Tropical Cyclone Gita Update - Issued 16th February 2018
By MetService Meteorologists Lisa Murray and Chris Noble.
Please note: all times listed below are New Zealand local time
At 1pm today, TC Gita was a Category 3 Cyclone (having further weakened since yesterday) with estimated winds of 80kt (~150km/h) and a central pressure of 956hPa, located over waters 300km south of Aneityum, Vanuatu, and moving west.
The latest track map from RSMC Nadi Tropical Cyclone Warning Centre based on the 1pm position of the cyclone is shown below, with a slightly closer approach to New Caledonia, and hence slower curve to the southwest. Based on the latest timings, we expect responsibility for official high seas warnings and forecast tracks to transfer from RSMC Nadi to TCWC Wellington (MetService) tomorrow Saturday evening or night.
Global ensemble models remain in general agreement that the cyclone will re-curve south of New Caledonia over the north Tasman Sea on the weekend, with a subsequent southeast track onto New Zealand next week (probably Tuesday) as a significant and deep extra-tropical cyclone bringing severe weather.
To highlight differences in potential tracks, the images below show the latest ensemble tracks from the ECMWF/IFS and the NCEP/GFS models. Because of these forecast track differences, local/regional impacts remain uncertain at this stage but likely significant.
The image above shows a range of possible tracks from one of the global weather models (ECWMF), which helps highlight the uncertainly associated with timing and track. Colours indicate intensity of the system based on windspeed. Note however, the TC is still expected to undergo a transition and reclassification process prior to reaching NZ.
The image above shows a range of possible tracks from one of the global weather models (this time the GFS) which favours a more southern approach to New Zealand added to the uncertainly of the track. As in the previous image, the colours indicate intensity of the system based on windspeed. Note however, the TC is still expected to undergo a transition and reclassification process prior to reaching NZ.
In addition to the spread in tracks shown above, it is worth noting that there are also timing differences in arrival. Of the three main global models used at MetService, the UKMO model is the fastest, while the more widely available NCEP/GFS model and the ECMWF model are both slower in bringing the cyclone towards NZ. Current MetService forecast policy favours a slower track and later arrival (again, probably Tuesday).
Impact on NZ
The latest information indicates that a deep low, currently Tropical Cyclone Gita, will impact New Zealand early next week. However, there is still a large spread of possible tracks and timing, leaving regional impacts uncertain at this stage - but likely significant. Although there is uncertainty regarding exactly where and when Gita will cross New Zealand, the models favour the centre moving over the North Island on Tuesday, with the effects starting during Monday and lasting into Wednesday.
MetService Senior Meteorologist, Lisa Murray, explained this uncertainty in a Facebook Live feed today. This can be found at https://www.facebook.com/MetService/videos/1743344949060889/
Tropical Cyclones have their most intense winds and rainfall close to the centre of the cyclone, while mid-latitude lows have a broad area of wind and rainfall which can cover thousands of kilometres. Tropical Cyclone Gita will transition into an intense mid-latitude low between 30S-35S before arriving on our shores, but it will still be a significant and potentially dangerous system bringing heavy rain, gale- to storm-force winds over a large area, large waves/swell and storm surge. Local impacts are highly dependent on the track it takes, but it is likely to be a high impact event which will most likely affect many regions across the North Island and upper South Island.
As the system transitions to an extra-tropical cyclone, current model guidance shows the main rain band becoming confined mainly to the south/southwest semicircle of the low centre (which is relatively common for a system under the influence of west/northwest shear causing the transition process). The implication of this is that the heaviest rain and largest accumulations are likely near and south of the track. To highlight this, the maps below show the 24 hour accumulated rain for Tuesday 20th from the deterministic solutions of the three global models. Note here, because the NCEP/GFS has a more southern track over the upper South Island the largest rainfall accumulations are over the upper SI, while the ECMWF/IFS has larger accumulations over the central/lower North Island due to a track across the central NI.
In regard to the image above please bear in mind, the deterministic solution (rain accumulation shown here) is one possible outcome from the ensemble and not necessarily the mean of all tracks.
Regional and local effects are highly dependent on the track of the cyclone eventually takes across NZ. The final track will also have a large bearing on the storm surge associated with the cyclone due to the low central pressure. (Note, the arrival of the system will not occur with abnormally high or King Tides, like the Cyclone Fehi did.)
MetService advises people to take time over the weekend to prepare for potential severe weather impacts. Civil Defence’s Get ready Get thru website is a very helpful place to start. As always, MetService is working closely with regional councils and emergency management teams, and recommends people follow advice from their local Civil Defence and council.
MetService meteorologists will continue to monitor the cyclone and assess multi-model guidance for this system and provide tropical updates daily via the Tropical Cyclone Activity page on metservice.com, where you can also see hourly satellite imagery.
Currently the potential impacts are addressed in the Severe Weather Outlook at http://bit.ly/SWOutlook.
The coming days will bring more clarity to exactly where and when impacts are likely to be the most severe. Regardless, it is wise to be prepared.
MetService will keep you informed about any potential severe weather for your region through our Severe Weather Outlook, Watches and Warnings. All are available on the MetService websites, NZ Weather app, by email (subscribe here) and on Twitter and Facebook.
We’ll post further updates on TC Gita in the coming days, and as always, you can keep up to date with the latest forecasts and warnings at metservice.com
For how to prepare you can go to: http://getthru.govt.nz/