hours to noon Thursday 6 December, published on www.metservice.com at 9:11pm Wednesday 5 December.

Severe Thunderstorm Outlook for noon to midnight Thursday 6 December, published on www.metservice.com at 9:20pm Wednesday 5 December.

The Severe Thunderstorm Outlooks issued on MetService websites on the morning of Thursday 6 December, for that day, are below.

Severe Thunderstorm Outlook for Thursday 6 December, published on www.metservice.com at 8:39am Thursday 6 December.

Severe Thunderstorm Outlook for noon to midnight Thursday 6 December, published on www.metservice.com at 10:14am Thursday 6 December.

As these Outlooks show, on Thursday morning the Severe Weather Team chose to extend the risk of thunderstorm activity across the whole of the North Island.

As the line of showers (see the radar image at the beginning of this blog) approached the west coasts of Northland and Auckland, the thunderstorm risk for Auckland was escalated from moderate to high.

At this point (around 10:15 am), Auckland Council Civil Defence pushed out advice of thunderstorms in the Auckland area on their mobile phone app (see below) based on consultation with MetService’s Severe Weather Team.

This type of consultation is common practice; MetService works closely with Ministry of Civil Defence & Emergency Management and local authorities during all severe weather events as part of the National Civil Defence and Emergency Management Plan.

Civil Defence app screenshot

Also at this time, the Auckland forecasts were re-issued on MetService websites so that they now included the mention of thunderstorms (see below).

Auckland regional forecast issued at 10:08am Thursday 6 December:

Periods of rain, some heavy and thundery, clearing this afternoon and becoming mainly fine. However, showers developing from the west tonight. Northwesterlies freshening this evening.

Auckland urban forecast issued at 10:21am Thursday 6 December

Rain with heavy, thundery falls, clearing PM. Northwest.

Monitoring the storm

The Severe Weather Team began intensive monitoring and analysis of the incoming shower line as soon as the Auckland weather radar could resolve it well. With potentially severe weather, this monitoring and analysis process takes place every 7.5 minutes as new radar data become available.

Near midday on Thursday, analysis of the shower line revealed that it contained showers and thunderstorms with high rainfall intensities. At this time, there were none of the tell-tale signs that would indicate that approaching storms within the line were supercellular (that is, very likely to produce tornadoes). As the storms approached Auckland, radar indicated that neither localised rainfall nor hail size would be sufficient to justify the issue of a Severe Thunderstorm Watch. Modelled vertical soundings through the system as it passed across the Auckland area did not indicate the helicity (corkscrew-like motion) or CAPE (energy available for making thunderstorms) large enough to raise concerns about tornadic activity.

Below are a couple of radar images immediately before and approximately at the time of the tornado localised wind storm. Vertical cross-sections of these – which the Severe Weather Team were scrutinising at the time – show that the thunderstorm near Hobsonville “flares up” and then “collapses” within about a 10-minute period. The tornado localised wind storm is likely to have occurred during the few minutes in which this thunderstorm collapsed.

MetService learnt about the tornado localised wind storm roughly half an hour afterwards, through its social media monitoring. Once we had verified media reports and taken a look at the Whenuapai midday upper-air sounding (which contains detailed factual information about the vertical structure of the atmosphere over the Auckland area), we responded with a Severe Thunderstorm Watch at 12:59pm.

SEVERE THUNDERSTORM WATCH Issued by MetService at 12:59 pm Thursday 06 December 2012

Valid until 06:00 pm Thursday 06 December 2012

This watch affects people in the following weather forecast districts:

Northland

Auckland

Waikato

An active line of thunderstorms has been moving southeastwards across Northland and into Auckland and western Waikato. These thunderstorms are accompanied by heavy rain, hail and strong gusts. This watch is for the risk of damaging gusts in excess of 110km/h and possible small tornadoes.

Wind gusts of this strength can cause some structural damage, including trees and power lines, and may make driving hazardous. If any tornados occur, they will only affect very localised areas. Issued by: Mads Naeraa This watch will be updated by: 06:00 pm Thursday 06 December 2012

We followed this Watch with a Severe Thunderstorm Warning at 1.52pm

Severe Thunderstorm Warning for Auckland issued 1.52pm 6 December 2012

After passing northwest Auckland, the line of showers in which this storm existed continued on a southeast path that took it across northern Waikato, Bay of Plenty and across Mahia Peninsula (see map of lightning strikes, below). Subsequently, we issued Severe Thunderstorm Warnings on other thunderstorms in the same line. During the afternoon, high one-hour rainfall totals were recorded at various sites in these regions and a tornado was reported near Ngongotaha.

Interestingly, there are relatively few lightning strikes, and hence limited thunderstorm activity, in the greater Auckland area.

How does MetService warn people about severe thunderstorms?

MetService includes information about expected thunderstorm activity in its regular forecasts, and provides three types of warning messages as part of its Severe Thunderstorm Warning service:

Outlook:  Issued daily, describing the thunderstorm risk expected over the next 48 hours

Watch: Issued when there is a significant risk of severe thunderstorms – usually issued 1 to 6 hours ahead depending on the situation

Warning: Issued for specific thunderstorm cells that meet severe thunderstorm criteria and are within range of a radar

Outlooks and Watches are used to provide a “heads-up” of the potential for severe thunderstorm activity within a prescribed area.

Severe Thunderstorm Warnings are issued for thunderstorm cells that can be tracked on MetService’s radar network, and for which there is clear evidence that the cell is producing severe weather. This evidence typically comes from expert interpretation of radar data or direct observation from people on the ground.

As a National Meteorological Service designated by the World Meteorological Organization (WMO), MetService’s Severe Weather programme follows professional standards and best practice as prescribed by the WMO – the UN agency responsible for international cooperation in meteorology. All of our meteorologists are trained to WMO standards, and the Severe Weather Team is made up of highly-experienced meteorologists with specialised expertise in radar interpretation and severe thunderstorm forecasting.

MetService treats a thunderstorm as severe if it meets one or more of the following conditions. These thresholds have been developed to reflect the nature and impact of New Zealand thunderstorms, in consultation with the Ministry of Civil Defence & Emergency Management and the Ministry of Transport:

  • Rainfall of 25 millimetres per hour or greater
  • Hailstones 20 millimetres in diameter or greater
  • Strong wind gusts 110 kilometres per hour (60 knots) or greater
  • Damaging tornadoes: Fujita F1 or greater, where F1 is defined as having wind speeds greater than 116 kilometres per hour (63 knots)

As noted above, the tornado localised wind storm that affected Hobsonville on Thursday was not detectable on radar and was so short-lived that a warning for the associated thunderstorm cell was not possible. Warnings were issued for several thunderstorms within the same line later in the day based on corroborating data from MetService’s radar network.

How do I get information about severe storms?

Regardless of how you get your basic daily forecast (radio, TV, newspaper, web), you should always check the MetService website for the very latest information as other media sources are updated less frequently.

All in-force Outlooks, Watches and Warnings are flagged on the Home page of the website.

You can also sign up here to have Severe Thunderstorm Watches and/or Severe Thunderstorm Warnings (and other warnings) emailed to you as they are issued.

Latest information on North Island thunderstorms 6 December 2012

Around midday on Thursday 6 December an active trough line passed slowly through Auckland. One of these thunderstorm cells contained a tornado which touched down near Hobsonville, tragically killing three people.

This blog post has been set up to keep you informed of the latest developments in this weather system as it travels south-east from Auckland.

Auckland radar image at 12.15pm 6 Dec 2012

Auckland radar image at 12.15pm 6 Dec 2012

Situation update as at 9.30am 7 December 2012

Further squally showers and one or two thunderstorms are expected in the West from Auckland to the Kapiti Coast today. We could see gusts to 110km/hr accompanying heavy showers. There is also a potential for small hail to 10-15mm. Showers clear overnight and the weather looks mostly dry for Auckland over the weekend.

Update as at 6.00pm 6 December 2012

Severe Thunderstorm Watches and Severe Thunderstorm Warnings have been lifted. This is the last blog update for today, but keep an eye out for tomorrow’s weather as there are still Severe Weather Warnings and Watches in place for heavy rain and gales in places. The Severe Thunderstorm Outlook is still in place here: http://metservice.com/national/warnings/severe-thunderstorm-outlook

Update as at 5.00pm 6 December 2012

A report of a tornado in Ngongotaha refers to part of the same system that struck Hobsonville earlier in the day, and has been the subject of Severe Thunderstorm Warnings. This cell is now moving over Mahia and out to sea.

Update as at 4.30pm 6 December 2012

The last active thunderstorm is moving in towards Lake Waikaremoana and is expected to dissipate. The remainder of eastern Waikato and BOP are still experiencing some heavy showers and occasional thunderclap, but this is expected to clear during the evening.

Update as at 3.30pm 6 December 2012

The active line of showers and thunderstorms is expected to clear Auckland CBD by 3.30pm. Active thunderstorms are now moving into the Bay of Plenty and there are Watches and Warnings in place. The whole system is then expected to clear the North Island later this evening.

A different system is also expected to bring active showers and possible thunderstorms to Taranaki from late evening.

Severe Thunderstorm Outlook map issued at: 2:52pm Thursday 6 Dec 2012

Severe Thunderstorm Outlook map valid to Midnight Thursday 6 Dec 2012, issued at: 2:52pm Thursday 6 Dec 2012

Severe Thunderstorm Outlook map valid to Midnight Thursday 6 Dec 2012, issued at: 2:52pm Thursday 6 Dec 2012

We will update this blog again before 5pm. Further updates will then be available through Watches and Warnings, as well as on our Facebook page and on Twitter.

Where to find the latest information from MetService:

Latest Severe Thunderstorm Warnings here: http://metservice.com/national/warnings/severe-thunderstorm-warnings

Rain radar here: http://metservice.com/national/maps-rain-radar/rain-radar/all-new-zealand

Latest Severe Thunderstorm Watches here: http://metservice.com/national/warnings/severe-thunderstorm-watch

Latest Severe Thunderstorm Outlook here: http://metservice.com/national/warnings/severe-thunderstorm-outlook

Latest Severe Weather Warnings here: http://metservice.com/national/warnings/severe-weather-warnings

Latest Severe Weather Watches here: http://metservice.com/national/warnings/severe-weather-watch

Latest Severe Weather Outlook here: http://metservice.com/national/warnings/severe-weather-outlook

Check our Facebook page here: https://www.facebook.com/MetService

Follow @MetService on Twitter here: https://twitter.com/MetService

MetService’s role in monitoring volcanic ash

Monday night’s eruption at Mount Tongariro set in motion MetService’s volcanic ash monitoring process for the aviation industry. The Volcanic Ash Advisory process involves interaction between aircraft operators, Airways Corporation and MetService, with important volcanic information input from GNS Science.

This information is also provided to Civil Aviation (CAA) and is used by them to designate Volcanic Hazard zones around those volcanoes that are known to be Volcanic Alert Level 1 or higher.

MetService produces Volcanic Ash Advisories (VAA), volcanic ash SIGMETs (warnings of significant aviation hazards) and forecasts for the track of the ash cloud, using ash trajectory models and other supporting information, including reports of visible ash.

Airways Corporation notifies airlines about which routes and procedures will be affected by each level of volcanic activity, and aircraft require air traffic control clearance via specific request from the pilot to operate within a Volcanic Hazard zone.

MetService’s Volcanic Ash Advisory Centre (VAAC) is part of a worldwide chain of 9 such centres, all working together to pass on information about ash cloud as it moves around the globe. Only last year, MetService worked closely with Australia’s VAAC in Darwin to keep an eye of the movement of the ash from Chile’s Puyehue Volcano. You can read more about that event on our website here: http://www.metservice.com/ar-2011/case-study1.html

Please note that MetService does not provide ash forecasts direct to the public, as our monitoring is specific to aviation requirements. However, if you are interested in the direction of the ash cloud, you can view a graphic representation of the latest ash monitoring information on our Volcanic Ash Advisory Centre site here: http://vaac.metservice.com/wellington

GNS Science is the organisation responsible for issuing Volcanic Alert Bulletins (including the predicted ashfall area) and Civil Defence are also monitoring the situation and issuing updates as required.

All queries regarding commercial passenger flights should be directed to your airline, and your best source of information about the volcano itself is GNS Science’s GeoNet site: http://www.geonet.org.nz/volcano/activity/tongariro/