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.
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:
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 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.