Forecaster Blog: The Slight Return of a Tasman Low

24 Oct 2016 1

Ben Macartney

Chief Surf Forecaster

COASTALWATCH | FORECASTER BLOG

Issued Monday, October 24, 2016

As you read this, the subject of this blog (a big SSE groundswell) is already filling in across the southern half of the NSW coast – and hence this doesn’t really quality as a forecast as such. Indeed if you read last week’s Swell Alert, you’d already be well aware this event is slated to peak on Monday afternoon/ evening, and carry over into Tuesday morning under early offshore winds. This episode follows the evolution of a deep low pressure system over the southern Tasman Sea over the last 48 hours; initially coalescing east of Bass Strait on Saturday morning before meandering southeast during the day.

SEE ALSO: WA Surfer Flies Into Barrel This Week

With winds set to swing offshore by Tuesday morning, there are likely to be some big, challenging conditions on offer across the entire NSW coast. Photo: CW user Coogeephotos.

With winds set to swing offshore by Tuesday morning, there are likely to be some big, challenging conditions on offer across the entire NSW coast. Photo: CW user Coogeephotos.

However, the real point of this blog is the further evolution of the low over the region over the last 24 hours.  During the early hours of Sunday morning the now south-bound low decelerated before re-curving to the north-northwest as it further intensified during the day. This slight return of the storm-system towards its point of origin is known as retrograding – and when it occurs it often has a major compounding effect on associated surf-potential. That is definitely the case this time around. In the 24 hours between Sunday and Monday mornings the low’s central pressure fell from 994hPa to 988hPa – all the while supporting a broad SSE fetch exhibiting a compact area of core wind-speeds of 40 to 50 knots.

Monday morning's MSLP chart depicts the low at its peak intensity over the southern Tasman Sea. From here on it's set to slowly weaken and accelerate away to the east. Source: BOM.

Monday morning's MSLP chart depicts the low at its peak intensity over the southern Tasman Sea. From here on it's set to slowly weaken and accelerate away to the east. Source: BOM.

A satellite pass from Sunday picked up surface winds of 40 to 50kts wrapping around the low's western quadrant. Source: NOAA.

A satellite pass from Sunday picked up surface winds of 40 to 50kts wrapping around the low's western quadrant. Source: NOAA.

The slight return of the low towards the NSW coast means the swell producing winds have briefly travelled in the same direction and roughly at the same speed as the swell being-produced – a process known as a captured fetch. In this case, the compounding effects of the captured fetch manifest as area of 18 to 25ft seas and swell propagating up into the Tasman Sea as we speak. A mid-period SSE groundswell spreading away from this chaos is already starting to show up across Sydney and the South Coast – and it should continue to build in size as the afternoon progresses.

If you’re unable – or just plain unwilling to tackle your local break during the height of this episode, the good news is there will be plenty of opportunity to do so during the back-end of this event unfolding throughout Tuesday and Wednesday. Initially the bulk of groundswell should continue to see larger surf ranging from 4 to 6ft plus across the region on Tuesday morning, with very energetic surf persisting throughout the day as it slowly begins to ease. By Wednesday morning the tail end of the swell will be back down to a well-organised 2 to 4ft under light offshore winds – and this should see a much wider variety of surfable options on tap across the region all day.

Latest virtual buoy readings for Sydney show the associated groundswell peaking on Monday night before easing steadily throughout Tuesday and Wednesday as winds swing in our favour. Source: CW Wave Tracker.

Latest virtual buoy readings for Sydney show the associated groundswell peaking on Monday night before easing steadily throughout Tuesday and Wednesday as winds swing in our favour. Source: CW Wave Tracker.

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