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’.
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.
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
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).
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.
It’s early days in a weather event which is likely to be memorable for its coldness.
Below is a satellite image for midday Sunday. The wind flow over New Zealand is generally from the southwest; the coldest showery air has made its way onto Fiordland, Southland, Otago and south Canterbury.
It’s not raining or snowing over all of southern New Zealand because the wind flow is more or less lined up with the South Island, thereby sheltering some places, and because the precipitation is showery.
The surface temperature doesn’t necessarily tell the whole story. Below is a graph of the air temperature for the 24 hours from 1:00pm Saturday 13 August to 1:00pm Sunday 14 August. Note how the air temperature at Invercargill Airport and Nugget Point, both fairly open to the southwest, has been on a pretty steady downward trend. At 2:00pm Sunday, the temperature at Nugget Point was 1.0 C and the wind speed was 85 km/h. Brrr.
At Timaru Airport, on the other hand, the temperature rose sharply when the southwest change arrived mid Sunday morning – the reverse of what might be expected. The period overnight Saturday to dawn Sunday at Timaru Airport was one of clear skies and fairly light winds, so the quickly-cooling land surface during this period cooled the air immediately above it and an inversion formed. When the southwest change arrived, the air once again became well-mixed. But this southwest air is expected to also become steadily colder.
4:15pm Sunday 14 August 2011
Here’s another look at why the surface temperature doesn’t necessarily tell the whole story.
In my blog post about the winter storm of early July 2011, I partially explained how showers may form in cold air moving over a warmer sea surface. The (relatively) warm sea heats “blobs” of the air immediately above it; these blobs then ascend because they are less dense than surrounding air. For the ascending process to continue, the surrounding air must remain relatively cooler than the ascending warm blobs. Thus, it is important to have information – that is, observations and forecasts – about the vertical temperature structure of the atmosphere.
Observations of the temperature structure of the atmosphere are primarily made using weather balloons. Below is a graph of the temperature at three levels in the atmosphere above Invercargill, obtained from radiosonde balloon flights. The blue line is the temperature at about 5000ft, the red line is the temperature at about 10000ft, and the green line is the temperature at about 18000ft. These heights are approximate; the height of a given pressure level varies with the air temperature; here, we should probably discuss the idea of the thickness of an atmospheric layer – but I think we’ll do that some other time.
Anyway, the graph shows that the atmosphere above Invercargill has been cooling off steadily since the middle of Saturday 13 August. In depth, it is now very cold.
6:15pm Sunday 14 August 2011
It snowed quite heavily in Wellington City, above about 100 metres, from approximately 4:30pm for at least an hour. This is the heaviest and most widespread snowfall in Wellington City for at least 30 50 years.
At midday Sunday 14 August the freezing level around Wellington, obtained from the Paraparaumu radiosonde balloon flight, was just over 1000 metres and falling (it was around 1600 metres at midnight Saturday 13 August). But late on Sunday afternoon, it would still have been well above the level to which snow fell in Wellington. Snow starts melting once it falls below the freezing level – but the melting process draws heat from the surrounding air, which lowers its temperature; thus, the melting snow “drags” the freezing level down with it, at least for a while. How far the freezing level within the area of falling snow is dragged towards the ground depends mostly on the intensity of the snowfall and the vertical variation of temperature and humidity of the air it is falling into.
1:00pm Monday 15 August 2011
So far in this blog, I’ve been talking quite a bit about the temperature throughout the depth of the troposphere (the troposphere is the part of the atmosphere in which weather systems exist). Time, now for a picture. Below is a plot of:
Forecast temperature (colours) at the 500 hPa level (roughly 18,000 ft, or about halfway up the troposphere)
Forecast wind speed (black lines) at the 250 hPa level (near the top of the troposphere)
… for midday Monday 15 August.
The colours in this plot are forecast temperature; over most of New Zealand, the temperature at around 18,000 ft was forecast to be -30 C or lower. The important thing to note is that a large mass of Antarctic air covers almost all of New Zealand.
The red arrow on this plot shows the forecast position of the axis of strongest winds, near the top of the troposphere, at midday Monday 15 August. This is the polar jet, on the border between the deep pool of Antarctic air over New Zealand and the warmer mid-latitude air around it.
Incidentally, the forecast temperatures compare very well with the observed temperatures at midday Monday 15 August, as shown in the table below.
Forecast 500 hPa temperature (C)
Observed 500 hPa temperature (C)
4:30pm Monday 15 August 2011
Here’s a graph of how the freezing level over New Zealand has changed over the last few days.
As of midday Monday 15 August, the freezing level varied between about 1000 ft at Invercargill to about 2000 ft at Whenuapai. Snow has fallen to sea level in many parts of southern and central New Zealand – that is, to at least 1000 ft below the freezing level.
This is a classic example of the melting effect (see the post made at 6:15pm Sunday 14 August 2011, above). Over the last few days, MetService’s Severe Weather Forecasters have spent a lot of time considering how far below the freezing level snow would fall. This requires a good understanding of cloud physics.
9:30am Tuesday 16 August 2011
Here’s a few photos from the Wellington snow of June 1976.
10:30am Tuesday 16 August 2011
In southerly flows, the West Coast of the South Island is well sheltered by the Southern Alps. Since the southerly took hold on Sunday, the air on the West Coast has been very dry because of the Foehn Effect.
Below is a graph of the dew point temperature (the temperature which air must be cooled to for water vapour to condense into water liquid or water solid) at Hokitika Airport from 10am Sunday 14 August to 10am Tuesday 16 August. On the afternoon and evening of Sunday 14 August, there’s a huge change in dew point (around 13 degrees), down to around -10 C. Since then, the dew point has remained negative, generally fluctuating between about -3 C in the morning and -7 C in the afternoon. Such a low dew point makes the air feel much colder than its temperature would suggest. We take the dew point into account when calculating the “feels like” temperature.
2:30pm Tuesday 16 August 2011
This event has been characterised by many places having low daytime (maximum) temperatures.
Maximum temperature on
Lowest daily maximum temperature on record
Month / year occurred in
Monday’s max temperature is the lowest since …
15 August 2011
New Plymouth Airport
25 July 2011 and 12 July 1951
Napier (Nelson Park)
25 July 2011 and 17 July 1995
Monday night / Tuesday morning was very cold in some places, though. Here’s a few notable overnight minima from MetService automatic weather stations.
Waiouru Automatic Weather Station
-7.7 C (new record for August)
Blenheim Airport Automatic Weather Station
-6.2 C (new record)
Rotorua Airport Automatic Weather Station
-5.2 C (equals record)
Taupo Airport Automatic Weather Station
5:00pm Tuesday 16 August 2011
Below is a plot of where the air arriving at an altitude of 500 metres above Auckland at midday Monday 15 August came from. Four days previously, it was over the Antarctic landmass; two days previously, it was still over the Antarctic sea ice. The Antarctic sea ice edge is close to its northern-most extent and is near latitude 60 degrees South. Thus, the air arriving at Auckland passed very quickly over the relatively warm ocean between the Antarctic ice edge and New Zealand. In contrast, the air from the Southern Ocean which arrived over Auckland on Saturday 9 July (see my blog post on the stormy period of early July 2011) had travelled over a much longer stretch of ocean, over a longer period of time, and consequently was warmer and moister.
6:00pm Tuesday 16 August 2011
As of 2:00pm Tuesday 16 August 2011, the extent of snowfall in this storm is as shown in the image below.
4:00pm Wednesday 17 August 2011
During the next few days, while an anticyclone advances onto the country, the general wind flow will decrease in strength and the depth of cloud along eastern coasts gradually reduce. Near sea level the air over New Zealand remains very cold, and the advancing anticyclone more or less “traps” it in place. Very cold air, clear skies and light winds overnight are a recipe for hard frosts.
Hopefully, the diagram below – known technically as a tephigram – helps illustrate this. It is a plot, in the vertical, of the air temperature and the dew point temperature derived from the radiosonde balloon flight at Invercargill at midday Wednesday 17 August. At Invercargill there is already a large mass of sinking, warming (and drying) air above about 5000 ft (see text in red on diagram). This sinking air presses on the (relatively) colder air beneath it, trapping it near the Earth’s surface. In this particular case, the zone of transition between the two different air masses is known as a subsidence inversion. I’ve marked the subsidence inversion on the diagram; it’s the broad blue horizontal bar near the bottom.
The very cold air trapped below about 5000 ft at Invercargill is much less inclined to move around than the air further up in the atmosphere. As I’ve explained above, this is partly because of the advancing anticyclone. But it’s also partly because cold air is less “runny” than warm air. (Treacle flows much more readily when warm than cold). On the right of the diagram below are the winds in the vertical, as they were above Invercargill at midday Wednesday 17 August. Clearly (see text in green on diagram), the one wind barb shown below 5000 ft indicates quite a different flow from all the winds above 5000 ft: the flow near the surface has become decoupled from that above.
In a general sense, this vertical temperature and wind structure is expected to spread over eastern parts of New Zealand during the next few days as the anticyclone moves closer and pressures over the country rise.
Forecast surface pressure field for midnight Wednesday 17 August 2011.
Forecast surface pressure field for midnight Thursday 18 August 2011.
3:30pm Friday 19 August 2011
Finally today, cloud over the south of the South Island has cleared enough to reveal the extent of snow cover there.
Below are two visible satellite images. The first is for around 10:00am on the morning of Wednesday 10 August, some days before this extraordinary cold outbreak. The second is for around 10:00am on the morning of Friday 19 August. Nearly all of the white over Canterbury, Otago, Southland and Fiordland is snow. The imagery only shows the extent of the snow, not its depth.
Today’s weather map shows how this cold southerly is being produced by a combination of a HIGH or anticyclone in the Tasman Sea, and a LOW or depression between Canterbury and the Chatham Islands. For want of a better phrase, we could call this an eggbeater southerly.
Using the isobars as a proxy for the air flow we can see how the air moving onto New Zealand is coming from around 55 degrees south— and that’s the Southern Ocean, but NOT from Antarctica. This technique doesn’t show the actual trajectory of the air, but is a good-enough first approximation, and it is one of the ways that enthusiastic weather watchers use weather maps to glean more about the weather.
As this cold air moves northwards and encounters warmer surfaces it becomes more buoyant, bubbly and bouncy. It picks up some moisture and turns this into clouds, but, because these clouds are so buoyant, they produce a scattering of showers containing rain, hail, sleet and snow, rather than a canopy of widespread snow.
This southerly blast is serious enough to warrant a heavy snow warning for Southland and eastern Otago, and has already disrupted transport in the South and across Cook Strait.
The weather maps over the next few days show that this is just the beginning of several days of southerly and, IF THINGS EVOLVE THE WAY THEY ARE GOING, another Low may roll in from the east this weekend and give another, more compact, eggbeater southerly. See our 7-day weather maps.
NOTE that this week’s southerly LACKS THE MOISTURE of a large scale snow storm such as the one that hit Canterbury in 2006— in that event a low pressure system conveyed some moist air aloft from the tropics to the South Island which was undercut by very cold air from the south, and brought a period of rapidly dropping pressure to Canterbury, accumulating a canopy of widespread snow.
For more about southerly outbreaks read our Learning Centre Article on the Old Man Southerly .