Challenges in forecasting severe weather

The severe weather in various parts of the country over the last week or so has presented a significant challenge to some communities – and to staff at MetService as well. Getting the message out about severe weather, particularly when it involves rapid changes, requires excellent communication with the New Zealand public and many organisations managing weather-related risks. The message needs to be relevant and clear – not always an easy task, given that users of weather information have such diverse needs.

Last Thursday saw the beginning of a memorable outbreak of west to southwest winds over the country. Indeed, this event still has a couple of days to run.

In the days leading up to this event, well before any weather watches or warnings were issued, while monitoring the upcoming southwest outbreak MetService’s Severe Weather team recognised that severe weather was likely in some areas. By the afternoon of Sunday 12 September there was sufficient concern for MetService to mention the southwest outbreak in the Severe Weather Outlook, which is issued daily to media, the public, regional councils, outdoor leisure companies, etc. Over the next few days, as the picture became clearer, various outlooks and forecasts issued by MetService contained more detail about what to expect.

The projected severity of the event prompted MetService to issue a media release on the afternoon of Wednesday 15 September, several days before anything much had actually happened. MetService advised that “the westerlies are likely to be strong or gale force in many places … possibly reaching severe gale around central New Zealand late in the week,” and that “a gradual change to bitterly cold conditions is likely to spread onto southern New Zealand from late Friday and this may bring appreciable amounts of snow to low levels.”

This sequence of events reflects a typical approach that MetService takes to achieve the right level of public attention in the lead-up to a major weather event. Severe Weather Outlooks and press releases act as a “heads-up” that significant weather is on the way. As the event draws closer, Severe Weather Watches and/or Warnings are issued, targeted at specific regions, to ensure that people in affected areas get more detailed advice about what to expect.

The timing of these steps, and the language used in the various bulletins, are chosen carefully to ensure that the message is pitched appropriately for the level of severity of the weather, and is delivered with an appropriate lead time. If the message is under-stated, or arrives too late, people may get caught out; if it’s over-stated, or comes too early, people may not listen the next time. In some ways, the challenge of getting the communication right is even more difficult than getting the meteorology right.

During this particular event, shortly after the MetService press release on Wednesday, this communication process was thrown off kilter by a media article about “the largest storm on the planet”. The article was based in part on the MetService press release but included information from other sources as well as a measure of journalistic licence.

Within a matter of hours, MetService was fielding calls from people concerned about the “massive storm heading for New Zealand” and asking for clarification on various statements that MetService had apparently made. It was clear early on that people were confused about the source of the information they were receiving, and had been misled into thinking that the whole country was in for serious weather.

The article “went viral” overnight on Wednesday 15 September and by the following morning had generated an unusually high level of concern in the community and interest from the media, both local and international. From Thursday 16 September to Tuesday 21 September, MetService fielded a huge number of enquiries – primarily from media organisations, but also from worried farmers and business people, and frightened members of the public. Organisations responsible for public safety, such as the New Zealand Transport Agency, have also contacted MetService concerned about the confusing information in the media and seeking clarification.

Over the last few days there has indeed been some very severe weather in several parts of the country, as had been predicted in MetService’s forecasts and warnings. Unfortunately, MetService’s ability to get weather information to those who really needed to know was significantly hampered by media articles over-stating the area affected by the storm.

The effects of this event on those communities that bore the brunt of the severe weather were and are substantial, with widespread flooding, slips and damage to property in some areas, stock losses and disruption to transport. However, it is important to remember that many parts of the country have experienced benign weather at the same time, leaving many people wondering what all the fuss was about. The danger this raises is that some of those may simply ignore the next Severe Weather Warning they receive.

To date, this cold, wet, windy, and snowy outbreak has progressed pretty much as MetService’s Severe Weather Team thought it would. As well as issuing forecasts and warnings for various areas and monitoring the weather, the MetService team has stayed in close touch with hydrologists and emergency managers at territorial authorities and the Ministry of Civil Defence & Emergency Management. At the same time, it has dealt with a large number of media interviews and public queries to try and clarify the situation as much as possible.

Like many others, the team at MetService will be relieved when recent events finally draw to a close and spring resumes its progress towards summer.

Wave three

New Zealand has been in a strong west to southwest flow for a few days now. This weather regime looks as though it will continue for the next few days.

There are reasons why the weather gets into regimes like this. It has to do with what meteorologists call the “long waves”.

The map below is one way of depicting what the weather pattern about half-way up the troposphere (the troposphere is that part of the atmosphere in which the weather occurs) looked like at midday Thursday 16 September 2010 New Zealand Standard Time, just before the westerlies broke out across the country. In this pattern, there’s nothing that looks like a high, or low, or front. But there is a bunch of wavy lines … which look kind of regular, but not quite.

NCEP 500hPa reanalysis valid 0000UTC 16-Sep-2010
Image provided by Physical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, from their Web site at

Now what if those waves could be separated out into longer ones and shorter ones, so we could see what was going on “under the bonnet”? There happens to be a way of doing exactly this; it’s called Fourier analysis. (Here we stand on the brink of numerical weather prediction, but we’ll go there some other time).

The picture below is made from a Fourier analysis of the pattern 12 hours further on from the one shown above. It reveals that, in New Zealand’s latitudes, it’s dominated by a wave which has three troughs (blue) and three ridges (pink) wrapped around the hemisphere. Most important to us is the big area of blue shading – a trough – near New Zealand.

Hemispheric wave analysis for 1200UTC 16-Sep-2010
The "long" and "medium" waves about half-way up the troposphere at midnight Thursday 16 September 2010

The animation below shows how this trough in the three-wave pattern edges only slowly onto New Zealand during the few days following Thursday 16 September 2010. “Wave three”, as it’s often called by meteorologists, is one of the long waves. It tends to move only slowly – which is why, once it’s gathered a bit of energy, it can influence the weather over a period of days.

text to go here
What the "long" and "medium" waves did (and are forecast to do) for the few days following Thursday 16 September 2010

We live on the Earth’s surface, at or near the bottom of the troposphere. Down on the Earth’s surface, the isobar maps of the last few days (see animation below) have a certain sameness – they all show

  • Tightly packed west to southwest isobars over New Zealand, which is why the wind has been more or less from the same direction
  • Fronts whizzing across the country, which is one of the reasons why showers have come and gone, and the wind shifted around a bit in direction, and gone up and down a bit in strength.
Isobar maps, at 6-hour intervals, from midnight Thursday 16 September 2010 to 6am Monday 20 September 2010

Going Viral

A story titled “Massive storm heading for New Zealand” went viral in the online media overnight Wednesday 15 September.

The expression “Massive storm heading for New Zealand” didn’t come from MetService. It conjures up impressions of a huge low swooping down on the country. Today, Thursday 16 September, we’ve had calls from media, business people and members of the public expressing their concerns about a “massive storm” and seeking more information. We’ve advised some of the media outlets covering this story of our dislike of the emotive language being used and asked that they attribute the source of these quotes.

For the next few days, the weather over New Zealand is expected to be severe in some places, at some times. But not everywhere. Early on the afternoon of Wednesday 15 September, MetService issued a media release about this. Finer details – the what, where and when – of the expected severe weather are described in the various Watches and Warnings routinely issued by MetService.

Take a look at the weather maps below. The first is the situation as it was at 6:00am Thursday 16 September. There’s a low over the eastern Tasman Sea; it’s moving quickly southeastwards and will be well out to the east of the country by midnight Thursday 16 September (see second map). This low is neither massive nor particularly deep. Over the last few months, many like this one have crossed New Zealand from the Tasman Sea.
Weather maps

The important things on the second and third maps (midnight tonight and midnight tomorrow, respectively) are:

  • The broad belt of tightly packed westerly isobars covering the country; this is the source of strong winds expected in many places over the next few days.
  • The way that these westerlies extend right back across the Tasman Sea and then over the Southern Ocean. This broad area of strong winds is generating big sea waves which are expected to arrive on New Zealand’s western coasts on Friday 17 September and then last for a few days, easing off slowly.
  • The way that the “source air” in the westerlies is from the Southern Ocean. This is why snow to low levels is being forecast over western and southern parts of the South Island.

Not obvious from the weather maps are the squally showers and thunderstorms that we expect will be carried into western parts of the country by these winds. However, you can see these in the satellite picture below: they’re in the circled area, still (at the time of posting this) out over the southern Tasman Sea.

Infra-red satellite image for 1:00pm Thursday 17 September 2010. Courtesy of Japan Meteorological Agency.
Infra-red satellite image for 1:00pm Thursday 17 September 2010. Courtesy of Japan Meteorological Agency.

Stay tuned to the MetService forecasts and warnings.

Charles Kingsford Smith

A recent visit to Sydney International Airport has inspired me to write a little about another famous aviator with a New Zealand connection. Last year I wrote a post about Jean Batten who, in her youth had met the topic of this post, Charles Kingsford Smith. Auckland International Airport is named in honour of Jean Batten and likewise Sydney International Airport is named after Charles Kingsford Smith.  

A view from Sydney's "Kingsford Smith Airport"

Charles Kingsford Smith was born in 1897 (12 years before Jean) and, after serving in the First World War (including Gallipoli), developed an interest in aviation. He became one of the earliest airline pilots in Australia. In 1928, in the Southern Cross, he led the first successful flight across the Pacific Ocean from USA to Australia.  

Later that year he led the first successful return flight from Australia to New Zealand, a journey that in the modern age of jet aircraft we take for granted. It took him 14 and a half hours to complete the flight from near Sydney to Christchurch, a distance of 2670 km (1660 milies). You can read more about this achievement in the previous link.  

Five years later Kingsford Smith made the first commercial trans-Tasman flight and, like Jean Batten he obtained weather information from our then Director of the New Zealand Meteorological Service, Dr Edward Kidson. As I explained in the post about Jean Batten, good up-to-date meteorological information is critical for air safety; but this importance extends beyond safety to also include flight efficiency.

We hold a few precious letters from Kingsford Smith, including the following.  

An early letter from Charles Kingsford Smith to Edward Kidson of the NZ Meteorological Service (23 Feb 1933)

The letter reads:  

Grand Hotel,
Palmerston North,
Feb 23rd 1933

Dr E.Kidson,
Meteorological Office,

Dear Dr Kidson,  

Will you please accept my sincere thanks for the useful information in your letter which reached me prior to my departure by steamer from Sydney.  

I will be making contact with you in the near future as I am still determined to spend some time in awaiting a favourable opportunity for the return trip, for which I hope to be ready not later than the 26th of March.  

Again thanking you very much,  

I am,
Yours very sincerely,

(signed) C Kingsford Smith”  


And a later letter:  

A letter from Charles Kingsford Smith to Edward Kidson (28 Feb 1933)

Midland Hotel,
Feb 28th 1933

Dr E.Kidson,
Meteorological Branch,

Dear Dr Kidson,  

In reply to your Query re petrol supply I beg to inform you that I will be carrying a minimum of 20 hours petrol at a cruising speed of about 88 miles per hour.  

Thanking you for your interest,  

Yours faithfully,  

(signed) C Kingsford Smith” 

Contrast his aircraft’s cruising speed of 88 mph (142 km/hr) back in 1933 with the typical cruising speed of a Boeing 767 aircraft, 550 mph (about 900 km/hr and at a much greater height too, of course). These days you can expect to fly from one of the eastern Australian airports to New Zealand in three and a half hours, and even less if the upper winds are favourable.

These upper winds are often in excess of 100 kt (115 mph, 185 km/hr) within narrow zones called jet-streams, usually blowing in the direction from Australia towards New Zealand. For today’s aircraft they can have a significant impact on flight duration, shortening the flight to New Zealand with a favourable tail-wind, and prolonging a return flight to Australia with an unfavourable head-wind.

Westward-bound flights to Australia plan their routes to try and avoid, as much as possible any such jet-stream zone, not only to shorten the flight but also to make more efficient use of fuel.