Latest information on North Island thunderstorms 6 December 2012

Around midday on Thursday 6 December an active trough line passed slowly through Auckland. One of these thunderstorm cells contained a tornado which touched down near Hobsonville, tragically killing three people.

This blog post has been set up to keep you informed of the latest developments in this weather system as it travels south-east from Auckland.

Auckland radar image at 12.15pm 6 Dec 2012

Auckland radar image at 12.15pm 6 Dec 2012

Situation update as at 9.30am 7 December 2012

Further squally showers and one or two thunderstorms are expected in the West from Auckland to the Kapiti Coast today. We could see gusts to 110km/hr accompanying heavy showers. There is also a potential for small hail to 10-15mm. Showers clear overnight and the weather looks mostly dry for Auckland over the weekend.

Update as at 6.00pm 6 December 2012

Severe Thunderstorm Watches and Severe Thunderstorm Warnings have been lifted. This is the last blog update for today, but keep an eye out for tomorrow’s weather as there are still Severe Weather Warnings and Watches in place for heavy rain and gales in places. The Severe Thunderstorm Outlook is still in place here: http://metservice.com/national/warnings/severe-thunderstorm-outlook

Update as at 5.00pm 6 December 2012

A report of a tornado in Ngongotaha refers to part of the same system that struck Hobsonville earlier in the day, and has been the subject of Severe Thunderstorm Warnings. This cell is now moving over Mahia and out to sea.

Update as at 4.30pm 6 December 2012

The last active thunderstorm is moving in towards Lake Waikaremoana and is expected to dissipate. The remainder of eastern Waikato and BOP are still experiencing some heavy showers and occasional thunderclap, but this is expected to clear during the evening.

Update as at 3.30pm 6 December 2012

The active line of showers and thunderstorms is expected to clear Auckland CBD by 3.30pm. Active thunderstorms are now moving into the Bay of Plenty and there are Watches and Warnings in place. The whole system is then expected to clear the North Island later this evening.

A different system is also expected to bring active showers and possible thunderstorms to Taranaki from late evening.

Severe Thunderstorm Outlook map issued at: 2:52pm Thursday 6 Dec 2012

Severe Thunderstorm Outlook map valid to Midnight Thursday 6 Dec 2012, issued at: 2:52pm Thursday 6 Dec 2012

Severe Thunderstorm Outlook map valid to Midnight Thursday 6 Dec 2012, issued at: 2:52pm Thursday 6 Dec 2012

We will update this blog again before 5pm. Further updates will then be available through Watches and Warnings, as well as on our Facebook page and on Twitter.

Where to find the latest information from MetService:

Latest Severe Thunderstorm Warnings here: http://metservice.com/national/warnings/severe-thunderstorm-warnings

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

Latest Severe Thunderstorm Watches here: http://metservice.com/national/warnings/severe-thunderstorm-watch

Latest Severe Thunderstorm Outlook here: http://metservice.com/national/warnings/severe-thunderstorm-outlook

Latest Severe Weather Warnings here: http://metservice.com/national/warnings/severe-weather-warnings

Latest Severe Weather Watches here: http://metservice.com/national/warnings/severe-weather-watch

Latest Severe Weather Outlook here: http://metservice.com/national/warnings/severe-weather-outlook

Check our Facebook page here: https://www.facebook.com/MetService

Follow @MetService on Twitter here: https://twitter.com/MetService

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.