Author Archives: Bob McDavitt

A Southerly ‘Buster’

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On Monday 28th November, a south to southwest change swept its way northwards across Otago and Canterbury during the afternoon.   Temperatures soared to 28 C preceding this change then rapidly plummeted to around 16.  This was a good example of what is known in Australasia as a ‘buster’.

Temperature traces on 28 November in degrees Celcius. DNA=Dunedin, OUA=Oamaru, TUA=Timaru, CHA=Christchurch and KIA=Kaikoura. Timestamp is in UTC; 27 0000 is 1pm NZDT.

The weather map for 1pm Monday 28 November 2011 showed a typical trough moving across New Zealand. The last of a series of fronts within this trough was the one responsible for this dramatic drop in temperature.

Weather map for 1pm NZDT Monday 28 November

The reasons for temperatures soaring to between 26 and 28 C ahead of this southerly change are:
•    Northwest winds ahead of the trough warmed by around 5 to 10 degrees Celsius as they descended down the eastern slopes of the Southern Alps
•    Sunny conditions in the relatively clear skies over the Canterbury Plains – on a date less than one month ahead of the longest day.

These warm temperatures combined with falling air pressure to produce a zone of relatively low density. Higher density air in the cooler southerly flow that followed this cold front accelerated into this zone of low density air producing a squally “gust front” with the wind change. This “gust front” built in size and intensity during the afternoon as can be seen from the tweets sent from @metservice during the afternoon

  • Southwest change arrived Dunedin Airport around 11:50am. Temp dropped from 22 to 14 C , gusts to 50 kph , and its on its way north. ^BM
  • Southerly change got to Oamaru about 1:30 pm, temp. dropped from 22 to 14 C, initial gusts were 54 kph . South Canterbury’s next ^BM
  • Southwest change arrives in #Timaru just before School’s out, Temperature drops from 28 C at 2:30 pm to 16 C by 3 pm , Gusts to 70 kph ^BM
  • Southerly change reached #Ashburton between 3:30 and 3:45 pm, temp. dropped from 26 to 16 C, gusts 70 kph see http://t.co/YhuAEZ4L ^BM
  • Southwest reached #Christchurch at 5 pm in time for evening commute, temps 28 C to 17 C in 20 minutes. with wind gusts 75 kph , ^BM
  • Hi #Christchurch be quick and look at wind blown dust of that southerly change on MetService radar at http://t.co/lmw4BcYA past hour ^BM

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September to November is the season for the strongest of these southerly busters (but they can occur at any time of the year).  Spring brings the strongest westerly winds of the year to South Island and it is also a time of relative cold offshore sea temperatures.  The temperature difference between the heated air over the Canterbury Plains and seas in the Canterbury Bight is what feeds the wind gusts of a buster. The coldest sea temperatures of the year occur in early spring, and they only just start rising in November.   You can find the latest reading from metservice.com by clicking on ‘marine’ and then ‘beach’ and selecting a suitable site.  The one shown below is Jack’s Point near Timaru (timestamp is 10am Wed, 30 Nov 2011).

Jack's Point on the metservice.com Marine & Surf section. Wind, Wave ands Sea conditions are now available on metservice.com

 

The showers with this buster occurred mainly along the coast and at sea.   In the drier air over the Canterbury Plains the southwest wind change picked up dust and dirt, especially over the Rakaia River, and lifted and blew these as a “dust storm” into Christchurch.  This can clearly be seen on the animation below, taken from the high frequency Christchurch rain radar site at metservice.com.

Story behind polar outbreak of August 2011

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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

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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.

Shortest day today. Coldest day still to come.

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Last night was the longest of the year, and today is the shortest day.

The time from sunrise to sunset is shorter today, by a few seconds, than the surrounding days.

The winter Solstice, or that point in time at which (from our viewpoint) the sun was furthermost to the north, occurred at 5:16am New Zealand local time.

Click here to see a previous MetService blog that has a graph of the sun’s changing position from our viewpoint during the year.

In Coordinated Universal  Time (UTC), this winter solstice was at 17:16 (hours: minutes) on Tuesday 21 June.  Some calendars printed overseas have June 21 marked as the shortest day, but for us it is Wednesday 22 June 2011.

The actual date of the solstice varies a little from year to year, and  wanders towards the 22nd as we  approach a leap year.  After the leap year adjustment is made and the solstice and equinox are corrected back toward their natural date, the 21st.

There is an old saying that goes something like this: “when the days start to get longer, the cold gets stronger”.  To help appreciate what that means, look at the past data page for a site near you from our ‘Towns and Cities’ or ‘Rural’ page.    Auckland is shown below as an example:

On our website you can read individual data points by using the ‘mouse-over’ option.

Notice how the long-term average of the minimum temperature for Auckland,computed monthly, reaches its lowest value of 1.9C in July, while the long-term average of the maximum temperature , computed monthly, reaches its lowest of 17.6C in August.  The coldest sea temperatures of the year in Auckland are just below 14C and usually occur in late August.  For Auckland, the warmest air temperatures usually occur on or within a few days of Waitangi Day and the warmest sea temperatures follow around ten days later.

In New Zealand, the coldest air temperatures of the year occur between mid-July and early August, around 3 to 6 weeks after the solstice.   This seasonal lag varies around the country, and is longer in the north than in the south.  The lag reflects the time required for the sun to warm the earth’s surface. In New Zealand, because of the strong effect of ocean temperature on our climate, the lag is strongly tied to the response of the oceans to changing sunlight.  Click here to read more. However, this lag is also seen in places with a continental climate – i.e. not directly affected by sea surface temperatures – where it reflects the time it takes to warm up the ground.

The graph above shows that, in Auckland, May 2011 was warmer than normal, about as warm as an average April.  So far, June has also been warmer than normal.  This has been caused by the frequent presence of  strong low pressure systems in the Tasman Sea, which have fed persistent, moist, mild northerly winds onto NZ, with very few days of cooler southwest winds.  There is still time for this pattern to change.  The latest seasonal weather outlook for July, located here, indicates that some parts of NZ are likely to see a change in weather pattern over the next few weeks.  For them, the ‘cold will get stronger’.

World Met Day 2011

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23rd March is World Meteorological Day.

Each year meteorologists around the world celebrate a chosen theme together to commemorate the anniversary of the founding of the World Meteorological Organization (WMO) on this day in 1950.

The theme this year is “Climate for You”.

[Click on the diagram to link to the WMO ‘Climate for You’ web site - where available information includes a brochure (2.7MB) and poster (0.4MB).]

WMO held an important Climate conference at Beijing in November 2005 — the WMO Technical Conference on Climate as a Resource — to consider how climate can be used as a resource, and how best to make climate data available to benefit society by improving economic decision-making.  These data are important for they shape the availability of natural and renewable energy resources.  A knowledge of rain and temperature trends is required to optimise agricultural performance, water management and food security.

The WMO Conference on Living with Climate Variability and Change: Understanding the uncertainties and managing the risks (held in Espoo, Finland in July 2006) underscored that while climate is indeed a critical resource, we are also especially vulnerable to its variability and change.  Some actions need be taken urgently to manage the risks of climate variability and change impacts.  Others should be increasingly implemented to harvest the benefits to be derived from climate information and services, by helping socio-economic sectors maximise their efficiency and productivity.

In March 2007, WMO organised in Madrid the International Conference on Secure and Sustainable Living: Social and Economic Benefits of Weather, Climate and Water Services, which provided an outstanding opportunity for a wide exchange of views, expectations and knowledge across various societal sectors to optimise the decision-making process, and formed the Madrid Action Plan.

Moreover, 2007 was the year when the WMO co-sponsored IPCC, released its fourth Assessment Report and received the prestigious Nobel Peace Prize, as well as when the fifteenth World Meteorological Congress agreed to convene with partners at a World Climate Conference-3, or WCC-3, in the spirit of the previous two historic World Climate Conferences which WMO had organised in 1979 and 1990.

Not long before the WCC-3 began, the WMO Executive Council decided at its 61st session in June 2009 that the theme of the World Meteorological Day for 2011, commemorating the coming into force of the WMO Convention on 23 March 1950, would be “Climate for You”.

At WCC-3, a High Level Taskforce was mandated to prepare a report which will be among the key issues to be considered by WMO Members during the sixteenth World Meteorological Congress, to be held at Geneva in May 2011.  This report includes a proposal for establishing a Global Framework for Climate Services, or GFCS.  The New Zealand delegation to Congress will be led by Dr. Neil Gordon from MetService, who is the Permanent Representative of New Zealand with WMO.

Dr. Neil Gordon from MetService, the Permanent Representative of NZ with WMO.

As WMO has recently reported, 2010 ranked together with 1998 and 2005 as the warmest year on record, which only confirms the observed long-term warming trend highlighted by the IPCC report.  All of the ten warmest years on record were experienced since 1998.  Additionally, over the ten years elapsed since 2001, global temperatures averaged almost half a degree above the 1961-1990 mean, the highest ever recorded for any 10-year period since the beginning of instrumental climate observations.

WMO activities in the area of climate are focused on human safety and well-being and the realisation of economic benefits for all.   This is in line with the spirit of the WMO Convention which came into force on 23 March, sixty one years ago, as well as the patrimony of the former International Meteorological Organization IMO established by the First International Meteorological Congress in Vienna, Sept 1873.

Please join with the meteorological community around the world in a round of applause and a toast to celebrating World Meteorological Day 2011.

————————————————————————————————————————————————————————————

The above text is based on a special message from Michel Jarraud, Secretary-General of WMO,  to mark World Meteorological Day 2011.

Michel Jarraud, Secretary-General of WMO

 

Information about WMO activities related to climate is available at: http://www.wmo.int/pages/themes/climate/index_en.php .

The WMO online youth corner, created recently, also includes a section on climate relevant to the theme of World Meteorological Day 2011: http://www.wmo.int/youth/climate_en.html

 

Storm Surge

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On 23 January 2011, widespread flooding affected places from Wanganui to Hawke’s Bay northwards. Coastal sea inundation affected several parts of Auckland including Queen Street, Tamaki Drive, the northwest motorway, Herald Island, Maraetai and Miranda (western side of Firth of Thames).

SH16 Auckland NW motorway city-bound buslane, Sun 23 Jan 2011. Photo courtesy of Peter Mitchell, Auckland Motorways.

This was a case of storm surge associated with a passing low pressure system. The system formed in the tropics around New Caledonia, but left the tropics before it had time to deepen into a tropical cyclone.


Weather map for noon on Sunday 23 January 2011. The region marked DQ is explained in the text below. Click on the map for a loop of animated maps in 12 hour time steps covering Sun-Mon 23-24 January 2011.

Storm surge is the name given to the situation when the sea floods inland along the coast. It has three components.

1. Wind setup.  When strong winds blow from the sea to the land (“onshore”), a wind setup is formed: under some circumstances, the sea is pushed onto the land faster than it can drain away, and waves penetrate beyond the high water mark. Wind setup depends on the size and shape of the strongest wind zone and the land it encounters. It is accentuated in shallow basins such as the Firth of Thames.

Strong winds are often associated with low pressure systems. Storm movement – that is, the movement of the low pressure system itself – influences the wind strength.

In the Southern Hemisphere, where the wind flow around a low is clockwise, storm movement adds to wind strength on the left side of the storm track.  For a low travelling southwards, this means that winds are stronger in its eastern semicircle.  This is why many sailors call this the dangerous semicircle and its front-left quadrant the dangerous quadrant.

On 23 January, when the low on the weather map above approached Northland, its dangerous quadrant (labelled DQ on the map above) brought the most pronounced onshore winds and highest wind setup to the Coromandel – Bay of Plenty area. NIWA measured storm surges of 370 mm around Coromandel and 590 mm at Moturiki Island near Tauranga.

2. Low air pressure. The inverse-barometer effect occurs whenever air pressure over the ocean differs from normal. Each hectoPascal of air pressure below the norm (of 1013 hPa) may raise the sea level by ten millimetres. So, a large low pressure system is accompanied by a dome of elevated  sea surface.

On 23 January, when the pressure dropped to 986 hPa around Auckland, the maximum inverse barometer effect was around 270mm.

3.  The tide comes and goes around once every 12 hours 20 minutes. The tidal range – that is, the difference in height between low and high tide – is largest a few days after the moon reaches its perigee (closest point of its orbit to earth) within a day or so of reaching a full or new phase. This happens only a few times per year,  and is sometimes called a King tide.

Auckland has a reasonably large tidal range of around 3 m. On 23 January the high tide height was 3.5m – only about 100mm lower than a King tide.

Strong onshore winds from the tropics often produce heavy rain on any hills and ranges. If this drains off quickly (as is usually the case in New Zealand) and arrives at the coast at the same time as a high tide,  the chances of flooding near river mouths and estuaries is increased.

Auckland/ mm/23 Jan ~noon ~midnight
1. Wind setup

449

270

2. Inverse barometer

 180 250
3. Astronomical tide

3500

3400
Total =  measured storm tide 4129 3920

Auckland’s tide gauge showed a peak of 4.129m between 11am and noon on 23 Jan 2011.  Data are courtesy of Ports of Auckland.

When Tropical cyclone Yasi made landfall onto Queensland,  Townsville (with a two-metre tidal range) was in the dangerous quadrant.  Yasi’s two-metre storm surge arrived on the outgoing tide, reducing its impact.  The highest-ever  measured storm surge was 8.5 metres at St Louis Bay in Hurricane Katrina (2005).  The worst storm surge, in terms of loss of life, was the Bhola cyclone which hit East Pakistan (now Bangladesh) on 12 Nov 1970, here an estimated 500,000 people perished.

The online nautical almanac published by Land Information New Zealand explains more about how tidal predictions only give the astronomical component,  and how strong meteorological effects need to be taken into account.  During a period of King tides, if there is an anticyclone around or strong offshore winds, extra care is needed around rocks, reefs and sandbars, as the actual water level may drop to near or below chart datum – which is the level that charted depths displayed on a nautical chart are measured from.  Each nautical chart carries a note defining its chart datum and most New Zealand nautical charts use the lowest astronomical tide computed over the period of one entire SAROS cycle.

The  next  King tides are on 21-22 March and 19-20 April.