NZ Weather Forum

Report of an apparent small tornado in Te Aroha West at about 4.30pm on Friday.

'Roof sailed in tornado air' - Stuff

Thanks for that damage report Nev.

Looks to have been a gust of wind and not associated with any convection.

Yes, agree. More likely a victim of the 'Kaimai Buster' and, as the story suggests, just one of at least 40 other roofs lifted around Te Aroha between Tuesday night and Saturday evening.

On July 13, Nev wrote:Ended up with 57mm yesterday, but we're at the western side of Waiheke. It looks like more eastern parts got much more…

Wow! NIWA's observer in the more sparsely populated geographical centre of Waiheke recorded about 2.5 times more than I did after that Saturday afternoon deluge - 88m compared to 35mm here, plus 36mm earlier that morning, compared to 21mm here.

A few more details (minus some amazing flood photos in the print version )..

'Deluge brings floods, slips and road closures' - Gulf News

I also read that the northern hunua area had 130mm and flooding

I see MS got around to throwing a blog together about this event…

'The Northland floods of July 2014'

I do notice a couple of mistakes...refers to things moving westwards when should be moving eastwards

Manukau heads obs wrote:
I do notice a couple of mistakes...refers to things moving westwards when should be moving eastwards

This sentence doesn't add up either,

Another aspect of note is that the slow-moving low has travelled over warmer seas in the north to reach Northland. It has thus picked up a lot of moisture on its journey and moving over the relatively colder sea near northern New Zealand, it is becoming destabilised in the lowest layers of the atmosphere.

From the blog

In this situation the blocking high pressure is having the effect of delivering an extended settled spell to the east coast of both islands

There was cloud/some showers down the east coast with the onshore flow, probably means west coast

the western tasman does have warmer water than this side
I guess that is what is meant?

NZstorm wrote: This sentence doesn't add up either,

Another aspect of note is that the slow-moving low has travelled over warmer seas in the north to reach Northland. It has thus picked up a lot of moisture on its journey and moving over the relatively colder sea near northern New Zealand, it is becoming destabilised in the lowest layers of the atmosphere.

It makes sense to me?

I interpret that as the system having picked up a lot of moisture over the warmer subtropical waters north of NZ, then becoming unstable when it moved down over the cooler waters around NZ.

Why mention cool water ? The cool water is not creating the instability

Relatively colder seas stabilise the atmosphere in the lowest layer, not destabilise. So that part of the story is wrong.

I would think that an air mass that is moving over progressively colder water, which then would cool that airmass, would mean it would make the air colder, and if that air mass was already saturated , then it has to rain out...
which actually creates latent heat, and also drops the air pressure (as the air mass gets colder)
that will be the mechanisms that enhance the instability

also there was two airmassed colliding of different origins and different dew points...one air mass coming around the top of the high, that was relatively stable, and the other air mass that had a higher dew point that was being dragged down from the sub tropics...those two air massed did not want to mix at all...and so hence the strong wind belt just ahead of the heavy rain

Oh, I see. What exactly did cause enough instability to result in so much heavy rain?
Was there a lot of upper level divergence or something like that?

note that it was also the duration of the rain event...basicly the series of fronts took 4 days to travel through due to the blocking high..i.e 4 days of moderate to heavy rain
there was not alot of orographic rain involved ...as it was rain from a relatively high based cloud...
we drove over the coromandel range as the first rain band moved through there..was no real evedence of orographic lift to enhance the rain....was quite strange acually...the cloud base was only just touching the top of the ranges

David wrote:Oh, I see. What exactly did cause enough instability to result in so much heavy rain?
Was there a lot of upper level divergence or something like that?

A North Tasman jet stream/divergence aloft activated the trough on the 8th with a deepening low.

As Brian has noted, persistence was a big factor in the big totals.