Forecaster Blog Updated: Tropical Cyclone Gita is Heading Our Way
COASTALWATCH | FORECASTER BLOG
Issued Monday, February 12, 2018
There’s now little doubt the entire East Coast will light up under many days of powerful easterly groundswell; generated by the now severe category 4 Tropical Cyclone Gita.
The former tropical depression TD07F was named Tropical Cyclone Gita (TC Gita) by Fiji’s Meteorological Service on Saturday. Since then the storm has closely followed forecast guidance; initially curving poleward over the weekend before beginning to track west on Monday morning. Prima facie, latest model runs suggest we’ll see a sustained run of large, long-period easterly groundswell hammering the entire Eastern Seaboard; probably the most powerful event to impact the coast since the similarly long-period groundswell generated by Tropical Cyclone Winston back in February 2016.
This is all based on TC Gita’s long-lasting westward movement across the Pacific Ocean over the course of this week – a trend that all the major computer models are now in tight agreement upon. TC Gita is forecast to continue on its westward journey; initially tracking immediately below Fiji as a category 5 system over the next two to three days before commencing a gradual south-westward curve as it approaches New Caledonia on Friday and Saturday;
A note on swell potential
Throughout this time-frame, TC Gita will be generating sustained, clockwise winds of 110 to 115 knots around its core, with stronger gusts projected to hit 145 knots. However, these phenomenal wind-speeds belie acute asymmetries in the wind-field. The relatively small size of tropical cyclones means there’s a rapid drop-off in wind-strength as one moves away from TC Gita’s eye; falling to 30 to 50 knots within just 60 to 120 nautical miles of its centre and below 35 knots outside of that.
So while this all looks pretty incredible in theory, it’s difficult to lean on virtual buoy readings associated with TC Gita with a whole lot of confidence. After going back through my files on TC Winston, it’s fair to say this similarly severe TC of February 2016 is a good proxy for what to expect. On the one had, TC Winston was supported by a stronger sub-tropical ridge to the south as it moves above NZ and weakened into a sub-tropical storm over the northern Tasman Sea; thereby supporting a broader pressure gradient situated a little further south than where TC Gita is forecast to be later this week.
However, if latest model guidance proves correct, TC Gita could prove a more substantial source of easterly groundswell. This is in part linked to its sustained intensity as it moves between NZ and New Caledonia this weekend; spawning several, overlapping pulses of E to ENE groundswell. It’s also linked to captured fetch, whereby the storm’s inexorable westward movement carries the strongest swell-producing winds along with the associated wave-field; thereby compounding the size of the swell being generated. Over the next few days TC Gita is forecast to maintain speeds of between 10 and 15 knots before it begins to accelerate with its south-westward curve later this week. This in turn is forecast to carry maximum deepwater wave-heights of 40 to 50ft into the north-eastern Tasman Sea this weekend.
For now, this weekend’s second and potentially largest easterly pulse is contingent on TC Gita’s behaviour as it moves below Fiji from Wednesday to Friday, so stay tuned for more on how this is going to play out as the week progresses. However this weekend pans out, there’s also likely to be plenty of E to SE swell left in the tank following the storm’s extra-tropical transition over the Tasman Sea. Latest model guidance presents pretty good chances for a new round of ESE to SE swell arriving across the region from Wednesday 21 to Friday 23 February. Stay tuned..
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