Re: Ex-Tropical Cyclone Gita impacts NZ Feb 18th-21st

[Chris Raine]

In addition the Tasman Sea is warm than normal so the degeneration into a mid latitude cold core low may not be as fast as expected .Gita may still have tropical characteristics at landfall which means the wind and rain will be closer to the centre and not unravelled hundreds of kilometres from the centre (i.e a wide wind field) plus and minuses with this scenario

[melja]

From memory the wind damage on the coast a few years back was from the easterly quarter, think it's the reverse of a NWer for the coast meaning similar wind acceleration as we get here in Canterbury.

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[Richard]

From memory the wind damage on the coast a few years back was from the easterly quarter, think it's the reverse of a NWer for the coast meaning similar wind acceleration as we get here in Canterbury.

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Yes thats right, hence way easterly quarter winds are never damaging on this side.

[harleyb]

Latest prediction from JTWC. This could be the first time in years that anything interesting has happened in Palmerston North. (Cue torches and pitchforks).

[Tornado Tim]

Latest prediction from JTWC. This could be the first time in years that anything interesting has happened in Palmerston North. (Cue torches and pitchforks).

The track map alludes to a intact Tropical cyclone, but this is not true as system will quickly enter extra tropical transition and start to enter a high shear environment (which warm cored TC's do not like).
Despite the warmer than normal Tasman Sea temps, it is still far too cold to hold an intact warm cored TC.
Sea Surface Temps around the Northern half of NZ are only 21-23C, where a TC requires a temp of 26C+
Its after the ex-tropical transition that the rapid deepening is expected to occur (and have associated high Wind, storm surge etc).

[harleyb]

Latest prediction from JTWC. This could be the first time in years that anything interesting has happened in Palmerston North. (Cue torches and pitchforks).

The track map alludes to a intact Tropical cyclone, but this is not true as system will quickly enter extra tropical transition and start to enter a high shear environment (which warm cored TC's do not like).
Despite the warmer than normal Tasman Sea temps, it is still far too cold to hold an intact warm cored TC.
Sea Surface Temps around the Northern half of NZ are only 21-23C, where a TC requires a temp of 26C+
Its after the ex-tropical transition that the rapid deepening is expected to occur (and have associated high Wind, storm surge etc).

The question will be what kind of interaction we might get with the jet stream, and where the remains will track. Still quite a bit of uncertainty with this one but it's looking more and more likely that NZ is going to see some reasonably severe weather early next week.

PN wisecracks aside, we rarely get much severe weather here, even with ex-cyclones, unless the centre passes north of here and we get winds from E or SE (downslope winds off the Tararuas/Ruahines). Cyclone Ita did that in 2014, which cost us our front fence (concrete posts snapped off at ground level). At this stage though it looks as though Gita may head further S, possibly towards where Fehi went?

[David]

Latest ECMWF has shifted landfall a bit south to Taranaki.

[Blowy]

JTWC now calling a cross near Wellington...
REMARKS:
150900Z POSITION NEAR 22.0S 172.9E.
TROPICAL CYCLONE 09P (GITA), LOCATED APPROXIMATELY 407 NM EAST OF
NOUMEA, NEW CALEDONIA, HAS TRACKED WEST-SOUTHWESTWARD AT 08 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MSI SHOWS A SYMMETRICAL SYSTEM WITH
WARMING CONVECTIVE TOPS AND A WELL-DEFINED, ALBEIT ENLARGED, 55NM
EYE. THERE IS HIGH CONFIDENCE IN THE INITIAL POSITION BASED ON THE
EYE IN THE MSI LOOP THAT ALSO LINES UP WELL WITH A MICROWAVE EYE
FEATURE IN THE 0458Z AMSU-B IMAGE. THE INITIAL INTENSITY OF 105 KNOTS
IS BASED ON THE LOW END OF AGENCY DVORAK ESTIMATES RANGING FROM T5.5
TO T6.0 TO REFLECT THE SLIGHT WEAKENING. ENVIRONMENTAL CONDITIONS
REMAIN CONDUCIVE WITH A POINT SOURCE ALOFT ENHANCING EXCELLENT RADIAL
OUTFLOW. SSTS ARE ALSO VERY WARM AT 28 CELSIUS. TC 09S IS TRACKING
ALONG THE NORTHERN PERIPHERY OF A DEEP-LAYERED STR TO THE SOUTH.
AFTER TAU 24, TC 09P WILL GRADUALLY TURN SOUTHWESTWARD THEN
SOUTHEASTWARD AS IT ROUNDS THE WESTERN EDGE OF THE STR. VWS IS
EXPECTED TO GRADUALLY INCREASE AS THE UPPER LEVEL WINDS BECOME MORE
OUT-OF-PHASE WITH STORM MOTION. ADDITIONALLY, ALONG-TRACK SSTS WILL
COOL AS THE CYCLONE MOVES POLEWARD. IN VIEW OF THESE DYNAMICS, TC
GITA WILL STEADILY WEAKEN AND BE REDUCED TO 50 KNOTS BY TAU 120 AS
IT CROSSES NEW ZEALAND NEAR WELLINGTON. CONCURRENTLY, THE CYCLONE
WILL BEGIN EXTRA-TROPICAL TRANSITION BY TAU 72 AS IT ENTERS THE
BAROCLINIC ZONE AND BECOMES A STORM-FORCE COLD CORE LOW WITH AN
EXPANSIVE WIND FIELD BY TAU 120. MAXIMUM SIGNIFICANT WAVE HEIGHT AT
150600Z IS 30 FEET. NEXT WARNINGS AT 151500Z, 152100Z, 160300Z AND
160900Z.//

[Chris Raine]

Had a chat with Metservice forecasters today .They reckon that process of decay may be slowed by the Tasman Sea warmth despite vertical wind shear expected. This storm will be worthy of a research paper when it has passed through as to what tropical characteristics were left when it makes landfall

[Ash]

Primarily a west coast event for the South Island?
Models seem consistent on this now.

[Razor]

GFS is stubbornly sticking to a South Island landfall and crossing on Tuesday. Followed by a secondary spinoff to the Otago area

[midgrove]

The GFS and European models seem to be sticking to their respective scenarios - ECMWF and UKMO moving it across the North Island which is what MetService is indicating with their longer range forecasts. CWU are staking their reputation on the GFS model while weatherwatch at least are looking at the ECMWF.

[spwill]

Using the GFS model there would be minimal affects for Auckland. Normally heavy rain would be welcome around here this time of year however the Auckland region is the greenest I have ever seen it in Feb.

[NZstorm]

I like these competitions between GFS and EC. Will be interesting to see which model wins out.

The system is looking a lot like Fehi. Perhaps not as strong.

[David]

EC has a ridge trying to push in towards NZ from the southwest, hence the North Island landfall, whereas GFS doesn't seem to think this will be the case. Interesting indeed.

12Z EC back to showing a Waikato landfall.

[TonyT]

EC has a ridge trying to push in towards NZ from the southwest, hence the North Island landfall, whereas GFS doesn't seem to think this will be the case. Interesting indeed.

12Z EC back to showing a Waikato landfall.

With a wibbly wobbly weather feature such as this you can't pin your expectations on a single model run, even if it is the control run. That control run is just one of multiple ensemble members, and you really need to cover off the possibilities covered by the majority. I hate the term landfall because this is not a tropical cyclone, but if that is the metric to focus on, then lets acknowledge that the EC model (and in fact ALL of the models) are still giving a wide range of possible tracks. The area from Waikato to Wellington has been consistently the preferred track over many model runs from many different models, but trying to tie it down more precisely than that is like trying to guess the exact score in a rugby game.

[David]

I don't think anyone here is pinning expectations on a single model run. I'm just mentioning what the models are showing over time to document how this all plays out.

Where is one to view ECMWF ensemble members? As I haven't been able to find that info online.

[TonyT]

I don't think anyone here is pinning expectations on a single model run. I'm just mentioning what the models are showing over time to document how this all plays out.

Where is one to view ECMWF ensemble members? As I haven't been able to find that info online.

Sorry, I wasn't intending to sound critical, just pointing out that a range of possibilities is on the cards with each model run, not just a single outcome which changes from run to run. Mr Tracey has been posting some output graphics of the EC spaghetti plots on the cyclone thread, but mostly they are locked up in paid for sites as the EC is more protective of its data than some. They also take about 12 hours after model time to arrive.

[David]

Metservice seem to be placing more faith in EC than GFS at this stage, based on their recently updated Tuesday forecasts for upper NI towns and cities.

[Bradley]

Metservice seem to be placing more faith in EC than GFS at this stage, based on their recently updated Tuesday forecasts for upper NI towns and cities.

I can see why too, EC is a much better performing model then GFS on the whole, I'd be very surprised if this makes landfall around Westport and not more towards New Plymouth/Palmerston North, which is what EC has been very consistent on the last few days...

[Razor]

Metservice seem to be placing more faith in EC than GFS at this stage, based on their recently updated Tuesday forecasts for upper NI towns and cities.

I can see why too, EC is a much better performing model then GFS on the whole, I'd be very surprised if this makes landfall around Westport and not more towards New Plymouth/Palmerston North, which is what EC has been very consistent on the last few days...

Why? I recall GFS being pretty good on ex TC Fehi

[Cyclone Tracy]

EC ensemble latest 1502 12Z - Very close alignment now on the SE turn around 30S 162E. GEFS ensemble similar but the turn is less sharp. IMO its now about the strength of the ridge and the polar upper level injection. The latest EC ensemble suggest the polar air is interacting further north than GEFS. MS will be looking very closely at this latest EC ensemble run due to it's explosive potential it has over large areas of the NI. We are still 100+ hours out, plenty can change

Black line is the average of EC ensemble members.

[TonyT]

EC ensemble latest 1502 12Z - Very close alignment now on the SE turn around 30S 162E. GEFS ensemble similar but the turn is less sharp. IMO its now about the strength of the ridge and the polar upper level injection. The latest EC ensemble suggest the polar air is interacting further north than GEFS. MS will be looking very closely at this latest EC ensemble run due to it's explosive potential it has over large areas of the NI. We are still 100+ hours out, plenty can change

Black line is the average of EC ensemble members.

I agree, I suspect the storm will slide into and southeastwards along the polar trough. So tracking the position/movement of that will be key.

[Tornado Tim]

Looking at the Ensemble Spread from the ECMWF Model, its fairly confident of the outcome, 51 ensembles are in this output variation. Fairly little spread.
UKMO and ECMWF have had a more agreement the GFS over the past 24-48 when looking prognostics past 72h+.

[Orion]

From memory the wind damage on the coast a few years back was from the easterly quarter, think it's the reverse of a NWer for the coast meaning similar wind acceleration as we get here in Canterbury.

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Which coast do you mean?