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

Christmas Weather Daily Update

Ex-Tropical Cyclone Evan is playing a part in New Zealand’s weather this Christmas.

Now that it has passed across the seas to the north of the North Island, the heaviest of the rain over northern New Zealand is over. This will be the last update of the “Evan”  blog.

Merry Christmas.

Update: Tuesday 25 December

Heavy rain in Northland has eased. It’s still raining on and off there, though, as it is over about the northern half of the North Island. A Severe Weather Watch covers the possibility of a few further bands of heavy precipitation rotating around Evan.

Today, afternoon and evening showers and thunderstorms – mostly inland – are part of the weather picture. The action is expected to be from Taranaki through to Wellington and Wairarapa, and in Buller, Nelson, Marlborough and north Canterbury. Keep an eye on the Severe Thunderstorm Outlook; Severe Thunderstorm Watches or Warnings may follow.

Also today, it’s fine (or becoming fine) and not windy over about the southern half of the South Island. It’s muggy, so please remember the Cancer Society’s message about being out in the sun.

On Boxing Day, Evan is expected to lie off to the northwest of Taranaki. At this time it will maintain a flow of very warm moist air over the North Island, and rain or showers are expected in many places. Heavy falls are possible in eastern Bay of Plenty and in thundery showers over high ground. See the Severe Weather Outlook for more detail.

Meanwhile, a southerly change with rain is expected across the South Island on Wednesday. This should cool things off a bit – and lower the humidity.

Where to find important forecast information

Severe Weather Warnings and Severe Weather Watches:

http://metservice.com/national/warnings/severe-weather-warnings

http://metservice.com/national/warnings/severe-weather-watch

Three day rain forecast: http://metservice.com/maps-radar/rain-forecast/rain-forecast-3-day

Rain radar: http://metservice.com/maps-radar/rain-radar/all-new-zealand

Tropical Cyclone Evan: Special Weather Bulletin from Fiji Meteorological Service

Now that Evan is south of Viti Levu and moving away, this will be the last blog post relaying Fiji Meteorological Service’s Special Weather Bulletin.


Special Weather Bulletin Number THIRTY FOUR FOR FIJI ON SEVERE TROPICAL CYCLONE EVAN ISSUED FROM RSMC NADI at 9:31am on Tuesday the 18th of December 2012 TROPICAL CYCLONE WARNING

A GALE WARNING REMAINS IN FORCE FOR KADAVU, BEQA, VATULELE AND NEARBY SMALLER ISLANDS .
A STRONG WIND WARNING REMAINS IN FORCE FOR THE REST OF FIJI.
A DAMAGING HEAVY SWELL WARNING REMAINS IN FORCE FOR FIJI.

SEVERE TROPICAL CYCLONE EVAN CENTRE [970HPA] CATEGORY 3 WAS LOCATED NEAR 19 DECIMAL 7 SOUTH 177 DECIMAL 2 EAST OR ABOUT 220 KM SOUTH OF NADI OR ABOUT 130 KM SOUTHWEST OF KADAVU OR ABOUT 220 KM SOUTHWEST OF SUVA AT 10:00AM TODAY. THE CYCLONE IS MOVING SOUTH AT ABOUT 14 KM/HR.
CLOSE TO ITS CENTRE THE CYCLONE IS EXPECTED TO HAVE AVERAGE WINDS UP TO 150 KM/HR WITH MOMENTARY GUSTS TO 210 KM/HR.

ON THIS TRACK, THE CYCLONE IS EXPECTED TO BE LOCATED ABOUT 380 KM SOUTH OF NADI OR ABOUT 260 KM SOUTH-SOUTHWEST OF KADAVU OR ABOUT 360 KM SOUTH-SOUTHWEST OF SUVA AT 10:00PM TODAY AND 375 KM SOUTH-SOUTHWEST OF KADAVU OR 475 KM SOUTH-SOUTHWEST OF SUVA AT 10:00AM TOMORROW.

DESTRUCTIVE WINDS MAY BEGIN SEVERAL HOURS BEFORE THE CYCLONE CENTRE PASSES OVERHEAD OR NEARBY.

FOR KADAVU, BEQA, VATULELE AND NEARBY SMALLER ISLANDS:
DAMAGING GALE FORCE WINDS WITH AVERAGE SPEEDS UP TO 85 KM/HR AND MOMENTARY GUSTS TO 110 KM/HR. PERIODS OF HEAVY RAIN WITH SQUALLY THUNDERSTORMS WITH FLOODING OF LOW LYING AREAS. DAMAGING HEAVY SWELLS WITH SEA FLOODING OF LOW LYING COASTAL AREAS.

FOR THE REST OF FIJI:
STRONG WINDS WITH AVERAGE SPEEDS TO 55 KM/HR AND MOMENTARY GUSTS TO 85 KM/HR. PERIODS OF RAIN, HEAVY AT TIMES AND SQUALLY THUNDERSTORMS.
FLOODING INCLUDING SEA FLOODING OF LOW LYING COASTAL AREAS.

The following information is provided especially for the mariners:

HURRICANE FORCE WINDS AND PHENOMENAL SEAS NEAR THE CYCLONE CENTRE. STORM FORCE WINDS WITHIN 30 NAUTICAL MILES OF CENTRE AND VERY HIGH SEAS. GALE FORCE WINDS AND HIGH SEAS TO ABOUT 100 NAUTICAL MILES FROM CENTRE. ELSEWHERE, STRONG WINDS AND ROUGH TO VERY ROUGH SEAS. HEAVY SWELLS.

The next Special Weather Bulletin for Fiji on Severe Tropical Cyclone Evan will be issued at or around 12:30PM today.

MetService in the Tropics

Beach weather or a tropical cyclone?

 

If you’re going on holiday in the southwest Pacific, you should check out the MetService TV Pacific Weather update. But it isn’t always “beach weather” in the tropics. Severe weather in the southwest Pacific can have very significant consequences.

Image courtesy James Lunny

Above: Late afternoon cloud build-ups at Henderson Airfield, Guadalcanal, Solomon Islands.
Severe weather on the way?

 

South Pacific Guidance

 

Since 1 November 2009, RSMC (Regional Specialized Meteorological Centre) Wellington, of MetService, Lead Meteorologists have produced twice-daily broadscale guidance for expected severe weather over the next 5 days in the tropical southwest Pacific. This type of guidance is known as the South Pacific Guidance charts, or SPG.

 

South Pacific Guidance produced by Lead Forecaster at RSMC Wellington

Above: From July 2012, an example of South Pacific Guidance produced by a Lead Forecaster at RSMC Wellington (MetService). It highlighted areas and probabilities of severe weather developing 4 days out.

It’s important to note that we are not doing the job of forecasters in each country. This South Pacific Guidance (SPG) chart is another piece of information that they can use to write their forecasts, but should not be used in isolation.

 

Why do we do this?

 

SPG charts are one of the key products of the Severe Weather Forecasting and Disaster Risk Reduction Demonstration Project,  (SWFDDP). The project is overseen by the World Meteorological Organization (WMO), based in Geneva. What is it trying to do?

 

Goals for the South Pacific SWFDDP Project

Above: The goals for the project.

 

 

The project is not unique to our part of the world. It began in Africa in the mid-2000s, and there are now two separate African projects covering 16 countries in Southern Africa and 6 countries in Eastern Africa. RSMCs in Pretoria, La Reunion, Nairobi and Dar es Salaam lead these projects. There are future plans for similar projects in Southeast Asia and the Bay of Bengal.

The southwest Pacific project includes 9 Pacific Island Countries ( Solomon Islands, Vanuatu, Fiji, Tuvalu, Kiribati, Tonga, Samoa, Niue and the Cook Islands) and 2 Territories (American Samoa and Tokelau) .

 

What do we mean by Severe Weather?

 

“Severe weather” needs to be defined carefully and in a way that is worthwhile and meaningful for all participating countries. Differences in latitude and terrain across the SW Pacific make this a tricky task. The current thresholds for severe weather are shown below:

 

Above: Severe Weather Thresholds for the SWFDDP.

 

Large waves, strong winds and heavy rain can all exist outside of tropical cyclones, whereas tropical cyclones can include all of the above.
For example, large waves have had a significant impact on low lying atolls in Kiribati and Tuvalu, strong trade winds have contributed to damage in Rarotonga and sinking of a vessel in the Solomon Islands, and heavy rain caused major flooding in Fiji in March 2012; all without the presence of a tropical cyclone.

As shown in the goals above, a major aim of the project is to raise awareness and capabilities in dealing with severe weather outside of tropical cyclones.

The first image in this post showed cloud build-ups about the higher ground of Guadalcanal. These may produce rainfall in excess of 100mm in 24 hours, but if they cover a very small area are not currently included in the SWFDDP. The project is really focussed on broadscale weather, which can be predicted with some accuracy days ahead of time.

 

What else does MetService do in the tropics?

 

The Lead Forecaster draws the “SPG” charts we saw above. Other duties of the Lead Forecaster in the tropics include regular discussions with meteorologists in RSMC Nadi regarding future tropical cyclone formation, production of bulletins for the New Zealand Ministry of Foreign Affairs and Trade detailing future tropical cyclone potential, and drawing streamline analyses.

 

Above: South of 25S, the familiar isobars are drawn. To the north, the teal-coloured lines are streamlines which show the windflow. The double black lines north of the equator mark the ITCZ, a major area of wind convergence. For more on convergence, see http://blog.metservice.com/2012/10/convergence-lines/

 

Streamline analyses look quite different to the regular “weather maps” shown in New Zealand newspapers, the MetService website, and on TV.

This type of chart is drawn because as you get further into the tropics (especially between the latitudes of 15S and 15N) drawing of isobars becomes increasingly difficult, ultimately impossible, and the wind no longer behaves with the same relation to isobars as it does in temperate regions.

 

In producing SPG charts, lead forecasters have to be aware of what is happening right now (using satellite data and surface observations and combining these into a streamline analysis) and what is expected to happen during the next 5 days. This requires combining meteorological understanding with numerical weather prediction (NWP) products from supercomputers around the world. In particular, products known as ‘ensemble prediction systems’ are used.

 

Ensemble products provide a way of dealing with the uncertainty in the atmosphere. The atmosphere is large and extremely complicated, hence it’s impossible to perfectly know everything about it at any given moment. But the problem is that the atmosphere is very susceptible to very minor changes in what is happening right now. These minor changes are proven to have a huge impact in the future. This is known as Chaos Theory and has been popularised with the well-known “butterfly effect”.

 

Global Ensemble Forecast System forecast tracks for SANDY (courtesy NOAA)

Above: Global Ensemble Forecast System forecast tracks for Hurricane SANDY (courtesy NOAA). Ensemble forecasting aims to find out the impacts of initial uncertainties in the atmosphere on the future conditions of the atmosphere – in this case the position of a hurricane.

 

Understanding, and interpreting, ensemble prediction systems is a delicate operation, but it is entirely essential in modern forecasting. Without it, forecasters are severely restricted in their  outlooks.

MetService also runs a website called MetConnect Pacific. This is a portal for forecasters involved in the SWFDDP to access information, observations and the South Pacific Guidance charts drawn by MetService Lead Forecasters.

 

Climate Change Development Fund

Thanks to financial aid from the Climate Change Development Fund administered by the New Zealand Ministry for the Environment, MetService has been able to conduct in-country training between May 2012 and November 2012 for all nine participating Pacific Island Countries. On each trip, two meteorologists from MetService have provided training, along with meteorologists from the National Weather Service in Hawaii for the trips to Samoa and Fiji.

What does this project, which focusses on the weather over the next 5 days, have to do with climate change, which looks into seasonal, interannual to decadal changes timescales?

We believe that dealing with extreme weather is the first vital step towards managing the risks of extreme events and disasters to advance  climate change adaptation at the community level. According to many climate scientists, climate extremes (extreme weather or climate events) such as substantial warming in temperature extremes, heavy precipitation or the proportion of total rainfall from heavy falls, average tropical cyclone maximum wind speed are likely to become more common in the future as the earth warms up. The recent Hurricane Sandy that struck the eastern United States could  be an example of this.

 

MODIS image of Hurricane SANDY on 28th October 2012

MODIS image of Hurricane SANDY on 28th October 2012 (image courtesy NASA). According to many climate scientists, average tropical cyclone maximum wind speeds are likely to increase as the earth warms up.

 

What we did in our training during 2012

The training has partly focussed on the practical use of NWP products, especially from ensemble prediction systems. We’ve also covered reporting of severe weather events (which is required every 4 months), case studies (looking at a past weather event and trying to find out what happened and why) and verification (go here or here to see how we do it in NZ).

In all countries, either a half-day or full-day workshop devoted to Disaster Risk Reduction has been held. This has been a practical, interactive day with representation from the national meteorological service and many other agencies such as (but not restricted to) the National Disaster Management Offices, Civil Aviation, Maritime, Fire Service,  Health departments and NGOs such as the Red Cross.

On this day we’ve run a mock-up severe weather exercise and asked participants to play their typical roles in responding to the expected weather impacts and formulating a plan of action.

 

SWFDDP Workshop in Apia, May 2012

Above: A mock-up severe weather exercise in Apia, May 2012 as part of the SWFDDP training.

 

Below are some images of the training in various countries over the last seven months:

 

 

SWFDDP Workshop in Niue, November 2012

Above: Group photo of facilitators and participants at the Niue SWFDDP Workshop, November 2012.

 

Training Met staff in Tuvalu

Above: Consulting Meteorologist Mark Schwarz discusses an exercise with staff at the Tuvalu Meteorological Service.

 

SWFDDP DRR Workshop in Vanuatu, 2012

Above: Running a practical exercise at the SWFDDP DRR Workshop in Vanuatu, 2012.

 

Met exercise in Cook Islands MetService

Above: Manager RSMC Wellington, Steve Ready runs through a practical exercise on MetConnect Pacific with CIMS Directors Arona Ngari and staff.

 

 

Met staff in the Solomon Islands in a practical exercise

MetService WMO Manager James Lunny facilitates a practical exercise with staff at the Solomon Islands MetService as part of the SWFDDP training 2012.

 

 

And finally….we’ve been lucky to visit some unusual places and meet friendly people (from top to bottom: Funafuti lagoon, Tuvalu; Ministry football team, Solomon Islands; Schoolkids, Kiribati).

 

 

Tropical cyclones: extra-tropical transition

On average, about nine tropical cyclones form in the South Pacific tropics between November and April. Three or four of these leave the tropics and nearly all of them undergo a marked transformation to a mid-latitude depression – a completely different weather system – before they reach New Zealand.

For a while after this extra-tropical transition, the system may be referred to as “low formerly cyclone so-and-so”. At this stage of its life, the system may still have considerable potential for severe weather, despite its name change – for example, the low that was formerly tropical cyclone Bola, in March 1988.

So, how does this extra-tropical transition take place?
When a well-developed tropical cyclone reaches its peak in the heart of the tropics, it has an eye. The eye is often fairly cloud-free, nearly circular, and surrounded by a ring of very active thunderstorms. In the early and middle parts of their lives, tropical cyclones stand up quite vertically in the atmosphere, like large columns.

Infra-red satellite image for 10pm 02 February 2011 (New Zealand Daylight Time); this image displays colour-coded cloud top temperature. Tropical cyclone Yasi is just about to make landfall on the Cassowary Coast of northern Queensland.

Besides encountering cooler seas, tropical cyclones heading towards New Zealand eventually come under the influence of the westerlies. The westerlies of the mid-latitudes increase in strength with height, a phenomenon known as vertical wind shear. This shear almost literally chops off the upper part of the tropical cyclone and sweeps it away, not unlike a woodcutter chopping off the upper part of a coconut tree to leave a section just above the ground (except it’s a much more gradual and subtle process). Along with the lower sea temperatures of the mid-latitudes, this destroys the positive feedback processes within the cyclone. What remains is the former cyclone’s low-level circulation, which may get carried off in the westerlies or become the focus of further development if conditions are right. Either way, tropical cyclones approaching the New Zealand area undergo drastic changes of structure and appearance as they undergo this extra-tropical transition.

Let’s take a look at the extra-tropical transition of Wilma, which affected New Zealand in late January.

While in the tropics, Wilma had a similar look to Yasi, but was a less intense tropical cyclone. In the above infra-red satellite image (for 2pm Wednesday 26 January 2011 New Zealand Daylight Time), we can see a well-developed eye and the ring of thunderstorms surrounding it.

24 hours later (2pm Wednesday 26 January 2011 New Zealand Daylight Time), at the margin of the tropics, in response to increasing wind shear, the eye of Wilma became obscured amongst a broader mass of colder topped clouds.

And 24 hours further on, Wilma’s cloud structure bore little resemblance to that of 26 January. The eye and eye wall are no longer identifiable.


Finally, we take a look at the visible image for the same time as the last infra-red image above. This shows the more active, colder cloud tops (bright white) are now east of Wilma’s low-level centre. Therefore, Wilma is on the verge of losing its tropical cyclone status and becoming classified as a depression. Like all other cyclones before Wilma, Wilma lost its tropical cyclone status before reaching New Zealand.

The Structure of Lows – part II

In my previous blog post I pointed out that tropical lows and cyclones don’t have fronts like the lows we’re used to around NZ, but rather, a core of warm air near the centre. I’d like to follow up by further contrasting tropical and mid-latitude lows, and looking a bit more closely at tropical cyclones and how they can affect our weather in New Zealand.

When tropical lows fully develop into cyclones they become the most damaging of tropical weather features. There are three main reasons for this:

  1. damaging winds at, or very close to the Earth’s surface
  2. intense rainfall leading to flooding
  3. near the coasts, the sea can be driven inland as a storm surge.

In an earlier blog post we saw that, on a weather map, the closeness of the isobars is related to the strength of the wind. There is a latitude effect too: for a given isobar spacing, if you were nearer the equator the wind would be stronger than if you were nearer the poles. This is an important point, because a tropical cyclone has very closely packed isobars near its centre and is relatively close to the equator.

Also, as you get nearer the equator the wind blows more across isobars, in contrast to the mid-latitudes where the wind blows approximately parallel with the isobars (like slot cars on a track). So in tropical cyclones there is a strong inflow into the centre as in the diagram below.

Inflow into a tropical cyclone (southern hemisphere). The innermost circle represents the "eye".

Inflow into a tropical cyclone (southern hemisphere). The innermost circle represents the "eye".

I don’t want to overcomplicate things, but there is a further effect based on the curvature of the isobars. If you’re interested send me a comment and I’ll explain it.

 

At the innermost circle in the diagram, the air is racing so fast that it doesn’t make it all the way into the centre. Inside this circle is the eye and a phenomenal contrast between the wind and rain at the outer wall and the relative calm of the interior.

The rainfall is usually at its most intense just outside the eye, and the cloud is very deep with extremely cold tops – see for example the infra-red satellite pictures described in the previous post. I remember being in Darwin many years ago when Cyclone Rachel was over the Timor Sea. We were some distance away from the centre of Rachel, but the cloud overhead was so dense that we needed lights on during the daytime if we wanted to read. We set a 24hr rainfall record for Darwin of 290mm, although the wind wasn’t particularly strong (compared with what I’m used to in Wellington).

Tropical cyclones sometimes affect New Zealand during our warmer months – a couple of examples were mentioned here. You may well recall some more recent examples, e.g. Drena and Fergus. After they form in the tropics they typically move quite slowly and erratically, then usually curve southeastwards into the mid-latitudes.

Once they move over colder water in mid-latitudes they undergo a transition in structure from the tropical to the extra-tropical (meaning outside the tropics, like “extraordinary” means “outside the ordinary”).

At this stage the cyclone tends to expand by drawing in colder contrasting air and evolving into a more typical mid-latitude storm complete with warm and cold fronts. On a few occasions (e.g. the Wahine Storm in 1968) they interact with a vigorous mid-latitude low to form a new system, complete with active fronts. Either way, the new storm isn’t a tropical cyclone anymore in terms of structure but can still be very damaging as a major mid-latitude depression.