Better detection of low cloud and fog
Fog and low cloud – which can be particularly disruptive to aviation – are difficult to detect in the overnight and early morning hours, the time when airlines are planning the coming day’s flights. In the overnight and early morning hours, there are at least four polar-orbiting satellite passes over the New Zealand area.
Better analysis of severe weather
Polar-orbiting satellite passes complement weather radar, providing high-resolution information about incoming weather features beyond the footprint of the radar network and which are too small to resolve using imagery from geostationary satellites.
The two images immediately below demonstrate the difference in resolution, of visible imagery, between two of the weather satellites that view New Zealand. As explained later in this article, data from each type of satellite has its advantages.
Using the new receiver, MetService currently gathers data from the Terra, Aqua and NOAA series of satellites. Their passes across the New Zealand area are generally clustered around early morning and late afternoon. Data from other polar-orbiting weather satellites is a work in progress.
Polar-orbiting satellites continuously orbit the Earth from pole to pole at low altitude (about 800 km). Because they are much closer to the Earth than geostationary satellites (about 36,000 km), they yield data at much higher resolution – that is, information from them is much more detailed.
On the other hand, information from any given sun-synchronous polar-orbiting satellite is available much less frequently than from a geostationary satellite and covers only a limited area. This is because each polar-orbiting satellite crosses the New Zealand area twice per day and is too close to the Earth’s surface to take a “full disk” view.
To receive data from a polar-orbiting satellite, the receiver must point at the satellite as it moves across the sky. This requires a precise knowledge of the orbit and being able to finely control the movement of the receiver. It’s a bit more complicated than setting up a satellite dish to receive a signal from a television satellite.
Final assembly of the satellite antenna, receiver and dome was carried out by MetService and SeaSpace engineers in the MetService head office car park over a period of three days.
The lift of equipment onto the roof of the MetService building required an early morning start and very little wind. Things went extremely well and the HeliPro helicopter successfully lowered the 364 kg load accurately onto the roof platform.
For about the last month, anticyclones have dominated the New Zealand area. Many places have had little or no rainfall since early February.
The map below shows the average mean sea level pressure over the New Zealand area over the last month or so. There’s no doubt about the pressure being high and not changing very much. Because this map shows averaged pressures, we don’t see the few troughs that have passed across the New Zealand area in the last 28 or so days.
There are reasons why the weather gets into “regimes” like this. It has to do with what meteorologists call the “long waves”. If you’ve been reading the MetService blog for any length of time, you might notice that this article is very similar to one published in September 2010 titled “Wave Three“.
The map below is one way of depicting what the average weather pattern about half-way up the troposphere (the troposphere is that part of the atmosphere in which the weather occurs) looked like over the last month or so. In this pattern, there’s nothing that looks like a high, or low, or front. But there is a bunch of wavy black lines … which look kind of regular, but not quite. The upper ridges are marked with red dashed curves. Note that there’s been one over the New Zealand area for most of the last month.
Now what if those waves could be separated out into longer ones and shorter ones, so we could see what was going on “under the bonnet”? There happens to be a way of doing exactly this; it’s called Fourier analysis.
The picture below is made from a Fourier analysis of the pattern at midnight Saturday 23 February 2013. It reveals that, in New Zealand’s latitudes, the pattern is dominated by a wave which has one ridge (pink) and one trough (blue) wrapped around the hemisphere. Most important to us is the big area of pink shading – the ridge – near New Zealand. Further south, in the Southern Ocean, the pattern has three waves with a remarkably strong ridge south of South America.
The picture below shows a 12-day history and 5-day expected movement of these longer waves. While shorter waves (some of them corresponding to weather features) are travelling steadily eastwards, the underlying pattern in the New Zealand area remains ridged.
If you were looking at radar imagery overnight Thursday 21 February 2013 or this morning (Friday 22 February 2013), you could be forgiven for thinking that there was quite a lot of light precipitation over the northern half of the North Island and west of Auckland.
Except for parts of Gisborne and Hawkes Bay, we know that there was almost no precipitation over the North Island in the period covered by the radar imagery above.
So what was all that yellow radar echo over parts of the central North Island and around Waikato and Auckland?
We’re not completely sure, but we strongly suspect the echo is swarms of insects. To show up in radar imagery like this, they must have some of the characteristics of precipitation particles: similar size, fairly numerous, enough water content (or perhaps coating).
The overnight period Thursday 21 February / Friday 22 February 2013 isn’t the only time lately radar echoes like this have been observed over the North Island. They’ve shown up, to a greater or lesser extent, most evenings in February. The weather over the North Island has been settled — and very dry — during this time.
Interestingly, there is quite a lot of literature on the subject of radar entomology.
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.
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.
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:
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.
This tragic event has been widely reported as a tornado. On Friday 7 December, as I wrote this blog, there certainly was an absence of evidence of a tornado. But absence of evidence is not evidence of absence. Over the last few days it has become increasingly clear — from inspecting aerial views of the damage swath, and from correspondence from people in the area at the time — that this was much more likely to have been a localised wind storm known as a microburst. Corresponding amendments have been made below, and post-event analysis will clarify this further.
Few weather events are as dramatic, dangerous or challenging to predict as localised wind storms, including tornadoes.
Around midday on Thursday 6 December an active trough line passed slowly through Auckland. A thunderstorm in this line produced a tornadolocalised wind stormthat touched down near Hobsonville, tragically killing three people.
Could this tornadolocalised wind storm have been forecast?
No. Technology to forecast the very small small and short-lived wind storms like tornadoes or microbursts – as we see them in New Zealand – does not yet exist.
Expert meteorologists are able to identify areas where there is a significant risk of small-scale severe weather, or where small-scale severe weather is already occurring. MetService is able to forecast the conditions favourable for the formation of (severe) thunderstorms, and sometimes we can anticipate the likelihood of tornadoes that have nothing to do with severe thunderstorms. It all comes down to being able to represent phenomena at the time and space scales upon which they occur – which in this case is a few minutes and a few hundred metres respectively.
A little bit about tornadoes in New Zealand
Tornadoes in New Zealand are quite different from those that occur in the Midwest of the United States primarily in the warm part of the year. In New Zealand, tornado occurrence is primarily related to convection along strong cold fronts – and thus they are largely a “cold season” phenomenon. New Zealand tornadoes are also very small and short-lived in comparison to US tornadoes, and in most cases form and dissipate within minutes.
Some notable recent instances include the Waitara tornado of 15 August 2004, the Greymouth tornado of 10 March 2005 (very likely a cold season event, even though it occurred in March), the New Plymouth tornadoes of 3-4 July 2007, the Cambridge tornado of 17 October 2008, and the Avondale tornado of 11 September 2011. The Albany tornado of 3 May 2011 formed under different circumstances.
About the storm of Thursday 6 December
For the few days leading up to this event, it was clear that a large area of rain-bearing air would move from the northern Tasman Sea across the North Island on Thursday 6 December – and this was communicated in forecasts.
Late on Thursday morning, an active cold front approached Auckland from the northwest with a line of showers ahead of it. Ahead of this line the winds were moderate northeasterlies; behind it, they were moderate northwesterlies. Along the line, the winds converged – that is, pushed against each other.
The Severe Thunderstorm Outlooks issued on MetService websites on the evening of Wednesday 5 December, for Thursday 6 December, are below.
The Severe Thunderstorm Outlooks issued on MetService websites on the morning of Thursday 6 December, for that day, are below.
As these Outlooks show, on Thursday morning the Severe Weather Team chose to extend the risk of thunderstorm activity across the whole of the North Island.
As the line of showers (see the radar image at the beginning of this blog) approached the west coasts of Northland and Auckland, the thunderstorm risk for Auckland was escalated from moderate to high.
At this point (around 10:15 am), Auckland Council Civil Defence pushed out advice of thunderstorms in the Auckland area on their mobile phone app (see below) based on consultation with MetService’s Severe Weather Team.
Also at this time, the Auckland forecasts were re-issued on MetService websites so that they now included the mention of thunderstorms (see below).
Auckland regional forecast issued at 10:08am Thursday 6 December:
Periods of rain, some heavy and thundery, clearing this afternoon and becoming mainly fine. However, showers developing from the west tonight. Northwesterlies freshening this evening.
Auckland urban forecast issued at 10:21am Thursday 6 December
Rain with heavy, thundery falls, clearing PM. Northwest.
Monitoring the storm
The Severe Weather Team began intensive monitoring and analysis of the incoming shower line as soon as the Auckland weather radar could resolve it well. With potentially severe weather, this monitoring and analysis process takes place every 7.5 minutes as new radar data become available.
Near midday on Thursday, analysis of the shower line revealed that it contained showers and thunderstorms with high rainfall intensities. At this time, there were none of the tell-tale signs that would indicate that approaching storms within the line were supercellular (that is, very likely to produce tornadoes). As the storms approached Auckland, radar indicated that neither localised rainfall nor hail size would be sufficient to justify the issue of a Severe Thunderstorm Watch. Modelled vertical soundings through the system as it passed across the Auckland area did not indicate the helicity (corkscrew-like motion) or CAPE (energy available for making thunderstorms) large enough to raise concerns about tornadic activity.
Below are a couple of radar images immediately before and approximately at the time of the tornadolocalised wind storm. Vertical cross-sections of these – which the Severe Weather Team were scrutinising at the time – show that the thunderstorm near Hobsonville “flares up” and then “collapses” within about a 10-minute period. The tornadolocalised wind storm is likely to have occurred during the few minutes in which this thunderstorm collapsed.
MetService learnt about the tornadolocalised wind storm roughly half an hour afterwards, through its social media monitoring. Once we had verified media reports and taken a look at the Whenuapai midday upper-air sounding (which contains detailed factual information about the vertical structure of the atmosphere over the Auckland area), we responded with a Severe Thunderstorm Watch at 12:59pm.
SEVERE THUNDERSTORM WATCH Issued by MetService at 12:59 pm Thursday 06 December 2012
Valid until 06:00 pm Thursday 06 December 2012
This watch affects people in the following weather forecast districts:
An active line of thunderstorms has been moving southeastwards across Northland and into Auckland and western Waikato. These thunderstorms are accompanied by heavy rain, hail and strong gusts. This watch is for the risk of damaging gusts in excess of 110km/h and possible small tornadoes.
Wind gusts of this strength can cause some structural damage, including trees and power lines, and may make driving hazardous. If any tornados occur, they will only affect very localised areas. Issued by: Mads Naeraa This watch will be updated by: 06:00 pm Thursday 06 December 2012
We followed this Watch with a Severe Thunderstorm Warning at 1.52pm
After passing northwest Auckland, the line of showers in which this storm existed continued on a southeast path that took it across northern Waikato, Bay of Plenty and across Mahia Peninsula (see map of lightning strikes, below). Subsequently, we issued Severe Thunderstorm Warnings on other thunderstorms in the same line. During the afternoon, high one-hour rainfall totals were recorded at various sites in these regions and a tornado was reported near Ngongotaha.
Interestingly, there are relatively few lightning strikes, and hence limited thunderstorm activity, in the greater Auckland area.
How does MetService warn people about severe thunderstorms?
MetService includes information about expected thunderstorm activity in its regular forecasts, and provides three types of warning messages as part of its Severe Thunderstorm Warning service:
Outlook: Issued daily, describing the thunderstorm risk expected over the next 48 hours
Watch: Issued when there is a significant risk of severe thunderstorms – usually issued 1 to 6 hours ahead depending on the situation
Warning: Issued for specific thunderstorm cells that meet severe thunderstorm criteria and are within range of a radar
Outlooks and Watches are used to provide a “heads-up” of the potential for severe thunderstorm activity within a prescribed area.
Severe Thunderstorm Warnings are issued for thunderstorm cells that can be tracked on MetService’s radar network, and for which there is clear evidence that the cell is producing severe weather. This evidence typically comes from expert interpretation of radar data or direct observation from people on the ground.
As a National Meteorological Service designated by the World Meteorological Organization (WMO), MetService’s Severe Weather programme follows professional standards and best practice as prescribed by the WMO – the UN agency responsible for international cooperation in meteorology. All of our meteorologists are trained to WMO standards, and the Severe Weather Team is made up of highly-experienced meteorologists with specialised expertise in radar interpretation and severe thunderstorm forecasting.
MetService treats a thunderstorm as severe if it meets one or more of the following conditions. These thresholds have been developed to reflect the nature and impact of New Zealand thunderstorms, in consultation with the Ministry of Civil Defence & Emergency Management and the Ministry of Transport:
Rainfall of 25 millimetres per hour or greater
Hailstones 20 millimetres in diameter or greater
Strong wind gusts 110 kilometres per hour (60 knots) or greater
Damaging tornadoes: Fujita F1 or greater, where F1 is defined as having wind speeds greater than 116 kilometres per hour (63 knots)
As noted above, the tornadolocalised wind storm that affected Hobsonville on Thursday was not detectable on radar and was so short-lived that a warning for the associated thunderstorm cell was not possible. Warnings were issued for several thunderstorms within the same line later in the day based on corroborating data from MetService’s radar network.
How do I get information about severe storms?
Regardless of how you get your basic daily forecast (radio, TV, newspaper, web), you should always check the MetService website for the very latest information as other media sources are updated less frequently.