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:

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.

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The last week in January 2010 will be remembered by many over the central North Island for the frequent thunderstorms that developed in the afternoon, often lasting well into the evening. Conditions changed little during the week with a slack pressure gradient over the North Island allowing afternoon sea breezes to combine with abundant low level moisture, triggering heavy showers and thunderstorms inland. Many of these storms were slow moving, prompting a number of Severe Thunderstorm Warnings as radar detected torrential rain and hail in some cells.

To illustrate the impressive number of lightning strikes that resulted, and just how widespread the thunderstorms were, here’s the North Island strike maps (courtesy MetService and Transpower) covering the afternoon and evening periods (all about 12 hours) from Monday 25th through to Thursday 28th January.

Monday 25th – more than 9,900 strikes:


Tuesday 26th – more than 32,200 strikes:


Wednesday 27th – more than 18,900 strikes:


Thursday 28th – more than 17,100 strikes:


The colours of the lightning strikes give an indication of when the strikes occurred, with each colour on these maps representing two hours worth of lightning. The times are pink and blue for early to mid-afternoon (about midday to 4pm), green and brown for late afternoon and early evening (about 4pm to 8pm) and orange and red for late evening and night (about 8pm to midnight). Armed with this knowledge you can now deduce where the lightning started in the afternoons and where the last strikes were in the evening. For example, on Wednesday some of the first strikes occurred on a line from South Taranaki to Gisborne with the last strikes of the day occurring in Waikato near Hamilton.

In addition to the colours for each lightning strike, you’ll also note different symbols on the maps. These tell us something about the nature of each strike including the charge. Cloud-to-ground strikes are plotted as a ‘plus’ or a ‘circle’ for positive and negative strikes respectively, while cloud-to-cloud strikes are plotted as a ‘square’.

While it’s not unusual to get afternoon and evening thunderstorms in summer, this week certainly stands out as a particularly active period with a large number of strikes on four consecutive days. And as I write this on Friday afternoon we are again expecting more thunderstorms inland over the North Island today (but perhaps not as many strikes as the above examples).

Lastly, I’ll leave you with a nice example of a large thunderstorm with anvil seen over the Wairarapa on Wednesday 27 January. I took this  panorama from the roof of the MetService building at about 7:25pm:

Click for full size image.