Deep Subtropical low to pass through NI, 10/7-13/7

[Dale]

The GFS run has well and truly gone now but tri stack winds 850, 500 & 350hPa were showing an awesome exhaust.. should have saved the file but never mind. The turning wasnt anything to talk about but the speed shear up top simply ripped it apart and sent it awol. Would love for the experts to come and have a chat about this one.

[spwill]

cold dense air over the auckland area, in behind the coromandel ranges ,was the reason for the gales not reaching ground level here in the auckland area

You do this on some cool calm mornings with wind obvious at higher levels( moving clouds) and the warming surface temps bringing the wind down.

[Dale]

Kind of like my fireplace.. if you stack coal and some kindling in the right places, you can create an awesome updraft and downdraft effect. Now.. delete a portion of the sky we are looking at and watch the instability unfold.. move another portion over a couple of inches, add a trigger & a fire tornado is created in the corner.

Smoke pisses off the neighbour but too bad, its atmospheric modelling on a small scale.
Or just complete boredom of decent storms..

[Tornado Tim]

Quite a low humidity here, only 74% when it is normally is around 98% by now. Obviously a dry airmass is over us atm.

[NZstorm]

Quite a low humidity here,

Due to the fohn effect, air is losing moisture on the windward east coasts.

It will be good to see this low finally gone. I don't like low pressure systems passing to our north as you tend to get only the rubbish weather from them, cold and wet.

[Nev]

Here's that quote from Bob McDavitt in the NZ Herald this morn...

Brief break before heavy rain hits again
Updated 7:04AM Monday Jul 13, 2009

...Auckland City was protected from gale-force winds that battered Northland over the weekend ... by a dense layer of protective air downwind of the Coromandel, said Mr McDavitt. The air barrier pushed southeasterly gales over the top of the city, grazing the tip of the Sky Tower but leaving city-dwellers unruffled, he said. ...

Mr McDavitt said 112mm of rain fell around Whangarei and winds reached 140km/h...

Close to a month's rain fell in 36 hours on hills above Kaeo, or up to 170mm of rain, compared with the usual total of 250mm for all of July...

[Manukau heads obs]

so what did the sky tower record?
I see that taranaki got good winds too yesterday once it went SE
from the description it sounds like 50 to 60 knots easily, higher even, as one reports of shed roofs being blown off (that survived bola)

I have a report of a car door being ripped off a car up on mt ruapehu too (they went to open the door to retreive a hat that blew away! (brand new car too!))

[Dale]

That wouldnt surprise me Brian.. and Bob might very well be correct, going back through previous days it does seem some sort of inversion effect protected the lowlying areas of Auckland.. akin to having a big invisible balloon and the good stuff everyone was expecting just bouncing off the top.. different story up here being elevated as high as the Bombays & Pukekohe Hill so it fair ripped through. Like Ruapehu, car doors have been very easy to open and close the last couple of days

[Manukau heads obs]

yes, I would say that the Hunua ranges was able to bring some of the upper winds down to lower levels, hence why the bombay/puke area got stronger winds (I see my brothers place in ramarama got 32 knots, and that i sbehind a row of trees)

[Nev]

so what did the sky tower record?
I see that taranaki got good winds too yesterday once it went SE
from the description it sounds like 50 to 60 knots easily, higher even, as one reports of shed roofs being blown off (that survived bola)

I think Dale mentioned winds of 43 kts at 2000 ft early Sat evening. Skytower is just over 1000 ft asl.

Yes, that Stuff article mentions gusts to about 55 kts at New Plymouth Aero yesterday. The shed-roof would have been around the N'ern base of the Mt Taranaki, where winds had been 'funnelling down the mountain'. The shed owner's neighbour estimates gusts would've been about 75-85 kts. ... And then there was this rather amusing comment...

A second man had been in the toilet when a giant eucalyptus tree came crashing through the front [driveway] of his Junction Rd home.

[Myself]

There's another explanation here:
http://blogs.nzherald.co.nz/blog/weathe ... id=1501977

"It was this jog to the east that saw the most severe winds remain JUST off Auckland's coast and JUST north of the city. If you looked at all the official weather stations we had winds gusting to 130 even 160km/h from Cape Reinga to the Hauraki Gulf. The weather stations at Whangaparaoa doesn't measure gusts but it showed a sustained wind of over 70km/h last night and I would guess that exposed parts had gusts to at least 110 or 120km/h. Waiheke Island took a battering and so too did the eastern Waikato and Coromandel Peninsula. Auckland city only reached about 30km/h sustained...although that has increased a little today."

But I don't think it's very convincing, it just sounds like join-the-dots meteorology.

[Paul Mallinson]

In many cases the position of the low can be critical, but in this case it was located quite a long way offshore. What was really controlling the winds over the upper North Island was the occluded front which lay from northwest to southeast over the northeast of the North Island with its back edge (the wind shift line to lighter winds on the low pressure side) also located a fair way offshore to the northeast.
When you look at a thermal profile through the occlusion over the very windy areas (have a look back at the Whenuapai soundings i.e. the midnight one on July 11) you see saturated cooler air (wet bulb potential temperature (WBPT) near 8C) in the low levels with a frontal mixing zone some way off the surface transiting to a WBPT of about 16 to 17C aloft. This was the sub-tropical air coming around the low. This is warm and less dense than the air below and so it is responsible (amongst other things) for the tight isobaric gradient beneath it where all the wind was. Looking at such a profile during this event, you see that the strongest winds are located through the mixing zone (55 to 60kts) in this case. The closer you look at such a profile to the wind shift line (in this case to the northeast towards low pressure), the lower these very strong winds are until right near the wind shift, the pressure gradient is very strong and these windspeeds are getting to the surface. Once across the wind shift, you are into the warmer air and the windspeed drops.
With the strong pressure gradient over the land, the topography distorts the pattern and tightens the gradient on the lee side of higher ground and so increases the windspeed, but in places, the isobar spacing is less and so the wind speed is also less. Throw into all this the idea that Bob talked about with the colder air near the ground being denser and somewhat harder to move, and so you have it.
Wind forecasting can be very fickle because it depends on a number of variables with the topography being a prominent one.
Hope this helps.

(Couldn't figure out how to paste an image in with this message).

[Dale]

Nailed it right on the head Paul.
Thankyou for your input.

[tgsnoopy]

Decent fall of snow on East cape higher points. The most I've noticed. I took a couple of quick shots from Matata on the way down to Whakatane today, but haven't yet bought the camera in to see if they are any good. I'll post them if they are tomorrow night.

[tgsnoopy]

Decent fall of snow on East cape higher points. The most I've noticed. I took a couple of quick shots from Matata on the way down to Whakatane today, but haven't yet bought the camera in to see if they are any good. I'll post them if they are tomorrow night.

Only one was worth showing, it was too far away and on 18x zoom without a tripod and the haze in the air contrast leaves a lot to be desired, but you can get the general idea.

[tich]

Seen from near Whakatane, they must have been the higher peaks of the Ureweras. Ranges closer to the BOP look lower than that, and I can't imagine much snow on them, unless in a very cold outbreak with moisture reaching the area. (not common at all)