Tropical cyclone season 2014

For the South West Pacific, tropical cyclone season is said to begin in November and continue right through to April the following year. However, the weather doesn’t follow a rigid calendar and tropical cyclones have been known to form as early as October and as late as the month of June.

Around the globe, the monitoring and forecasting of tropical systems is looked after by a Regional Specialized Meteorological Centre (RSMC) or a Tropical Cyclone Warning Centre (TCWC), depending on the location of the cyclone. TCWC Wellington, based at MetService, looks after the area that extends from 160E to 120W and between 25S and 40S. Although it is very rare for any tropical cyclones to form in TCWC Wellington’s area of responsibility, fully-fledged tropical cyclones do arrive from the Brisbane or Nadi areas and they may retain their cyclone status until 30S. Sometimes an ex-tropical cyclone will approach New Zealand and Severe Weather Watches and Warnings need to be issued. Even if land areas are not affected, warnings are issued for vessels over the open sea.

RSMC locations and areas of responsibility.

RSMC locations and areas of responsibility.

Our neighbouring TCWCs are Brisbane, run by the Australian Bureau of Meteorology, and RSMC Nadi, run by the Fiji Meteorological Service.

Cyclone categories

Cyclones are classified by a Category (Cat) system, numbering Cat 1 to Cat 5 depending on the strength of the winds near the centre of the system:

Cyclone Category

Average Wind Speed (km/h) Typical Strongest gusts(km/h) Central pressure(hPa)

1

Gale 63-88 Less than 125 Greater than 985

2

Storm 89-117 125-164 985-970

3

Hurricane 118-159 165-224 970-955

4

Major Hurricane 160-199 225-279 955-930

5

Major Hurricane Greater than 200 Greater than 279 Less than 930

Forecast for this season

Every year MetService works alongside NIWA as well as national meteorological services from other Pacific nations to produce a Tropical Cyclone Outlook.

Although this Outlook cannot say exactly when or where tropical cyclones will form, it is used as a guide to the expected activity over the South West Pacific. The outlook for 2013-2014 is for a near-average number of tropical cyclones (around nine) to form in the area from the Coral Sea to French Polynesia. Of these, four could potentially reach category 3 or 4. Under the forecast conditions for this season, a category 5 cyclone is unlikely but cannot be discounted. As with all forecasts, the best advice to stay up-to-date with the latest information.

The Australian Bureau of Meteorology (BOM) also produces forecasts for the areas to the north and west of Australia. You can find their latest forecast here.

How does this compare to normal?

On average there are nine tropical cyclones in a season but the actual number can vary greatly. The most active season in the SW Pacific since 1969 was the 1997-98 season when 17 tropical cyclones were recorded:

Number and intensity of Cyclones in the South Pacific

Number and intensity of Cyclones in the South Pacific

The Bomb

It’s been a while since a rapidly-deepening low passed close to, or over, New Zealand. I thought it might be interesting to take a quick look at why the “bomb” low of Saturday 03 March 2012 deepened so quickly and why the winds around it affected the areas they did.

First of all, here is a series of weather maps covering the period 1pm Friday 02 March to 1am Sunday 04 March.

Mean sea level analyses from 1pm Friday 02 March 2012 to 1am Sunday 03 March 2012, at six-hourly intervals.

A Bit of Background

Large-scale features on the weather map – that is, those systems which influence the day-to-day weather (highs, lows, fronts) – are driven by processes in the middle and upper parts of the troposphere. Lows become deep and anticyclones become intense when there is strong positive feedback between these processes.

In the case of lows, when the intensification (a change in central pressure) exceeds more than a certain amount over a given time, they are defined as “bombs”. Actually, it’s not quite that simple: if you really want to know more, see “Technical Stuff” at the end of this article.

Between 1am Friday 02 March and 1am Saturday 03 March, the central pressure of this particular low fell from 1002 hPa to 975 hPa: it qualifies.

The term “weather bomb” has come into popular usage in New Zealand to describe dramatic and/or destructive weather events – but very seldom is a “bomb” low the cause. “Bomb” lows aren’t all that common in the New Zealand area.

The Low of Saturday 03 March

Below is the weather map for 1am Saturday 03 March, superimposed on a satellite image. At this stage the low was still west of Taranaki, deepening rapidly and heading more or less straight for Palmerston North (but it never got there). I’ve drawn in some mauve arrowed lines to indicate the axis of strongest winds in the upper troposphere, known as the jet. The relative locations of the low and the jet, and the shape of the jet, strongly favour further deepening of the low and its movement towards the eastsoutheast – which is what happened. Note: this is a simplified explanation ahead of a bit more case work.

Mean sea level analysis 1am Saturday 03 March 2012, together with infra-red satellite image. Satellite image courtesy Japan Meteorological Agency.

A few hours later, at 6am Saturday 03 March, the low had a central pressure of about 971hPa and was not far south of Patea. Below is a portion of the working chart for 6am, drawn by one of MetService’s Severe Weather team. There’s a lot of isobars around the low – and over Taranaki, Wellington and the Marlborough Sounds in particular. The closer together the isobars, the stronger the winds. Also note that there’s a front drawn curling around the low. Tucked in on the southern and western side of this front is a zone of very strong winds, shown by the blue arrow. It’s mostly this zone which did the damage as it moved across the southern part of the North Island.

Hand-drawn mean sea level analysis for 6am Saturday 03 March 2012. This is an internal working chart, drawn by a member of MetService's Severe Weather team. Image copyright Meteorological Service of New Zealand Limited 2012.

Some Interesting Observations

At Hawera, the wind increased quickly during the early morning hours of Saturday 03 March.

Time Wind direction Mean speed (km/h) Highest gust last hour (km/h)
Midnight Fri-02-Mar Northnortheast 35 57
1am Sat-03-Mar North 26 44
2am Sat-03-Mar Northnortheast 24 43
3am Sat-03-Mar Northnortheast 53 83
4am Sat-03-Mar Northnorthwest 53 87
5am Sat-03-Mar Northwest 85 122

No observations were received between 6am and 11am because power to the reporting site was lost.

At Wanganui Airport, the temperature climbed steadily from 12.7 C at midnight Friday 02 March to 17.5 C at 5am Saturday 03 March. (The temperature behaved similarly at Hawera a few hours earlier). This happened because the relatively warm moist air flowing around the northern side of the approaching low was warmed and dried – the Foehn effect - as it passed across the high country to the north of Wanganui.

At Ohakea and Palmerston North, the winds were reasonably strong easterlies for some hours before and after dawn, but blew from the northwest for a while around dawn. Two switch-arounds (not quite 180 degrees) of steady to strong-ish winds in a short time is remarkable.

Between 4am and 3pm Saturday 03 March, the south to southeasterly wind at Brothers Island had a mean speed of 108 km/h. During this time, the highest gust was 142 km/h.

Crossing the Country

I mentioned earlier that at 6am the low was not far south of Patea and heading for Palmerston North but never got there. This is because it didn’t physically cross the North Island. Nevertheless, a graph of mean sea level pressure at Palmerston North (below) for Saturday 03 March 2012 shows that the pressure certainly reached a minimum around dawn before rising steeply again from mid morning.

Mean sea level pressure at Palmerston North.

While lows have distinct structures, they’re best not thought of as rotating solid bodies of air – because they’re not. Rather they are the manifestation, at the Earth’s surface, of processes which have produced a local minimum of pressure. Looking back to the working chart above, we see that there are two lows at 6am: one south of Patea, and another just east of Hawke’s Bay. The low south of Patea came ashore east of Wanganui and then decayed, while the “new” low continued to deepen and move away to the east. This decay/development happens because the “driving” atmospheric processes are largely above the Earth’s surface and moving with the general flow: they left the Patea low behind and powered the development of the “new” low east of the North Island.

Technical Stuff

Finally, the definition of a “bomb” low is technical and not whimsical. As far as I know, the first mention of the term “bomb” was in a paper by two distinguished meteorological researchers, Fred Sanders and John Gyakum, titled “Synoptic-Dynamic Climatology of the “Bomb”", published in the October 1980 issue of Weather and Forecasting, a journal of the American Meteorological Society. Because of their destructive potential, rapidly deepening lows have been the subject of many a research paper.

The Big Chill

2:30pm Sunday 14 August 2011

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.

Infra-red satellite image for midday Sunday 14 August 2011. Image courtesy Japan Meteorological Agency.

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.

Air temperatures at Invercargill Airport, Nugget Point (Otago coast) and Timaru Airport for the period 1:00pm Saturday 13 August 2011 to 1:00pm Sunday 14 August 2011.

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.

"Upper air" temperatures obtained from balloon soundings at Invercargill Airport at midnight Fri-12-Aug, midday Sat-13-Aug, midnight Sat-13-Aug and midday Sun-14-Aug.

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.

Snow in Clifford Road, Johnsonville (altitude approximately 200 metres) early evening, Sunday 14 August 2011. Snow depth 3 to 5 cm.

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.

Wellington radar image, 5:05pm Sunday 14 August 2011.

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.

Plot of (a) forecast temperature (colours) at the 500 hPa level (roughly 18,000 ft, or about halfway up the troposphere), and (b) forecast wind speed (black lines) at the 250 hPa level (near the top of the troposphere), for midday Monday 15 August 2011. Data courtesy European Centre for Medium-Range Weather Forecasting.

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)
Whenuapai -29.3 -29.9
Paraparaumu -38.9 -39.1
Invercargill -34.4 -35.1

 

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.

Freezing levels at Whenuapai, Paraparaumu and Invercargill from midnight Friday 12 August 2011 to midday Monday 15 August 2011, derived from radiosonde flights.

9:30am Tuesday 16 August 2011

Here’s a few photos from the Wellington snow of June 1976.

Wellington snow of June 1976: looking approximately southwest from the MetService building.

Wellington snow of June 1976: Karori Road, looking southwest.

Wellington snow, June 1976: MetService building, from Salamanca Road. Look at those cars ...

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.

Dew point temperature at Hokitika Airport, 10am Sunday 14 August 2011 to 10am Tuesday 16 August 2011.

2:30pm Tuesday 16 August 2011

This event has been characterised by many places having low daytime (maximum) temperatures.

Location Maximum temperature on
Mon-15-Aug 2011
Lowest daily maximum temperature on record Month / year occurred in Record starts Monday’s max temperature is the lowest since …
Auckland Airport
8.1 8.1 15 August 2011 1966 New record
New Plymouth Airport
8.8 6.0 July 1951 1945 25 July 2011 and 12 July 1951
Napier (Nelson Park)
7.8 3.6 June 1969 1940 June 2006
Paraparaumu Airport
6.1 5.4 August 2004 1972 July 1986
Kelburn, Wellington 5.1 4.5 August 1938 1931 25 July 2011 and 17 July 1995
Christchurch Airport
4.6 1.7 August 1992 1954 July 2007


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     -5.1 C  


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.

"Backward trajectories" of the air at 500 metres above mean sea level arriving at Auckland midday Monday 15 August 2011. The red line traces the path of the air over the period from midday Friday 12 August to midday Monday 15 August. Triangles denote the location of the air mass at 12-hourly intervals. Data courtesy NOAA Air Resources Laboratory.

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.

The scalloped areas delineate where significant snowfall is known (or believed) to have occurred. The dashed areas delineate where lesser amounts of snow have been observed and where little or no snow is believed to by lying. The period is from the beginning of the event on Sunday 14 August 2011 to 2:00pm Tuesday 16 August 2011.

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.

Invercargill tephigram for midday Wednesday 17 August 2011.

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.

Before: MODIS-Terra visible image for about 10:00am on Wednesday 10 August 2011. Data courtesy NASA/GSFC, Rapid Response.


After: MODIS-Terra visible image for about 10:00am on Friday 19 August 2011. Data courtesy NASA/GSFC, Rapid Response.

A Winter Storm

Since Wednesday 6 July, stormy westerly conditions have affected New Zealand. In this blog, we’ll look at why.

The “Long Waves”

Below is the mean sea level analysis – the weather map – for 6am Sunday 10 July. In between big highs over the mid South Pacific and south of western Australia is a really large trough; it’s the area shaded light blue. The weather map has looked like this, more or less, since Wednesday 6 July: that is, the big features on it aren’t moving much.

MetService mean sea level analysis for 6am Sunday 10 July 2011.

There’s good reasons why these big features aren’t moving much. They reflect the so-called “long waves” in the troposphere (the troposphere is that part of the Earth’s atmosphere in which the weather occurs), which are stationary at the moment. Below is an image made from a Fourier analysis of the wave pattern in the Southern Hemisphere at midnight Saturday 09 July. There’s a trough (blue) in this wave pattern more or less in the same place as the one shaded light blue on the weather map above. And either side of this trough, there are ridges (pink) in about the same place as the big highs on the weather map above. For more about this, see my blog post on Wave Three.

The "long" and "medium" waves about half-way up the troposphere at midnight Saturday 09 July 2011.

Polar Air

Below is a plot of where the air arriving on New Zealand’s west coast at midnight Saturday 09 July came from. The air is from the Antarctic.

"Backward trajectories" of the air at mean sea level arriving at Cape Reinga, Farewell Spit and Secretary Island at midnight Saturday 09 July 2011. Each red line traces the path of the air over the period of a week. That is, the air arriving on New Zealand's west coast at midnight Saturday 09 July left the Antarctic about a week previously. Data courtesy NOAA Air Resources Laboratory.

On its way to New Zealand, this air travelled over a long stretch of ocean. It will have been colder than the surface of the sea it passed over, and will therefore have taken up heat from the sea surface. As heat transfers from the sea surface to the air immediately above it, “blobs” of air become warmer than their surroundings and rise upwards. This process is known as convection (see Chris Webster’s blog post about predictability and popcorn). If convection continues for long enough, showers and/or thunderstorms are the result. In the satellite picture below, more or less all of the clouds over the Tasman Sea, New Zealand and the seas to the south of the country are “blobs” of air which is rising (or has recently risen) convectively.

MTSAT-1R infra-red satellite image for midnight Saturday 09 July 2011. Image courtesy Japan Meteorological Agency.

The Jet

A major flood of air out of the Antarctic region, like this, has other consequences. The northern boundary of the cold air pushes against the warmer air further north. Thus, the north-south temperature contrast increases and simultaneously the strength of the westerly winds increases – not just at the Earth’s surface, but throughout the depth of the troposphere. (How this works might be the subject of a future blog post). This is at the heart of why many places have been windy over the last few days.

The axis of the strongest winds, in the mid- to upper troposphere, is known as the jet stream. In the plot below (for midnight Saturday 09 July), the polar jet has two branches (arrows in black): one curving across the south Tasman Sea and over the South Island, and the other crossing the south of the North Island. Over central New Zealand, wind speeds above 10,000ft were generally 80 kt (150 km/hr) or more.

Wind speed at 500 hPa (approximately 18,000 ft) in the New Zealand region at midnight Saturday 09 July 2011. Warmer colours are stronger winds. Data courtesy European Centre for Medium-range Weather Forecasting.

Thunderstorms and Tornadoes

Air ascending into convective clouds (showers and thunderstorms) comes down again. When a convective cloud collapses, the resulting downdraft hits the Earth’s surface and spreads out, much like water does when tipped from a bucket onto the ground. When the showers and thunderstorms are themselves fast-moving (on the afternoon of Saturday 09 July, storm motions on the Kapiti Coast were 70 to 90 km/hr), the winds near the Earth’s surface can become very strong when downdrafts occur: this is very likely to be the cause of some of the wind damage on the Kapiti Coast on the afternoon of Saturday 09 July. And this is one of the reasons why the various Severe Thunderstorm Outlooks, Watches and Warnings issued over the last few days have included the mention of damaging wind gusts.

Imagery (see below) from the Wellington radar for 4:00pm Saturday 09 July shows a line of thunderstorms extending across Cook Strait onto the Kapiti Coast. The tornado is very likely to have been associated with the strong thunderstorm shown just east of Waikanae at 4:00pm; eye witness reports suggest that the tornado crossed State Highway 1 around 3:55pm. The tornado’s genesis remains unclear: it may be that a low-level vortex was “spun off” the northern end of Kapiti Island just at the time that this thunderstorm passed by, and the strong ascending motion in the thunderstorm developed it into a tornado between there and landfall on the Kapiti Coast. Once again, it is remarkable that there was no loss of life.

Reflectivity image from the Wellington radar, 4pm Saturday 09 July 2011. Colours represent how strongly precipitation bounces the radar signal back to the radar.

Snow

The Southern Alps are a significant barrier: in westerly airstreams, generally only a moderate amount of precipitation falls any distance east of the Divide. How much precipitation falls east of the Divide, and how far east of the Divide it reaches, depends on a number of factors. Not the least of these is the Foehn Effect. Overnight Saturday 09 July and on the morning of Sunday 10 July, a reasonable amount of snow fell east of the Divide, in parts of Otago, to below 500 metres, in a northwesterly airstream. This is an uncommon occurrence, and reflects how deeply cold and showery the air passing across Otago was at the time.

Large Sea Waves

Since about the middle of the first week of July, sea waves arriving on New Zealand’s western coasts have been notably large. In some places, they have probably attained heights observed only once every year or two – and will remain high until late in the week ending Fri-15-Jul. The weather map below, from about the time large waves began arriving, shows why:

  • The fetch – that is, the expanse of ocean over which waves arriving on New Zealand’s western coasts have been generated (pink arrow) – is very long
  • The waves are still growing as they reach New Zealand’s western coasts, because the winds across New Zealand are themselves strong.

MetService mean sea level analysis for midnight Thursday 07 July 2011. The red arrow shows the approximate path sea waves arriving on New Zealand's western coasts have travelled.

Storm Surge

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.

The Storm of Late May 2010

What a week it has been weatherwise. 

Today MetService has issued a severe weather warning for heavy snow over parts of South Island. Looking back over the past week, it is unusual for a single weather event to result in severe weather warnings for all of widespread heavy rain, severe gales and heavy snow. But this event has been far from typical in its severity and longevity. 

The media have extensively reported the full impacts of the storm, so I won’t go into that here. Instead I’ll look a little more deeply into the meteorology of what’s been going on, starting from day one. 

Here is an animation of the surface weather maps starting midnight on Friday 21 May – frames are every six hours throughout the event, and the isobars are at 2 hPa spacing: 

Loop of isobaric weather maps from midnight 21 May to midday 27 May 2010. Data courtesy US Global Forecast System (GFS)

There is a lot going on in these surface maps. To give a full account of what’s been happening I should really include a discussion about the weather in the upper air – but to keep this post short I’ll concentrate just on the surface here. Points to note from the maps are: 

  • at the start of the loop, an old Low over the south Tasman Sea weakens
  • a new Low forms off the south Queensland coast, then moves southeastwards towards New Zealand and deepens
  • a second Low centre forms west of North Island, then a third forms over Bay of Plenty
  • the third Low takes a very unusual track, moving south then southwestwards towards Canterbury
  • a High moves onto the south Tasman Sea and stays there for the rest of the period
  • a very strong east to southeasterly airstream develops over the bottom half of South Island
  • the Queensland Low eventually moves eastwards over North Island

The centre of the Queensland Low took a rather convoluted track as it approached us: 

Track of the low that formed on 21 May 2010

I guess you could say it “looped the loop” near longitude 170E. This was caused by the formation of the secondary and tertiary Low centres that shifted the “centre of gravity” of the broader system away from the originating Low. 

If you look again closely at the loop of weather maps above, you’ll notice that just after halfway through, the second and third Lows are dumb-belling cyclonically around each other over New Zealand. This motion occurs when a multi-centred Low develops – we sometimes call these systems complex lows. The combined motion plus the spiralling bands of precipitation were well captured by our current radar network. 

The following animation shows where our weather radar is detecting rainfall-sized drops, and the shape of the rain areas illustrates the position and movement of the Lows. Pictures are seven minutes apart and cover the period from the evening of 25 May to the early hours of 26 May. Light falls are yellow and heavier falls blue

Click to view animation. Note – the animation is a large gif file: 2.9MB 

The radar pictures also show the change in texture from north to south, with the northern precipitation looking more speckly (showery) and the southern precipitation more uniform (rainy). 

As I mentioned at the beginning, this storm is not over yet, with snow still to come in the south. You can keep up to date with the latest weather by continuing to check out metservice.com.