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

On the Cusp

MetService issues severe weather warnings to New Zealanders whenever widespread heavy rain, heavy snow or damaging winds are expected. There are formal criteria that events have to meet or exceed for the forecast to be called a success; the wind criteria are in a previous post, while the rain and snow criteria relate to the total falls required within set periods (write a comment below if you want to ask for details).

Sometimes a warning is considered a false alarm because the event was “on the cusp” of meeting the criteria but its intensity fell just short. These marginal events can be trying for forecasters. However, there has been nothing marginal about the rainfalls that have occurred over the West Coast of the South Island last week. Rainfall amounts have far exceeded warning criteria, and there have been some very large totals. For example:

rainfall Sun 25 Apr
(midnight to midnight)
Mon 26 Apr
(midnight to midnight)
West Homer (Fiordland) 213 mm 119 mm
Cropp Hutt (Westland) 382 mm 155 mm

The rainfall has been associated with an active front along which a large depression (or Low) subsequently formed over the Tasman Sea. Here is a sequence of satellite pictures showing the Low’s development and movement. The enhancement is the same as in an earlier post:

Infra-red satellite image, noon NZ Standard Time, 25 Apr 2010

Initially the band of deep clouds (purple, red and yellow) extend almost in a straight line from New South Wales to Fiordland.

As previous but midnight 25 Apr 2010

As time passes the cloud band perturbs and eventually takes on a comma shape.

Noon 26 Apr 2010
Midnight 26 Apr 2010
Noon 27 Apr 2010

Note how the clouds spiral in towards the centre of the depression in the last two pictures. The pointy part of the cloud near the centre is called a cusp. Its shape results from the flow that is rotating clockwise around the developing depression, with a maximum speed just out from the centre. To the southeast of the centre the flow is splitting and then moving in the opposite direction, creating an abrupt southwestern edge to the cloud there.

In the last satellite picture I’ve drawn arrows to show how part of the flow in the upper troposphere is wrapping around the depression while another part is being drawn into the westerly airstream at higher latitudes.

The animation below illustrates how a depression shapes the air around it, with the red and purple areas resembling the areas of cloud in the satellite pictures.

How regions of cold dry air (blue) and warm moist air (red) are shaped by a depression

Once depressions reach the stage of forming a cusp they are becoming mature. There is little further scope for them to develop, although a new development often occurs downstream as the older system fizzles.

By looking at animations of satellite images on TV or on the internet you may be able to identify systems that complete a full life cycle, and others that are disrupted in some way.