New Plymouth tornado, Sunday 19 June 2011

The tornado in New Plymouth early on the morning of Sunday 19 June 2011, like every tornado that passes through an area where people observe it, was certainly dramatic.

Like many tornadoes that affect Taranaki, this tornado formed out to sea – not very far to the northwest of New Plymouth – in a line of thundery showers.

Radar imagery

At 4:15am, a mesocyclone (to be explained later in this blog) is identifiable over New Plymouth in imagery from the New Plymouth radar. Radar imagery 7.5 minutes earlier, at around 4:07am, hints vaguely at the mesocyclone’s existence but is far from conclusive. Radar imagery at around 4:22am suggests the mesocyclone either has decayed or is rapidly decaying. In other words, conditions supporting the development of a tornado in the New Plymouth area were favourable only for a very short time.

Reflectivity image from the New Plymouth radar for 4:16am Sunday 19 June 2011. Colours represent how strongly precipitation bounces the radar signal back to the radar.

Colour key for the above image. The further to the right along the scale, the more strongly precipitation reflects the radar signal.

Severe Weather Forecaster John Crouch has extracted radar data from the bottom few “sweeps” of the radar beam. The mesocyclone was sufficiently close to the New Plymouth radar for these sweeps to provide a reaonable view of it. Here they are below, animated. Each time step is 33 seconds; the first sweep is at about 100 metres above the ground near downtown New Plymouth, while the last is about 1000 metres above the ground. So, we’re looking at successively higher layers of the mesocyclone as time goes forward; the reflectivity pattern can be seen wrapping around the circulation associated with the mesocyclone.

Radar reflectivity loop covering the (just over) three-minute period from 4:15am to 4:18am Sunday 19 June 2011. A hook shape can be seen moving from the northwest across New Plymouth.

It’s important to note that this doesn’t mean it’s possible to produce 33-second radar imagery routinely, or in “operational time”. It’s only when phenomena very close to the radar are moving and evolving sufficiently rapidly that data from subsequent sweeps of the radar beam might be coherent enough for animations of them to be meaningful.

Life cycle

Note, in the above animation, how quickly the hook-shaped pattern moves and changes. It suggests strongly that the whole tornado event was over in a couple of minutes, which appears to be consistent with reports in the media. In the animation, there’s evidence of only one hook-shaped pattern in the New Plymouth area: it may be that there was only one tornado, and it wasn’t always reaching down to the surface along its path.

Warm seas to the northwest of New Plymouth provided some of the “fuel” for the thundery showers which passed across Taranaki in the early hours of the morning of Sunday 19 June 2011. This is very likely why the tornado was short-lived: when the mesocyclone came ashore, its fuel supply was cut off.

A little bit about mesocyclones

In brief, a mesocyclone is a local rotation and ascent of air about a vertical axis.

Hook-shaped patterns in radar reflectivity imagery are not uncommon – but are nothing like conclusive evidence of the presence of tornadoes. There also needs to be strong, coincident, rotation: this is one of the reasons why weather radars measure the inbound / outbound speed of the echo using the Doppler Effect.

In the case of the tornado in New Plymouth early on the morning of Sunday 19 June 2011, there was a strong velocity couplet observable at 4:15am, but not either side of this time.

Further reading

For more on tornadoes in New Zealand, see the blog about the Albany tornado of Tuesday 3 May 2011.

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About Peter Kreft

Peter has a BSc(Hons) in mathematics and has been a meteorologist for over 30 years. He's spent about half of this time working in the National Forecast Centre, as both a forecaster and a manager, and the other half recruiting and training New Zealand's meteorologists. These days, Peter gets his practical weather experience by trying to estimate the wind strength while trying to run up Wellington's hills. Peter's particular area of expertise is broad-scale weather systems and he has been an honorary teaching associate at Victoria University of Wellington. He is Chief Forecaster at Meteorological Service of New Zealand Limited.

2 thoughts on “New Plymouth tornado, Sunday 19 June 2011

  1. Peter,
    Thank you for this fascinating write up. Were there any ground wind speed recordings obtained from within the tornado’s path? (I have access to NIWA’s weather database recordings, but am not sure whether they have any recording stations which were in the tornados path/would have survived the experience!) Failing that, what is the typical range during this type of tornado? Finally what would be the horizontal velocity profile you would expect to have seen? This is very relevant in allowing me to evaluate our present wind pressure calculation method for Transmission tower loadings. I appreciate that answering all these questions may take significant effort, but any information which you have easy access to or suggestions on where to go looking would be greatly appreciated.

  2. Since writing this blog on Sunday 19 June 2011, I’ve become aware of media reports of damage paths in Omata and Bell Block as well as in New Plymouth itself. On the radar imagery, we’ve identified a hook echo offshore to the north of Omata; this may well have been where the tornado which destroyed the clubrooms of the New Plymouth Clay Target Club came from. We’ve not been able to identify any other mesocyclone signatures; damage at Bell Block may have been caused by very strong in-line winds.

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