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.
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.
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:
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.
Thank you for all your feedback on the new MetService website. The website is an evolving platform and we will continue to make enhancements once we have reviewed all feeback.
The majority of feedback received has been about four main points:
1. The website colours. We have some design ideas to lighten the colours on the website. Please tell us which one of the four concepts you like best by ‘liking’ the image on Facebook. We will review the feedback and any changes will be made in January.
For those who don’t have access to Facebook, here are the images. You can provide feedback at the blue link below.
2. The hourly/two-hourly graphs and table data. A lot of people have told us they prefer to read temperature in a line graph and rainfall in a bar graph. We will take a look at how we will address this in the new year.
3. Past Weather information – please note this will be available in January.
4. Rural temperatures – please note this will be available early next year.
If you’re going on holiday in the southwest Pacific, you should check out the MetService TV Pacific Weather update. But it isn’t always “beach weather” in the tropics. Severe weather in the southwest Pacific can have very significant consequences.
South Pacific Guidance
Since 1 November 2009, RSMC (Regional Specialized Meteorological Centre) Wellington, of MetService, Lead Meteorologists have produced twice-daily broadscale guidance for expected severe weather over the next 5 days in the tropical southwest Pacific. This type of guidance is known as the South Pacific Guidance charts, or SPG.
It’s important to note that we are not doing the job of forecasters in each country. This South Pacific Guidance (SPG) chart is another piece of information that they can use to write their forecasts, but should not be used in isolation.
Why do we do this?
SPG charts are one of the key products of the Severe Weather Forecasting and Disaster Risk Reduction Demonstration Project, (SWFDDP). The project is overseen by the World Meteorological Organization (WMO), based in Geneva. What is it trying to do?
The project is not unique to our part of the world. It began in Africa in the mid-2000s, and there are now two separate African projects covering 16 countries in Southern Africa and 6 countries in Eastern Africa. RSMCs in Pretoria, La Reunion, Nairobi and Dar es Salaam lead these projects. There are future plans for similar projects in Southeast Asia and the Bay of Bengal.
The southwest Pacific project includes 9 Pacific Island Countries ( Solomon Islands, Vanuatu, Fiji, Tuvalu, Kiribati, Tonga, Samoa, Niue and the Cook Islands) and 2 Territories (American Samoa and Tokelau) .
What do we mean by Severe Weather?
“Severe weather” needs to be defined carefully and in a way that is worthwhile and meaningful for all participating countries. Differences in latitude and terrain across the SW Pacific make this a tricky task. The current thresholds for severe weather are shown below:
Large waves, strong winds and heavy rain can all exist outside of tropical cyclones, whereas tropical cyclones can include all of the above.
For example, large waves have had a significant impact on low lying atolls in Kiribati and Tuvalu, strong trade winds have contributed to damage in Rarotonga and sinking of a vessel in the Solomon Islands, and heavy rain caused major flooding in Fiji in March 2012; all without the presence of a tropical cyclone.
As shown in the goals above, a major aim of the project is to raise awareness and capabilities in dealing with severe weather outside of tropical cyclones.
The first image in this post showed cloud build-ups about the higher ground of Guadalcanal. These may produce rainfall in excess of 100mm in 24 hours, but if they cover a very small area are not currently included in the SWFDDP. The project is really focussed on broadscale weather, which can be predicted with some accuracy days ahead of time.
What else does MetService do in the tropics?
The Lead Forecaster draws the “SPG” charts we saw above. Other duties of the Lead Forecaster in the tropics include regular discussions with meteorologists in RSMC Nadi regarding future tropical cyclone formation, production of bulletins for the New Zealand Ministry of Foreign Affairs and Trade detailing future tropical cyclone potential, and drawing streamline analyses.
Streamline analyses look quite different to the regular “weather maps” shown in New Zealand newspapers, the MetService website, and on TV.
This type of chart is drawn because as you get further into the tropics (especially between the latitudes of 15S and 15N) drawing of isobars becomes increasingly difficult, ultimately impossible, and the wind no longer behaves with the same relation to isobars as it does in temperate regions.
In producing SPG charts, lead forecasters have to be aware of what is happening right now (using satellite data and surface observations and combining these into a streamline analysis) and what is expected to happen during the next 5 days. This requires combining meteorological understanding with numerical weather prediction (NWP) products from supercomputers around the world. In particular, products known as ‘ensemble prediction systems’ are used.
Ensemble products provide a way of dealing with the uncertainty in the atmosphere. The atmosphere is large and extremely complicated, hence it’s impossible to perfectly know everything about it at any given moment. But the problem is that the atmosphere is very susceptible to very minor changes in what is happening right now. These minor changes are proven to have a huge impact in the future. This is known as Chaos Theory and has been popularised with the well-known “butterfly effect”.
Understanding, and interpreting, ensemble prediction systems is a delicate operation, but it is entirely essential in modern forecasting. Without it, forecasters are severely restricted in their outlooks.
MetService also runs a website called MetConnect Pacific. This is a portal for forecasters involved in the SWFDDP to access information, observations and the South Pacific Guidance charts drawn by MetService Lead Forecasters.
Climate Change Development Fund
Thanks to financial aid from the Climate Change Development Fund administered by the New Zealand Ministry for the Environment, MetService has been able to conduct in-country training between May 2012 and November 2012 for all nine participating Pacific Island Countries. On each trip, two meteorologists from MetService have provided training, along with meteorologists from the National Weather Service in Hawaii for the trips to Samoa and Fiji.
What does this project, which focusses on the weather over the next 5 days, have to do with climate change, which looks into seasonal, interannual to decadal changes timescales?
We believe that dealing with extreme weather is the first vital step towards managing the risks of extreme events and disasters to advance climate change adaptation at the community level. According to many climate scientists, climate extremes (extreme weather or climate events) such as substantial warming in temperature extremes, heavy precipitation or the proportion of total rainfall from heavy falls, average tropical cyclone maximum wind speed are likely to become more common in the future as the earth warms up. The recent Hurricane Sandy that struck the eastern United States could be an example of this.
What we did in our training during 2012
The training has partly focussed on the practical use of NWP products, especially from ensemble prediction systems. We’ve also covered reporting of severe weather events (which is required every 4 months), case studies (looking at a past weather event and trying to find out what happened and why) and verification (go here or here to see how we do it in NZ).
In all countries, either a half-day or full-day workshop devoted to Disaster Risk Reduction has been held. This has been a practical, interactive day with representation from the national meteorological service and many other agencies such as (but not restricted to) the National Disaster Management Offices, Civil Aviation, Maritime, Fire Service, Health departments and NGOs such as the Red Cross.
On this day we’ve run a mock-up severe weather exercise and asked participants to play their typical roles in responding to the expected weather impacts and formulating a plan of action.
Below are some images of the training in various countries over the last seven months:
And finally….we’ve been lucky to visit some unusual places and meet friendly people (from top to bottom: Funafuti lagoon, Tuvalu; Ministry football team, Solomon Islands; Schoolkids, Kiribati).
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 tornadolocalised wind stormthat touched down near Hobsonville, tragically killing three people.
Could this tornadolocalised 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.
The Severe Thunderstorm Outlooks issued on MetService websites on the morning of Thursday 6 December, for that day, are below.
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.
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 tornadolocalised 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 tornadolocalised wind storm is likely to have occurred during the few minutes in which this thunderstorm collapsed.
MetService learnt about the tornadolocalised 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:
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
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 tornadolocalised 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.
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.
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.