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.

Cold Windy Friday

MetService News  Release issued at 12:10pm Thu 4 Nov 2010:

MetService has issued a Special Weather Advisory for the south and east of the South Island.  A cold front crossing the South Island on Friday is likely to be followed by chilly southerly winds, with a period of cold rain and snow to 600-700 metres from Southland through Otago and Canterbury to Marlborough.

MSL midday Thu-04-Nov 2010

Above: Mean sea level analysis and infra-red satellite image for midday Thursday 04 November 2010 New Zealand daylight time.

“After a few mild weeks, this cold change will be very noticeable,” commented MetService Weather Ambassador, Bob McDavitt.  “Farmers should note that the combination of wind and cold rain or snow is likely to put stress on vulnerable stock, and travellers are advised that snow may affect higher roads and mountain passes.”

Prognosis for midday Fri-05-Nov 2010

Above: Mean sea level prognosis for midday Friday 05 November 2010 New Zealand daylight time

“The weather is also expected to be unsettled over the North Island and the north and west of the South Island on Friday, as other fronts cross these areas,” said Mr. McDavitt.  “Check the weather forecast before heading out to that fireworks show.”

Latest weather forecast and warning updates are available on metservice.com, and, for mobiles on m.metservice.com .

Wet and windy around Friday the 13th

The wet and windy conditions that battered parts of New Zealand around Friday 13 August were produced by what looks on the weather maps to be just a ordinary passing front.

Click on the weather map below to watch the weather sequence.

Click to view full sequence of weather maps from 11 to 15 August 2010

Severe gales

In our brief mountain forecast issued on Wednesday 11 August, MetService increased the forecast winds for Thursday over the Canterbury alps to severe gale, the top of the scale for mountains.  Indeed, the winds whistled to over 200 kph at Mount Hutt.

Weather map for 12 Aug 2010

The explanation can be gleaned to some extent  from the weather maps.  Winds rotating counter-clockwise around the large high were being flung outwards at low levels (see rules number 3 and 4 for reading weather maps), encountering the winds rotating clockwise around the deepening low in the Tasman Sea.

These flows combined something like the arms of a kitchen eggbeater, making an accelerating northerly wind reach the Canterbury alps.  Also, the high-pressure system left behind layers of stable air which acted like a lid,  pressing down on that northerly wind as it  encountered the mountains.

When this wind found a gap between the mountain tops it shot out like water does through the nozzle of a hose on power flow.

Deluging rain

On Wednesday 11 August, MetService started a sequence of Severe Warning Watches and Warnings for heavy rain associated with an incoming active front.  Warnings for heavy rain were issued for western and northern areas of both the North and South Islands, as well as for Coromandel, Bay of Plenty and Gisborne mountains north of Ruatoria, with peak daily amounts of around 200 mm for the Westland and eastern Bay of Plenty ranges.

The map above shows the accumulation of rain measured in the MetService network for the week ending 9am Mon 16 August.  Most of the rain in that period was from the passing front.   MetService was picking early on that this front would be able to deliver heavier than normal amounts of rain.   This was because it was given a boost of water vapour from an unusual source —

Click on this image to watch loop of ten days ending 15 August (1.6Mb mpg).

The above image is from The University of Wisconsin Space Science and engineering centre data centre web site and shows a global composite of imagery from the water vapour channel of several meteorological satellites.  Clouds are full white and other parts of the atmosphere are shown in shades of grey according to the amount of water vapour they carry (black=dry air)..  Energy for our weather comes from the sun in part by evaporating surface water.  This imagery shows how evaporated water vapour makes its way around the planet in moving rivers of air, acting as fuel lines for weather.  The red circled area above, on day 222 (10 August) shows moist air departing the Coral Sea region and heading for the Tasman Sea.  It was this water vapour that made last week’s front somewhat out of the ordinary, as picked up in MetService’s warnings.

Saturday’s frontal lull

Winds observed Noon Sat 15 August. Each barb=10knots, wind flies from barbs to point.

Notice that as the front crossed the Auckland region on Saturday it started to buckle and twist. This buckle seemed to start when the front encountered Coromandel.  Winds measured at noon on Saturday were gale force about Great Barrier but there was a lull in the wind just following the front in the Hauraki Gulf.  This buckle could easily have stepped back towards Auckland bringing the gales with it, so a marine wind warning was kept in force for Hauraki Gulf for that afternoon, in spite of the frontal lull.   A fleet of 137 started the Shorthanded Sailing Association’s Shorthaul and Longhaul 60 yacht race, but this frontal lull killed their wind and only 29 could finish in the allotted time.  Such is the fun we get with sailing.

On Saturday afternoon, in particular, it was interesting watching the front via the MetService Tauranga web cam (click on web cam and move mouse left-right to time-track).

So this front posed several problems to people in both their work and play.  Sometimes a debrief such as this, after the event, is a useful adjunct to help our understanding of the weather.