G’day Squid,
Today’s swell was always going to be dependent on the final position and depth of the low near tassie, these things are notorious hard to try and get right.
Why didn’t we maintain the swell? Have a look at this MSL Anal loop for the last day or so here .
The first anal (00z - 19th) shows a reasonable fetch south of Tas. generating the swell for yesterday. Also worth looking at, the Quickscat for the 19th here
Jump forward to say 12z on the 20th (last night at 11pm) MSL has the low due south of Tassie producing a marked change in the wind field for the southern Tasman, compare the previous Quickscat with the one from last night here , notice the difference.
Go to the last MSL in the loop, the low has rapidly moved east (you can see on the charts that a rough speed is indicated on the fronts, in this case it’s sped up from a mean of 35 to 45kts). It’ll be interested to see the next round of Quickscat’s when they come in.
In hindsight a quick check of the charts and winds last night would have had the alarm bells ringing cause it was pretty obvious that the drop in swell size was going to be marked.
Shot in the dark for Sunday;
A weak low pressure system to form off the coast from Sydney early hours Sunday morning and as a result winds will be WSW initially tending SW by 7.32am, S by 9.05am and strengthening SE’ers to 25kts during the afternoon. Slight increase in swell overnight, mainly 3ft but occasional bomb to 5ft for the AM. Afternoon, short period wind slop to 4ft.
Looking back
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G’day Squid,
The wind is the result of the low’s position. The NW nature of the wind direction in the second quickscat plot (20th @12z) is a result of the low being positioned south and a little west of Tas.
My thinking, and remember that all this is pretty subjective and different people will have different views, was that after looking at the models late last week the low would;
Deepen more and linger more due east of Tassie over the last 24 hours.
Instead, it deepened 770hpa (0z on the 20th) down to 963hpa (06z on the 20th) but tended to hang west of Tassie longer than I thought it would.
Hope this is making some kind of sense; I’ve just finished a couple of doggos so I’m not sure I understand it
Ben or someone should be able to share their thoughts on the subject
The wind is the result of the low’s position. The NW nature of the wind direction in the second quickscat plot (20th @12z) is a result of the low being positioned south and a little west of Tas.
My thinking, and remember that all this is pretty subjective and different people will have different views, was that after looking at the models late last week the low would;
Deepen more and linger more due east of Tassie over the last 24 hours.
Instead, it deepened 770hpa (0z on the 20th) down to 963hpa (06z on the 20th) but tended to hang west of Tassie longer than I thought it would.
Hope this is making some kind of sense; I’ve just finished a couple of doggos so I’m not sure I understand it
Ben or someone should be able to share their thoughts on the subject
Kimmy/Squid
sorry for the short hand, you just get so use to abbreviating things you sometimes forget that people haven’t got a clue what your talking about.
Hpa - (Pa) The unit of pressure (force per unit area) Kg/m². atmospheric pressurs are usually given in hectopascals (100 pascals)
Z time - “Greenwich Mean Time (GMT). Centred on zero degrees longitude. For scientific purposes this time zone is also known as Universal Time (UTC). All meteorological observations worldwide are reported using this time (for ease of comparison and computation). This timezone is commonly suffixed with 'Z'.”
MSL - Mean Sea Level
Anal – analyses
So a MSL anal loop is just a loop of mean sea level analyses charts.
Kimmy you forgot to add that the “doggo” can only be caught late at night and the process involves lots of swearing and bumping into walls that seem to appear out of no where. Just last night after 12 hours I managed to catch another one! Bed time for me........
sorry for the short hand, you just get so use to abbreviating things you sometimes forget that people haven’t got a clue what your talking about.
Hpa - (Pa) The unit of pressure (force per unit area) Kg/m². atmospheric pressurs are usually given in hectopascals (100 pascals)
Z time - “Greenwich Mean Time (GMT). Centred on zero degrees longitude. For scientific purposes this time zone is also known as Universal Time (UTC). All meteorological observations worldwide are reported using this time (for ease of comparison and computation). This timezone is commonly suffixed with 'Z'.”
MSL - Mean Sea Level
Anal – analyses
So a MSL anal loop is just a loop of mean sea level analyses charts.
Kimmy you forgot to add that the “doggo” can only be caught late at night and the process involves lots of swearing and bumping into walls that seem to appear out of no where. Just last night after 12 hours I managed to catch another one! Bed time for me........
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thermalben
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The last couple of days have been very interesting indeed. I'm not overly surprised at the drop in swell on Saturday; it was generally anticipated, but not with the speed and to the degree that we saw along the Sydney coastline (keep in mind that there were quite a few places that reported plenty of surf early on Saturday though - "Bozo's Stanwell Report" and "Brian's Coffs Report" on R.S. both reported 3-5ft in the morning.
HSW has summed it up pretty well above
Just a few other things from my point of view though - IMO, the main attribution to such a quick loss in wave heights is the swell direction. This swell was coming straight out of the south (the true swell direction through the lower Tasman Sea was in fact SW thru' SSW), and as soon as the peak of a swell passes, you'll always notice a downwards trend first and most impressively at locations not exposed to the true swell direction (ie 80% of Sydney!). There was no shortage of swell-generating wind pushing into the Tasman Sea, with a long fetch extending down past 55S (I've stitched together the 4 Quikscat panels of interest from Thursday morning - I think - here).
However, as HSW noted, the front was pushing through the lower Tasman quite quickly, so the fetch wasn't give a huge amount of time to generate a sustained wave field - this also contributed to a short-lived event. But picking the magnitude of the drop was hard, especially seeing that it peaked at quite a significant size (I had plenty of reports of 6'+ sets along the coast on Friday).
There's several phases to a swell event, and predicting the size and scope of each if really hard. These include a few of the following:
Tying this all in together with local winds, current and weather patterns makes for more perplexing discussion. Sometimes, simply coming up with a single integer to represent one day of surf (ie "it'll be 4ft tomorrow") just doesn't do it justice. There is so much to take into consideration.
Anyway, enough rambling from me... sorry for those that dislike long posts. I'll try to condense a little better next time
HSW has summed it up pretty well above
Just a few other things from my point of view though - IMO, the main attribution to such a quick loss in wave heights is the swell direction. This swell was coming straight out of the south (the true swell direction through the lower Tasman Sea was in fact SW thru' SSW), and as soon as the peak of a swell passes, you'll always notice a downwards trend first and most impressively at locations not exposed to the true swell direction (ie 80% of Sydney!). There was no shortage of swell-generating wind pushing into the Tasman Sea, with a long fetch extending down past 55S (I've stitched together the 4 Quikscat panels of interest from Thursday morning - I think - here). 
However, as HSW noted, the front was pushing through the lower Tasman quite quickly, so the fetch wasn't give a huge amount of time to generate a sustained wave field - this also contributed to a short-lived event. But picking the magnitude of the drop was hard, especially seeing that it peaked at quite a significant size (I had plenty of reports of 6'+ sets along the coast on Friday).
There's several phases to a swell event, and predicting the size and scope of each if really hard. These include a few of the following:
- *swell event identification (where, when, how, why)
*arrival time of leading edge (if true long-range groundswell)
*upwards phase of first swell pulse - (ie how quickly will it build; when can we expect to see bulk energy)
*arrival of peak wave heights
*duration of peak wave heights/ETA on swell dropping trend
*downwards phase of swell (ie when will it drop, and how quickly)
*utility swell factor (is there any residual swell from other swell source(s) that might keep wave heights a little higher than expected once the primary swell has eased significantly?)
[note: this is just a rough guide off the top of my head.. there's quite a bit more to it than that, but I really don't want to bore everyone]
- *small+ but long-range (ie long period) southerly groundswell from a storm near Antarctica
*small leftover refracted SE short period swell from change earlier in the week
*building moderate to large shortrange E'ly groundswell as a significant fetch develops off coastline
*building moderate+ NE groundswell later in the week from a low near New Caledonia/Fiji. This low looks like turning into a Tropical Cyclone in the next few days, and this will make things even more interesting
Tying this all in together with local winds, current and weather patterns makes for more perplexing discussion. Sometimes, simply coming up with a single integer to represent one day of surf (ie "it'll be 4ft tomorrow") just doesn't do it justice. There is so much to take into consideration.
Anyway, enough rambling from me... sorry for those that dislike long posts. I'll try to condense a little better next time
Last edited by thermalben on Sun Feb 22, 2004 6:31 pm, edited 1 time in total.
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The Goat
1. Did the swell come up Friday (though not as huge as half of Sydney thought it would by looking at the raw data - which should only be used by consenting adults!)??
2. Did the swell go down Saturday (there were small waves Sat morning at places open to the south, then they went with the tide, then they were back later - average for the day around 1 m more or less... (If there were an hour by hour precise size prediction, no one would agree on it anyway, and no one would be prepared to pay for it! haha))!??
3. Did it then bounce around about 1 m in size still from the South for the rest of the weekend?
Close enough...
End of story....
2. Did the swell go down Saturday (there were small waves Sat morning at places open to the south, then they went with the tide, then they were back later - average for the day around 1 m more or less... (If there were an hour by hour precise size prediction, no one would agree on it anyway, and no one would be prepared to pay for it! haha))!??
3. Did it then bounce around about 1 m in size still from the South for the rest of the weekend?
Close enough...
End of story....
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