General December Weather

[NZstorm]

Looks like the first day of summer will be coolish with that SW. But a gradual warm up coming with gfs indicating hot temps possible later in the first week of December.

[Manukau heads obs]

some places though are hot in a SW wind, if there is good clearing skies over the land
e.g BOP

[David]

Perhaps some rain from a subtropical low in the first week. Would be good to get another burst of rain soon.

[NZstorm]

Yes, looks like a potential heavy rainfall event for northern areas mid week. Subtropical low will bring moist air. If last nights gfs run is verified then falls exceeding 200mm likely in the Coromandel and Bay of Plenty.

Auckland gets rain from later Tuesday.

[spwill]

The very moist air will spread across much of NZ.
dewpoint chart for later in the week

[Storm Struck]

Even looking good for inland thunderstorms for Marlborough/Canterbury Thursday with the mix of the air from the low to the north and another incoming low from the south tasman.
An interesting mix between those southern and northern lows attacking the country could make for an interesting month.

[Storm Struck]

Weak unstable skies on a southerly wind change just now, some large drops of rain and the odd intermixed small hail stone around 5mm in diameter so pea size.
Might be a weak storm for Kaikoura in an hour or so

[NZstorm]

LIfted index chart for Thursday.

[Nev]

Nice sun-halo overhead atm.

[NZ Thunderstorm Soc]

Weak unstable skies on a southerly wind change just now, some large drops of rain and the odd intermixed small hail stone around 5mm in diameter so pea size.
Might be a weak storm for Kaikoura in an hour or so

A bit similar to the southerly change that came over Oamaru on the Saturday of Show WE, there Jase.
Cold blustery wind and a few heavy drops falling from what scattered cloud there was.

[NZstorm]

Models still indicating good thunderstorm potential over the North Island on Thursday.
The surface NE will be a key wind and if that eventuates as modeled watch out western North Island!

Thunder also possible over eastern high ground of the South Island on Thursday.

[Tornado Tim]

Models still indicating good thunderstorm potential over the North Island on Thursday.
The surface NE will be a key wind and if that eventuates as modeled watch out western North Island!

There is better mid level moisture forecast in the lower half of the north island. The upper air is forecast to be too dry in the upper north Island, even dry enough for the saturated adiabatic lapse rate to wobble slightly!
The cap is not expected to be there, so I am not entirely optimistic for this setup, but then again forecasts change.....

[NZstorm]

Models still indicating good thunderstorm potential over the North Island on Thursday.
The surface NE will be a key wind and if that eventuates as modeled watch out western North Island!

There is better mid level moisture forecast in the lower half of the north island. The upper air is forecast to be too dry in the upper north Island, even dry enough for the saturated adiabatic lapse rate to wobble slightly!
The cap is not expected to be there, so I am not entirely optimistic for this setup, but then again forecasts change.....

For thunderstorms you need moisture in the lowest 100-150mb (surface moisture). Dry air in the mid levels is considered favourable for storms. You only need mid level moisture for elevated storms.

The looming problem for Thursday is the low clouds in a saturated NE flow. Its a case of seeing how bad the cloud situation is on the day. We need 24-25C to get the CAPE. If storms can develop they will move quickly SE but the upper flow/shear will help energize them.

[Manukau heads obs]

wow, that rain had really developed and pushed south fast over the far north
might get all the way to auckland by the late afternoon?

[Tornado Tim]

For thunderstorms you need moisture in the lowest 100-150mb (surface moisture). Dry air in the mid levels is considered favourable for storms. You only need mid level moisture for elevated storms.

I dont see how very dry air air is favourable for storm development, especially when the layer extends all the way to the trop. Strong lift can compensate for this, but this by no means does it rule out that the air won't mix out. The 26th of November was a prime example where the SFC moisture remained moist but the upper air mixed out, resulting in more showers than thundershowers.
Pockets of dry air are very common especially in caps and yes in those cases I would think that they enhance the convection, but because of temp inversion not the dry air.

For the interest of clarification: 100-150mb is the upper atmosphere not the SFC.

[David]

For the interest of clarification: 100-150mb is the upper atmosphere not the SFC.

Just to clarify the clarification
Steven said the lowest 100-150mb, i.e the range from sea-level pressure up the height which corresponds to 100-150mb below the SLP.

[Tornado Tim]

Steven said the lowest 100-150mb

Oh woops, I read too quickly........ haha

Mostly cloudy here with a mild 19.9 and a light northerly.

[Richard]

It was a cooker of a day here today with 33deg,cooled now to 27deg with a southerly breeze.

[Richard]

Valid to: Midnight Wednesday 4 Dec 2013

Issued at: 10:30am Tuesday 3 Dec 2013

A front is expected to move slowly south over the Tasman Sea just west of the North Island. There is a low risk of elevated thunderstorms over the North Island from about Auckland northwards.

Over northern Canterbury, sea-breeze convergence and daytime heating is likely to lead to showers with a moderate risk of thunderstorms developing in the afternoon and evening. Any thunderstorms that form will be slow-moving and may have rainfall rates of 10 to 25mm per hour.

Something for me to look forward to on Wednesday being in the middle of this moderate risk area,where i am is an area where two converging winds often meet,a NE /easterly over the northern Amuri plains meeting a southerly from the Waipara plains,certainly this sea breeze convergence has produced some memorable storms in the past.

[NZ Thunderstorm Soc]

Waiting with interest with your reports on this as you are right in the area

[03Stormchaser]

Thursday looks good for you Richard aswell.

[Richard]

Oh good,ive even planed my work so i can be home for the next two days

[Richard]

Waiting with interest with you reports on this as you are right in the area

yep,that arrow is point right to my place

[NZstorm]

For thunderstorms you need moisture in the lowest 100-150mb (surface moisture). Dry air in the mid levels is considered favourable for storms. You only need mid level moisture for elevated storms.

I dont see how very dry air air is favourable for storm development, especially when the layer extends all the way to the trop. Strong lift can compensate for this, but this by no means does it rule out that the air won't mix out. The 26th of November was a prime example where the SFC moisture remained moist but the upper air mixed out, resulting in more showers than thundershowers.
Pockets of dry air are very common especially in caps and yes in those cases I would think that they enhance the convection, but because of temp inversion not the dry air.

Dry mid level air is said to enhance convective buoyancy. I remember reading an explanation for it but can't quite remember how it goes.

The regions of the world that get severe thunderstorms essentially have dry upper air masses from the desert regions with tropical surface moisture under running. USA Midwest plains is the best example but eastern Australia is also a case of this.

[Tornado Tim]

The regions of the world that get severe thunderstorms essentially have dry upper air masses from the desert regions with tropical surface moisture under running. USA Midwest plains is the best example but eastern Australia is also a case of this.

Yea I do see that storms do develop in a dry upper atmosphere, but I think that lift in those case's is much stronger than what we get here in NZ.

The way I see it, dry air surrounding moist air containing water vapour (in liquid state) acts like a sponge and helps mix and transfer the water vapour from one body to another, depleting the saturated body of air. However from what I understand this doesn't happen as quickly(if at all) when water vapour is frozen.

Fluid dynamics and Meteorology I find is soo interesting, Id be very keen to read that paper. I love learning these things. If you dont find it Steven don't worry I'll do a bit googling.......

Perhaps I should have done Meteorology instead of I.T (Web Dev).....
Oh hang on..... Metservice (NZ).....Shift Work?!!!