Swell from a Tasman low with winds and swell best mid/late week

Steve Shearer picture
Steve Shearer (freeride76)

Sydney Hunter Illawarra Surf Forecast by Steve Shearer (issued Mon Sep13)

Forecast Summary (tl;dr)

  • Mostly short period S to SE swell Tues hampered by fresh S to SE winds
  • Stronger SSE/SE swell Wed/Thurs hampered by persistent S winds
  • Better quality SE/ESE swell Fri, reinforcing pulse PM, with lighter winds
  • Fun ESE swell Sat AM with good winds, winds tending NE and freshening PM
  • More S swell next week, stay tuned for details


Saturday’s S swell came in hot and quite a fair bit over f/cast expectations. Plenty of S exposed beaches saw size in the 4-5ft range (even some 6ft bombs reported!), although the normal size differential between the Hunter and Sydney basins was reduced suggesting a slightly more SSE angle to the swell. Conditions were clean with light WNW to NW winds. As expected size quickly dropped away on Sunday to drop below the 1-2ft range. Conditions today are very sub-par with a high in the Bight directing S to SE winds along the coast and a front now along the North Coast. Onshore winds and around 2ft of short period mixed NE to S to SE swell are providing surf for the very keen. 

This week (Sep 13-17)

This week will be dominated by a low in the Tasman, which is expected to form tomorrow along a NW-SE angled trough line extending from far NENSW across the Tasman sea down to the North Island. This low and a slow moving high pressure cell- now drifting across the Bight and expected to track over Tasmania mid-week will hold a pattern of S’ly winds and developing SE swell through the week. S’ly winds will ease from Thurs as the pressure gradients between the two systems ease. This system looks good in snap shots but has a few flaws which will limit surf potential, read on for details.

Short term looks ordinary with a regime of mod/fresh S’ly winds along the coast and short period windswell on offer.  With wind and swell from the same direction quality will be very ordinary through both Tues and Wed. Size will top out around 2-3ft Tues and build a few notches through Wed, mostly short period windswell from SSE/SE winds in the central Tasman, though swell periods are expected to bump up during Wed, keep expectations low due to prevailing S’ly winds slashing up surface conditions. 

The second half of the week looks a better bet.  Our Tasman low drifts ESE towards the North Island through Wed, in a favourable position for swell development but windspeeds are strongest on the western and north-western flank, aimed up the Tasman sea pipe at South Pacific targets. That will put a lid on wave heights for the East Coast. 

The ridge should slacken by Thursday, although there will still be a SE flow along the coast, odds are increasing for at least a good window of lighter winds if not a morning offshore breeze. We’ll dial that in on Wed. SE swell is expected to be in the 3-4ft range Thursday, possibly with some 5ft sets at more S exposed locations. Expect a roll off in size at more sheltered spots away from the wind. 

Friday should be the first proper clean offshore day of the week, based on current modelling. With high pressure SE of Tasmania and the low now close to the North Island, weak pressure gradients should see a regime of light land and seabreezes. Although the strongest fetch of the low is aimed up away from the East Coast, the supporting fetch of SE/SSE winds extending through the Tasman should see plenty of fun, 3-4ft surf through Friday. 

An afternoon pulse of more ESE angled swell is on the cards too as the low flares up next to the North island late Wed into Thursday. Stay tuned for details on that on Wed.

This weekend (Sep 18-19)

With the dominant high drifting away to the South of  New Zealand later next week, a small high cell drifts NE into the Tasman. This high then becomes squeezed by an approaching front, bringing freshening N’ly winds this Sat. Waves look fun for the AM session with ESE swell generated by the low directing winds out of or near Cook Strait later this week. That should see surf in the 3ft+ range, slowly easing through the day. Expect a NW flow early before freshening N to NE winds.

Swell continues to ease through Sun, dropping back below the 2ft mark through the day. Approaching frontal winds are likely to tend NW and W through the day, so some clean babyfood is on the cards.

Next week (Sep 20) and beyond

Back to S swells next week as a series of deep polar lows approach and pass the lower Tasman Sea. Models are offering divergent outcomes of these lows as they enter our swell window so we’ll need to revise estimates on Wed (and possibly Fri) but the basic pattern can be blocked out now. 

A broad, zonal fetch centred on Tasmania is expected to enter the swell window Sun, with a Bass Strait fetch likely to see a small increase in S swell either later Mon, or early Tuesday depending on timing.

GFS then progs a much more significant, and better angled fetch of SW gales to severe gales through Mon, from an intense polar low. That would suggest a strong pulse of S swell either later Tues and into Wed.

EC has a much more modest system entering the swell window, so we’ll need to keep expectations in check.

Further fronts are expected through next week, though at this stage the orientation of winds is very zonal (W-E), suggesting a few small S swell pulses into next weekend. High pressure is expected to be in the more seasonal position so N’ly winds are most likely.

Check back Wed for a clearer picture on the long and short term.









belly's picture
belly's picture
belly Tuesday, 14 Sep 2021 at 3:19pm

Looks like the low is sitting a smidge further south on the latest BoM 4-dayer compared to earlier BoM / GFS runs, for swell Weds / Thurs and those of us south of Sydney :-)