Story behind polar outbreak of August 2011

August 2011′s polar outbreak was a major weather event that drew media interest from around the world.

This event was notable in recent history, in terms of the coldness of the air and extent of its spread across New Zealand.  During the week of the ‘big chill’, MetService’s Chief Forecaster kept up an in-depth explanation of the polar outbreak as it happened.

Behind every forecast is a lot of work.

The starting point for an accurate forecast is an accurate representation of the atmosphere’s current state. This is why MetService operates extensive observing and weather modelling programmes and runs a large 24/7 forecasting operation. For much more about this, see the blog on MetService’s Investment in Forecasting and TVNZ’s recent Breakfast programme: “How does the weather work”.

The 7-day rain forecast on www.metservice.com is very useful for planning purposes, with the maps showing pressure, direction and strength of wind, and where rain is expected to fall. The forecast is in the form of a series of maps, which are also provided in a player, so you can animate the series of maps to see how the situation could change over the period of time.

Rainfall forecast (yellow shades) in the six hours from 6:00pm to midnight Sunday 14 August; forecast wind speed and direction (barbs) at midnight Sunday 14 August; mean sea level pressure (blue lines) at midnight Sunday 14 August, as displayed on www.metservice.com on Friday 12 August.

Our 3-day model provides a closer look at New Zealand. On these charts, the thick purple line is a guide to where precipitation may fall as snow.

Rainfall forecast (shaded) in the three hours from 9:00pm to midnight Sunday 14 August; forecast wind speed and direction (barbs) at midnight Sunday 14 August, as displayed on www.metservice.com on Friday 12 August. Snowfall is likely in the coloured areas enclosed by the thick purple line.

However, computers deal only in numbers and don’t give the complete story – this is where human expertise comes into play. Trained forecasters do things that weather models cannot, for example:

  • Consider, in real time, conflicting information – from models and observations – and determine which outcomes are more likely, based on an understanding of the weather situation
  • Steer a steady course when the model forecasts are “jumping” from run to run
  • Reconcile observed and forecast weather and recognise when the forecast needs changing (regardless of what the models say)
  • Explain the weather, particularly to those who are managing weather-related risks (people talk to people; computers talk to computers)

It is vitally important for forecasts (and updates as they happen) to be communicated in a measured, timely and credible manner.

On Wednesday 10 August, days before the outbreak, MetService published a media release and contacted farming organisations, to alert those with a real need to plan in advance about what forecasters were anticipating. Snow and cold would have a large impact to farmers in the lambing and calving season so this was highlighted in the news release.

Severe Weather Outlook map issued at 1:43 pm on Thursday 11 August.

To ensure that all sectors of New Zealand were kept up to date, a Special Weather Advisory was also issued (click here to see pdf) on Thursday 11 August. This is a special news release that draws people’s attention to particularly significant or widespread weather to come. On Friday 12 August another media release was issued to give a further update on the developing polar blast.

Because of the chaotic nature of the atmosphere (and the variation of predictability with time), the forecast for a given day (or time) in the future can change as we get closer to it. This is why the forecast issued on a Monday for the weekend to come might be very different to the forecast issued on, say, the Thursday of the same week.

In the days that followed Friday 12 August, Severe Weather Watches and Warnings (click here for a detailed account – a pdf document) were issued and updated as conditions warranted. As more data became available, site-specific urban forecasts were updated to include the risk for snow.

Mean sea level analysis for midnight Sunday 14th August, near the beginning of the coldest period during this event.

Snow fell in episodes over several days. There were some notable snow amounts – and reports of snow in places that had not seen it in a very long time.

Snow map showing reports of the depth of snow (in centimetres) from various parts of the country, during the event.

And here is a photo gallery showing images that were sent in from around New Zealand.

This polar blast was long lived and delivered snow to many regions of the country. While heavier snowfalls have been recorded before in some regions, this is undoubtedly the most widespread and prolonged event since 1939 – the subject of a blog by Erick Brenstrum.

Monday 25th July: 2011′s coldest day so far

In New Zealand, the coldest days of the year are usually in late July or early August.

The most significant cold snap since 1995 gripped the nation from late Friday 22 July until late on Monday 25 July. Significant amounts of snow fell in the south and east of both islands, with over 30 cm to near sea level reported in the Christchurch area. Many of the arterial routes through the North and South Islands were closed, including the Waioeka Gorge route from Gisborne to Opotiki.

On Wednesday 20 July forecasters advised, in a Severe Weather Outlook, of the potential for heavy snow during the upcoming weekend. As the weekend approached, various Severe Weather Warnings / Watches, Road Snowfall Warnings and Special Weather Advisories were issued, advising of significant snow from Southland to Hawkes Bay and describing accumulations down to near sea level likely in the South Island and down to 200m further north.

This cold episode was the result of a river of air flowing straight from the Antarctic ice shelf to New Zealand’s shores. For this to occur, a high pressure zone over Tasmania needs to occur at the same time as a low pressure system deepens over the Chathams area, so that between them a southerly flow extends from the Antarctic to New Zealand for a long enough time to allow polar air to reach the country.

MetService mean sea level analysis for midnight Saturday 23 July 2011.

As the weather map above shows, the “polar blast” was just arriving over the far south of the country at midnight Saturday 23 July (behind the old front); it spread across the country during the following two days. Because it is so cold, polar air is generally very dry. But as it passes over the (increasingly warmer) ocean between the Antarctic and New Zealand, it takes up heat and moisture from the sea surface: this is what drives the formation of showers.

Had the polar air arrived a day or so earlier it would have encountered warmer air associated with the low which brought heavy rain to parts of Northland, Bay of Plenty and Gisborne a few days earlier; the result would have been even more snow.

MTSAT-1R infra-red satellite image for 8:00am Monday 25 July as a good burst of snow was moving across Canterbury. The heaviest snow is likely to be falling from the coldest-topped (red) clouds. Image courtesy Japan Meteorological Agency.

MetService weather radar image for 8:00am Monday 25 July. Blue areas are heavier showers.

The coldest air made its way across the North Island early on Monday 25 July and a trough following on behind brought a further bout of heavy snow later that day. Heavy showers brought the snow level down as they passed: a centimetre or so of snow even fell in Greytown (about 100 metres above sea level) in the Wairarapa to the east of Wellington.

Location Maximum temperature on
Mon-25-Jul 2011
Lowest daily maximum temperature on record Month / year occurred in Record starts Monday’s max temperature is the lowest since …
Auckland 10.2 8.8 June 1976 1966 June 2002
New Plymouth 6.7 6.0 July 1951 1944 1944
Napier 7.4 3.6 September 1969 1940 May 1989
Paraparaumu 6.3 5.4 July 2004 1972 1972
Kelburn (Wellington) 5.6 4.5 August 1938 1931 July 1995
Christchurch 5.0 1.7 August 1992 1954 June 2007

 

Finally, this MODIS (Moderate Resolution Imaging Spectroradiometer) satellite image shows the snow coverage over New Zealand late on the morning of Tuesday 26 July. Snow areas are coloured red.