Swellnet Dispatch Narelle Aims To Please

In: Swellnet Dispatch by Ben Matson 4 Comments Fri 11th Jan '13

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Can one weather system deliver groundswell to five states of Australia and two nearby countries? With Severe Tropical Cyclone Narelle pushing down the Western Australian coastline and a dynamic prediction from the computer models, it may very well happen.

On January 5th, a weak tropical low formed in the Timor Sea and began tracking westward. Three days later the low had reached the status of a Category 1 Tropical Cyclone, and was named Tropical Cyclone Narelle.

Since then TC Narelle pursued a west-southwest then south-southwest track, as well as intensifying the Category 4 status, meaning it is now a 'Severe' Tropical Cyclone. STC Narelle is exhibiting sustained winds of 95 knots (175km/h), with gusts of 115 knots (212km/h) and maximum significant wave heights of 40 feet. The cyclone is currently tracking southwest at 8 knots and still intensifying.

For Western Australian surfers cyclone swells are a very rare occurrence, but have occasionally produced memorable swells from the north-west. In January 2011 Severe Tropical Cyclone Bianca produced an incredible groundswell that lit up the West Oz coastline with perfect six-to-eight feet waves across across the lower south-west. Almost a year later Tropical Cyclone Iggy delivered a smaller but equally impressive north-west swell that even saw the metropolitan Perth beaches come to life.

So, with a forecast southerly track almost parallel with the Western Australian coastline, will Severe Tropical Cyclone Narelle deliver anything worthwhile?

At this stage STC Narelle is expected to track closest to the WA coast between Exmouth and Kalbarri, whilst maintaining considerable strength (possibly still Category 4). This will kick up a very large swell for the NW coast however local winds are likely to be at hurricane strength, severely limiting surfing options.

As STC Narelle continues to head south it'll start to rapidly weaken in strength whilst also accelerating its southerly track. We're expecting a modest NW swell across southern WA coasts however no great size is likely due to the cyclone's fetch being focused mainly outside of the West Oz's swell window. In contrast, STC Bianca and TC Iggy initially tracked south-west out into the Indian Ocean, before recurving back into the mainland, developing a more focused fetch aimed squarely at the West Oz coastline.

Building NW swells are expected across the lower South-west late Monday ahead of a peak early Tuesday, with wave heights possibly in the 4ft range at exposed locations (smaller in the Perth and Mandurah regions). However a larger SW groundswell is also expected to arrive on Tuesday and it'll be a much more dominant event, with set waves pushing 6-8ft in the Margs region.

Tuesday looks to be the only day of note in West Oz during the first half of next week due to a favourable offshore airstream. During this time the remnants of STC Narelle will become absorbed by a low-latitude westerly flow, which will pull it quickly to the south-east. As such ex-STC Narelle will cross the lower SW of Western Australia sometime on Wednesday, bringing gusty south-west winds and choppy conditions to the region.

But, the story of STC Narelle doesn't end there. Computer model guidance is suggesting the ex-tropical cyclone will re-intensify in the Southern Ocean well to the west of Tasmania around Thursday. In particular, the models are forecast a 'bombing' low, whereby the central surface pressure drops by more than 24hPa in a 24 hour period. This will generate extremely strong winds within a short space of time, and will consequently develop a large new swell event.

At this stage we're expecting a solid increase in size across South Australia on Friday - albeit from a secondary cold front, and not the actual low pressure system - as well as Victoria and Tasmania. New South Wales will also see a small refracted southerly swell push up the coast later in the weekend. Probably the biggest beneficiary of ex-STC Narelle's redevelopment will be New Zealand, whose West Coast should see a large long period groundswell late Saturday or Sunday.

And for the record, even though we may not see anything amazing in the surf department, swell energy from ex-STC Narelle will probably reach the Fijian region during the middle of the following week.

In any case, we'll monitor the developments of Severe Tropical Cyclone Narelle over the coming days and update the situation in the comments below. //BEN MATSON

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