Atypical weather sequence for the Rolex Sydney Hobart Yacht Race is as follows:
Day 1: A cold front moves through the fleet sometime during the first day. One of the worst case scenarios would be for (he front to develop into a 'Southerly Buster', in which case the race would start in a N E sea breeze which would turn fresh to strong (sometimes gale force) and gusty southerly along t he NSW south coast behind the front. Strong to galeforce winds would blow from the west to southwest in Bass Strait and out into the western Tasman Sea.
Day 2; Southerly winds would ease along the NSW coast and turn more south easterly during the day. Winds would remain at least Fresh south westerly further south in Bass Strait and into the western Tasman Sea. The passage of a second cold front through Bass Strait is possible. In this situation the wind would turn more westerly and freshen over central and southern parts of the race course. Along the NSW coast, the wind would be a moderate east to northeasterly. As the front progessed eastward over southern areas, the wind direction would turn fresh to strong southwesterly in Bass Strait and in the western Tasman Sea behind this front. Winds along the southern NSW coast would eventually turn fresh to strong southerly behind this front.
Day 3: Winds would be southeast to north east along much of the NSW coast, tending south west to southerly Further south.
Day 4: South to southeast winds moderating during the day and turning east to northeasterly or light and variable. Coastal sea breezes are possible in the afternoon.
Day 5: Depending upon the position of the high pressure belt, amongst others, the winds could be light and variable or will commence to slowly freshen out of the north to northeast ahead of the next cold front ...
OK so much for the climatology (the average weather), now for a closer look at the 'nuts and bolls" of the weather along the race track.
For simplicity I have broken up the Sydney-Hobart race course into five segments, along with the suggested strategy and possible weather problems for each segment.
SEGMENT 1: Sydney to Gabo Island
The first leg of the race track requires a good understanding of both the weather and ocean currents, specifically the East Australia Current. Some of the main issues include:
(i) Southerly changes, of which the Southerly Buster is the worst case scenario. Winds behind fronts at this time of the year tend to blow more from due south along the NSW coastline. Be prepared because as these can be very nasty! Try to get a handle on precisely when the change will arrive (tricky at the best of times) and what wind speeds will accompany the change. Checking reports from automatic weal her stations along the coast via the internet will help you do this. Get the boat snug down early.
(ii) West to southwest changes: If a west to southwest change is forecast, it pays to be on or east: of the rhumbline to remain in truer wind flow. In westerly flow the area between Jervis Bay and Green Cape and offshore to at least Montague Island is notorious for very variable winds due to lee vortices that can be generated in the lee of the Great Dividing Range.
(iii) Southeast to east winds: In southeast to east winds it pays to be positioned east of the rhumbline, especially on the NSW south coast as the wind is often softer on the coast.
(iv) Pre-frontal troughs: A pre-frontal trough (trough lying north east of the cold front) can be a precursor to the formation of a Southerly Buster. They can also cause an earlier arrival of southerly winds than could otherwise expected from plots of the frontal movement. These changes will later be reinforced by the passage of the subsequent cold front.
(v) Explosive cyclogenesis: The rapid formation of a deep low, commonly known as an East Coast Low, means big problems. The extreme 1993 and 1998 race conditions were produced by explosive development of cut off lows.
(vi) Sea breezes: Some of the strongest sea breezes on the east coast of Australia can occur on this pail of the race course. This development can bring about some very hard running conditions. (vii) Thunderstorms: One of nature's terrors, thunderstorms can spell trouble at this time of the year. Apart from spectacular lightning displays, wind squalls can be a big issue here. Savage squalls (up to 60 per cent stronger than average wind speed) lasting only minutes and often from directions totally different to the prevailing direction, can create havoc on sailing yachts. There is also the risk of hail, waterspouts and heavy rain as well! Be well on your toes when a thunderstorm is around.
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(viii) East Australian Current: Apart from the obvious advantages in capturing a three to five knot current moving south with you (the sailor's 'magic carpet ride'), whenever the situation arises where the surface wind opposes the current, a very nasty seaway can be created. This is where the 'brains trust' on the boat will have to make some serious decisions regarding boat positioning, especially if a front is expected to pass through the fleet on this part of the course.
SEGMENT 2: Gabo Island to north east Tasmania
This part of the race crosses the far eastern parts of the infamous Bass Strait or The Paddock'. This water area between the north and south islands of Australia is one of the most changeable, and hence the most dangerous, stretches of water in the world. Any issues in Bass Strait can affect the adjacent western Tasman Sea area. If you are in Bass Strait and still racing you are too far west! Some potential problems could be:
(i) The wind direction behind cold fronts: They are usually west to southwest compared to a southerly along the NSW coast.
(ii) Funnelling between the Australian mainland and Tasmania: This will see wind speeds, associated with broad westerly flow at the eastern exit of Bass Strait, significantly stronger than those at the western entrance to the Strait. This funnelling effect can be more pronounced in Banks Strait (between Cape Barren Island and north east Tasmania).
(iii) The Corner Effect: This is the situation where the wind speed is enhanced around the outer edge of obstacles (capes, islands, etc) in its path. This is regularly observed in the vicinity of Gabo Island in a west to southwest airflow. This effect is also observed at Green Gape a little further north as well.
(iv) The shallow sea floor of Bass Strait: This will produce a dangerously steep sea very quickly after the onset of strong west to southwest flow that will spill over into the western Tasman Sea.
(v) Pre-frontal troughs: In the Strait and adjacent areas, there is a better chance of observing a roll cloud, marking the position of an approaching trough. This cloud can herald the approach of a 15 to 60 minute period of strong and squally winds, typically followed by sustained strong to gale force westerly winds that set in with the passage of a following cold front.
(vi) Explosive cyclogenesis: Explosive low development (or even a less intense low pressure system passing eastwards through Bass Strait) can be a problem in eastern Bass Strait. The 1993 and 1998 races, as noted above, were associated with explosive cyclogenesis situations.
(vii) Wave conditions in strong westerlies: Wave action can settle somewhat once a yacht is in the lee of Flinders Island, due to a decreased fetch. This can be a blessing for most competitors. The closer you are to the island, the more pronounced this effect will be.
(viii) Thunderstorms: They can still pose problems, but their severity is generally less than those along the NSW coastline.
(ix) East Australian Current eddies:
An eddy will circulate around Gabo Island and into the northeast part of Bass Strait. This can see unwary yachts being 'sucked' into the Strait, ending up further west than where they would like to be. If you can see the Bass Strait oil rigs, then you are definitely in trouble!
(x) Opposing waves and currents: When a swell wave train opposes a wind wave train and these converge with a strong ocean current, then very dangerous overall wave conditions are possible. The infamous 1998 race again provides dramatic proof of this.
SEGMENT 3: The East Coast of Tasmania:
One of the most complex and frustrating parts of the race track. This is where the 30/15 rule comes into play due to the lee trough effect set up by the Tasmanian Central Plateau (see below).
You don't have to be as fussy when the wind blows from the south through the east to the north. Complex tidal and ocean currents occuring inshore often draw yachts in closer than they would like to be. Be careful.
When in this segment, the navigator needs to be particularly vigilant and aware of the following:
(i) Under broad westerly (NW to SW) wind flow: Winds within a line running from 30 nm off Eddystone Pt to i5nm off Maria Island may be influenced by a lee trough (i.e. Light and very flukey). By keeping this distance off the coast at least, yachts have a much better chance of staying a in steadier and mostly stronger winds than closer inshore.
(ii) One would have to be at least 50-60 nm off the coast to sail in a true wind flow. This is just a little too far off under most circumstances unless you are going for the 'southeasterly blinder'. This is the situation where westerly winds go very quickly through the south to the southeast following the passage of a cold front and its associated low pressure system, which will occur closer to southern Tasmania than normal. Most of the time however, fronts are embedded in a very persistent west/southwest flow and the 'blinder' just won't happen. As yachts close on Tasman Island, the wind direction will tend to veer quickly to the south southwest and the speed can increase quite dramatically.
(ii) Approaching Tasman Island: In a west through to southerly flow it generally pays to lay inside of the Hippolyte (at least between the Little Hippolyte and Cape Hauy). This keeps you out of current that is generally setting northeastwards around Tasman Island and in flatter water. On starboard tack you can get a nice lift along Munroe Bight if you are fairly close in. There is normally a definite line of pressure on the water that you have to be inside to gain the maximum advantage.
(iii) At Tasman Island: When yachts are at Tasman Island, wind speeds can increase and decrease quite rapidly (gusts and lulls known as 'bullets') and the direction can also vary quickly. Be prepared! You can generally go very close to Tasman Island, apart; from its southern extremity where there is a reef.
(iv) Thunderstorms: Once again they can pose problems with the passage of a cold front. (v) Intense low pressure systems:
(v) Intense lows passing very close to Tasmania could produce gale to storm force wind conditions.
SEGMENT 4.Tasman Island to the Iron Pot (Storm Bay):
Once around Tasman Island and clear away to the west, wind conditions will generally tend to ease.
(i) Tasman Island to Cape Raoul: Under most conditions, straight-line it from Tasman Island to Cape Raoul, but give the Raoul a slightly wide berth (about mm off). The only situation that you would want to be slightly north of this rhumbline and closer to the Raoul would be in a sea breeze. In a sea breeze situation, the northeast ocean sea breeze can get squeezed between Tasman Island and Cape Pillar (Tasman Passage) and slightly more pressure can result, just to the north of the rhumbline.
(ii) Cape Raoul to the Iron Pot: Once you have cleared Gape Raoul it is generally a straight-line course to the Iron Pot.
(iii) Heavy Rain: If it has rained heavily in southern Tasmania in the week leading up to the race, then there will be a lot of extra water ebbing out of Fredrick Henry Bay.
(iv) Remember that Storm Bay can live up to its name!
SEGMENT 5: Iron Pot to the Finish (the dreaded Derwent River):
The other really complex and frustrating section of this race is the Derwent River. Definitely a place to love or hate. It all depends on what time of the day you reach the Derwent!
(i) Past the Iron Pot: Once past the Iron Pot it generally pays to work the eastern side of the river, staying about 100m off the headlands, especially at night with an ebb tide and after heavy rain. In situations where heavy rain has fallen and light winds prevail, you may have to scallop in and out of the bays. Bui watch the headlands, not too close please!
(ii) The River: Under most weather regimes the Derwent River "shuts down" wind-wise from about 2200 hours local and does not "open" until 0600 to 0700 hr.
(iii) The Western Side: It generally doesn't pay to work the western side of the river, especially from "White Rock to the John Garrow Light,
(iv) Broad westerly flow: Mt Wellington can induce standing wave and rotor activity in these situations. Standing wave activity will generally be felt the most north of the Garrow Light and can lead to very fluky winds from around the middle of the river westwards. Slightly stronger westerly winds will occur towards the eastern shore. Rotor activity can lead to light southeast to easterly winds being generated over much of the river, especially north of the Garrow Light. This situation is often mistaken as a weak sea breeze.
(v) The sea breeze: During the day it is mostly a south easterly and will draw more easterly out of the bays along the eastern side of the river. Under this situation, more pressure will occur eastwards of a line from White Rock to the Finish. At times the east coast northeast sea breeze may break through to the Derwent.
(iv) The Finish: We made it. Now it's time to party!
Now for some words of wisdom:
* Listen to all broadcast weather forecast sources. That crucial wind warning may be broadcast well before you get to hear about it from the official race "sked".
* The Bureau will issue wind warnings according to the following:
Strong wind warning: 10 minute average wind speeds from 25 to 33 knots.
Gale warning: 10 minute average wind speeds from 34 to 47 knots Storm warning: 10 minute average wind speeds 48 to 63 knots Hurricane wind warning: 10 minute average wind speeds of 64 knots plus. Remember wind gusts can be up to 40% more than the 10 min average.
* Wind speed and direction quoted in warnings, forecasts and observations are averages unless quoted otherwise.
* Wave heights in warnings, forecasts and observations are significant wave heights (Average height of the highest one third of waves in a wave train). Maximum wave heights can be double or more the significant wave height.
* Keep that weather eye open at all times.