Thunderstorms- 16-17th December, North Island

[Tornado Tim]

Looks like there is a low chance of storm today in the Waikato, looks a bit more favorable down south.

But it certainly seems tomorrow Waikato is in for a storm tomorrow.
Upper air looks unstable at -17 to -18°C, due to the trough of low pressure passing around then I think we should have a good chance of a storm in mid-northern Waikato to South Waikato.

One thing I have noticed for a quite some time now, storms seem develop quite late here especially if we have a NE.
This happened last week and it happened on the mid to late January Unstable period too.
This is most likely due to a Sea breeze coming in from the West and colliding with the NE.
The West Seabreeze (from what I have noticed from my weather station here) is that it carries most of the moisture and usually comes in around 5 PM (aka the Action hour )

Anyway here is a CAPE/LI chart for tomorrow on the 12Z run of the GFS:

[spwill]

Sunshine is what we want to see tomorrow to help set up convergence.
Perhaps we will see convergence between the SE flow and a moist SW seabreeze from the west coast mid to late afternoon / early evening ?

[Tornado Tim]

Severe Thunderstorm Watches have been issued for Napier, Buller, Hawkes Bay,Taihape,Manawatu and Tararua.
Overcast here with a shower here atm.

[jamie]

my station has had some good showers. Up to 14mm there now. Nothing hardly at work which is what Tims station is recording.

[borris89]

Little bit of thunder here today, stayed away from where i was though and didn't last long.

[NZstorm]

One thing I have noticed for a quite some time now, storms seem develop quite late here especially if we have a NE.
This happened last week and it happened on the mid to late January Unstable period too.

Storms over the central NI can send out outflow boundaries which arrive in the Waikato in the evening. I recall this happening in Jan 2010.

[jamie]

Looks like a decent setup. Shame about the timing. Will be an early afternoon show here I'm thinking. Chances of a twister would be a bit higher than normal wouldnt it? considering it's a low crossing so the is a lot more turning motion in the air than a usual front or sea breeze convergence.

[NZstorm]

It doesn't look good for tornadoes Jamie. In a nutshell there needs to be strong wind flow around the 850mb to generate helicity for a tornado. The 850mb flow tomorrow is about 5kts according to gfs.

[jamie]

Thanks for that nzstorm. What are ur thoughts on tomorrow. Will it be chasable stuff or just a big mess?

[Tornado Tim]

Interesting seeing the latest outlook from Metservice:
They added this at the bottom of the text which they usually don't do in thunderstorm outlooks:

People are urged to keep up to date with the latest forecasts, in case a SEVERE THUNDERSTORM WATCH or WARNING becomes necessary.

And the thunderstorms are mentioned in the Severe Weather Watch Waikato/BOP is under:

A low is expected to track slowly eastwards over Auckland during
Saturday afternoon. On the southern side of this low a band of heavy
rain with possible thunderstorms is expected to affect Waikato,
Western Bay of Plenty (including Tauranga) and southern parts of the
Coromandel Peninsula from late Saturday morning. The most likely
target area is the Kaimai Ranges. There is a risk of rainfall amounts
reaching short duration warning criteria ( 50-70mm in a 6-8 hour
period). Thunderstorms may bring localised downpours to these areas
Saturday afternoon.

From my perspective on things, the instability should be there and a decent lapse rate should be achieved if the sun comes out and plays its part. Morning may be a bit messy I am thinking but hopefully not in the afternoon.
CAPE should be good due incoming moisture from both sides of the island, NE breeze expected.

I'll keep an eye on things tomorrow but the set-up does look good

[NZstorm]

Currently temp 17C dp16C broken cu/stratocu. So a good start to the day as far as moisture goes.

Hopefully plenty of surface heating over the Waikato today. 23-24C would be possible if there is good sunny periods.

[spwill]

There are a lot of small showers showing on the radar which is a negative as they will dampen down temperature but hopefully there is some good heating in places today.

[Tornado Tim]

Its all go according to MS:

A low is expected to move slowly east across the upper North Island this afternoon and evening, while a trough should move north through the Waikato, Bay of Plenty and Rotorua regions this afternoon. There is a moderate risk of thunderstorms developing this morning in the area from Auckland to Waitomo and across Taupo and the Bay of Plenty to Gisborne. Most thunderstorms that develop through this area have the potential to produce localised rain of 10-25mm/hr and hail.
However from Waikato and north Waitomo across the Bay of Plenty, the risk of
thunderstorms developing is considered high for this afternoon and evening. Some
of these thunderstorms could become slow moving and severe for a time, resulting
in possible localised downpours of 30-40mm/hr, hail of 10-15mm in diameter and
unusually large hail accumulations in isolated places. A SEVERE THUNDERSTORMS
WATCH has been issued with respect to this.

[jamie]

warming up here. 18.3/17.1 so plenty of moisture. the cu seems to already be gelling together so showers will not be far from developing. i think it should be ok though. this time of year we can get to 22 23 quite easily. ie the sun doesnt have to come out for long.

[Manukau heads obs]

heavy onshore rain showers here in whakatane....but looks like its going to clear

[jamie]

19.4/17.2 now. sun has come out briefly and there are some dark clouds around now.

[jamie]

looks like it clears after these showers which just passed then we should get some real heating. im looks at the visible sat image and there is a big clear slot which is about to move over waikato so this will be the slot where the storms will get their fuel i think

[spwill]

Cloud/sunny spells here, 22C,dp17C.

[David]

A sea breeze is trying to get going here...that will help with convection

[spwill]

A few small showers around as well, just had a quite heavy one here just now, dropped the temperature 2 or 3C which might be a problem this afternoon..

[Tornado Tim]

22°C and dark cumulus bases around, odd shower to the south east and north west.

Here is the latest Water Vapor Sat Image, lots of incoming low level moisture
(courtesy of CIMSS)

[NZstorm]

Raglan showing 25C.

[Manukau heads obs]

gone easterly here now in whakatane and clearing

[Tornado Tim]

Just had a decent shower, now sun is out, making the DP even higher

Things are starting to organize a little more now, crisper T/Cu going up now

[David]

Not much happening here at the moment, scattered brief light showers. Dewpoint 16-17C.