Prepared by: Bruce Buckley (Weather Australia)

Weather situation
A low over central Australia extends a trough into the Bonaparte Gulf. This is producing some instability in the region from Darwin across the Arafura Sea to Timor. Isolated showers with one or two thunderstorms are expected across this region for the next 24 hours. For the remainder of today the winds are likely to be somewhat unsteady. Then the winds are forecast to settle down to a light to moderate SSE to SE trade wind airflow. Generally fine weather is expected through to Bali, although very isolated showers are possible near the islands.

Prepared by: Bruce Buckley (Weather Australia)

Weather situation
An upper level trough linked to a low pressure system that is expected to develop over central Australia during Wednesday and Thursday is expected to increase the general instability in the Arafura Sea during the first 36 hours of this leg of the World ARC. As a result shower and thunderstorm activity is expected to develop in this region, although the departure of the fleet is likely to occur just early enough to outpace this development. Winds will tend to be somewhat changeable for the first 36 hours then they should settle down for the remainder of the trip to Bali. After the first 36 hours the trip to Bali should be in quite benign weather conditions controlled by a relatively weak SE trade wind airflow.

The weather for the remainder of the trip to Darwin is likely to be under the influence of a moderate high centred over the east of Australia with a weak trough forming to the west of Darwin. This should produce fresh ESE winds off the northern coastline of the Northern Territory but these trade winds should weaken over the final run past Melville Island and in to Darwin Harbour where coastal sea breeze effects are likely to dominate the winds through the afternoon period.

The weather for the next few days should be quite good with a weak trough over the Darwin region and a moderate high pressure system moving across southern parts of Australia. Mid-range east to ESE trade winds, cycling through significant fresher then weaker cycles over approximately 24 hour periods should continue for the next few days. The trough development to the west is likely to lead to the development of some showers from Saturday evening onwards off the Northern Territory coastline.

Weather situation
The weather for the start of this leg should be controlled by a ridge of high pressure that extends up the Queensland east coast from a moderate strength high centred over the south eastern Coral Sea coupled with a poorly formed low centred over the south of New Guinea. This should initially produce fresh ESE trade winds in the Thursday Island and adjacent area. However the top part of Cape York Peninsula produces a marked wind shadow for the sector from due west of Thursday Island around to the SSW so if you sail directly west through WSW towards Darwin you are likely to encounter this region. An initial course along 280 degrees true or further to the north for the first 100km should avoid the greatest impacts of this wind shadow.

Weather situation

A strong high pressure system centred over the east of the Great Australia Bight extends a ridge over the north of the Tasman Sea. This is producing increasing SE trade winds over the route from the rear of the fleet through to Mackay. With a tropical convergent line forming over the north of the Coral Sea, the result should be quite persistent fresh to strong SE then ESE trade winds throughout the next 3 days. Weatherwise isolated, disorganised trade wind shower activity is expected, particularly near the Great Barrier Reef, with long fine spells between the passing showers.

A moderate strength high pressure system is moving eastwards across the north of the Tasman Sea. This should be replaced by a low and frontal system Monday night through Tuesday. As a result of this the trade winds should be at their peak today then are likely to progressively moderate during Monday and Tuesday morning. The passage of the top end of the trough line is likely to see the lead yachts experience a freshening SE change. However there is not expected to be much shower activity on the trough and generally the conditions for this passage should remain good.

A deep low pressure system is moving across the eastern Tasman Sea and this has weakened the ESE trade winds along the route towards Mackay. A moderate strength high currently centred over the inland southeast of Australia is forecast to move over the northern parts of the Tasman Sea during Saturday. The ESE trade winds should be at their weakest around sunrise then they should gradually start to increase in strength during this afternoon and particularly on Saturday. They should be at their strongest on Sunday morning. The weather itself continues to be quite benign with only passing trade wind showers about and long fine breaks between them.

The passage of a low pressure system into the Tasman Sea is expected to weaken the ESE trade winds for the first 24 hours of the crossing from Porto Santo to Mackay, although the low itself will remain well this route. Then a moderate strength high is forecast to move over eastern Australia during Friday and over the northern Tasman Sea during Saturday. As a result of this the ESE trade winds should gradually start to increase in strength during Friday afternoon and particularly on Saturday and Sunday. The weather itself should be quite benign with only passing trade wind showers about and long fine breaks between them.

The trade winds are expected to be entering a dying phase for the start of the coming leg from Musket Cove to Tanna. The development of a strong low pressure system to the north of the North Island of New Zealand is expected to significantly weaken the trade wind flow on Sunday afternoon and night. The remains of a frontal line feeding into this low are expected to move across the route during Monday morning with a gusty south westerly change expected with the change. The southerly winds forming behind the change should then become established later Monday and through Tuesday, although they should only reach moderate strength.

The weather between Nuie and Tonga should continue to be benign under the influence of a weak ridge to the south of the islands through to Sunday. A weak ridge is being maintained to the south of the route between the islands due to the presence of a very intense low in the central Tasman Sea with an extensive cloud band feeding into it but remaining to the west of Tonga, which is a very good thing. The minor trade wind surge mentioned in the previous forecast is now looking weaker, although it should still develop this evening.
Some minor trough lines are likely to develop during Sunday which are likely to make the wind directions more variable and could produce a shower or two but they should not be of any concern.

Conditions between Niue and Tonga should continue to be benign under the influence of a weak ridge to the south of the islands. Although there is a quite large low and trough to the northwest of Fiji that is producing extensive cloud over the Niue to Tonga route, it is expected to be relatively inactive along this route. The only direct impact is through the formation of a minor trough line that extends from 17:30S 175:00W to 20:00S 171:00W at the moment which is slightly disturbing the trade wind flow with a broken line of showers in its vicinity. This feature should dissipate within the next 12 hours. In general moderate east to EE trade winds can therefore be expected along this route through to Friday. A minor SE trade wind surge is likely to move over the route during the middle of the day on Friday.

Weather situation
A persistent ridge is expected to lie to the south of Nuie and Tonga for the next three days. This is expected to maintain a moderate east to east southeast trade wind regime across the route between the Niue and Tonga through to Wednesday and beyond. This trade wind regime is expected to be relatively free of trough lines with only relatively minor trade wind pulses expected. It should produce good sailing conditions between the two islands throughout the forecast period.

The remains of a weakening trough lies from 15S 175W to near Niue then it turns towards the southeast. Isolated thunderstorm activity lingers along this trough. However it is expected to dissipate during the next 6 to 12 hours. In general the ESE trade wind regime which spans the route is in a relatively moderate phase, weakest between Niuee and Tonga to the west of the trough line. As the trough line dissipates the ESE to SE trade winds should gradually build across the region between Nuie and Tonga. Once the trade winds have re-established themselves across the entire route, the weather should be confined to isolated showers.

The ESE trade wind regime is entering a relatively steady phase. There is general but disorganised instability over the region from 8S past Suwarrow and down to 15S. In this area there are clusters of scattered showers with a couple of well separated thunderstorms but with no major surface feature to organise them. South of 22S a couple of weak troughs are weakening the trade wind flow but they are also weak and unstructured. There is a stronger trough system to the southwest of Tonga but this should dissipate before it reaches Tonga as a significant feature.

This will lead to a mid range east to ESE trade wind regime across the route from Suwarrow to Nuie, weakening and becoming more variable towards Tonga. It should be a relatively safe weather pattern, although localised wind squalls with the well separated thunderstorms north of 15S will remain a hazard.

A weakening trough with a cloud band around 150nm wide containing scattered showers with a couple of clusters of thunderstorms lies from 15S 177W to 20S 167W and is moving towards the east at around 10 knots. This cloud band is weakening but some pockets of squally winds are possible in the regions east and southeast of the thunderstorms that lie just ahead of the wind change.

Winds shift from a Moderate NE direction near Suwarrow to a freshening northerly south of 15S with winds tending NW for a few hours ahead of the wind change. The wind change is a weak southwest then southerly that should gradually shift SSE.

A complex low and trough system is producing extensive rain and thunderstorm activity in a broad band between 170W and 178W in the latitude band between 13S and 20S. This system is expected to track eastwards during the next 48 hours with the leading edge of the heavier shower activity reaching Nuie close to 00UTC Tuesday. The rear edge of the band should clear Nuie around 15 hours later. The band of heavy showers and a few thunderstorms is likely to be around 180nm wide at this time. The strongest part of this system is expected to shift south of Nuie by the time the lead yachts approach the cloud band.

A double set of convergence lines looks likely to form in the trade winds, particularly near Suwarrow along 12S-13S west of 160W and also near 14:30S to 15:30S on the leg from Suwarrow to Nuie. The other convergence line appears likely to remain just to the north of Suwarrow. On the southern side of these convergence lines the winds are likely to be stronger than elsewhere in the trade wind flow with winds reaching 20/25 knots within 90nm on the southern side. The convergence lines themselves are likely to be showery bands with the possibility for a thunderstorm or two, which could produce local strong squalls. Neither of the lines are expected to develop and appear likely to weaken markedly on Saturday afternoon. This should be accompanied by a general decline in the strength of the trade winds. Overall apart for the squalls on the convergence lines the conditions along the route of this leg appear fairly benign.

The trade winds surge has moved just north of Suwarrow and is producing a band of showers with a couple of thunderstorms between 10S and 13S at the moment. These are likely to slowly shift towards the northwest during the next 24 hours. The other major weather systems in the South Pacific are well away from this leg with a low weakening near 20S 145W and another developing to the southwest of Tonga. This leaves the route from Bora Bora to Suwarrow and down to Nuie under a mid range trade wind regime at the current time. This are expected to remain relatively benign through the next 60 hours. A minor secondary of SE to ESE trade winds is expected to move across the fleet within the first 12-18 hours after departure. Then mid range trade winds should become established over the Bora Bora to Suwarrow route, providing improving sailing conditions after a slow start.

The ESE trade wind surge has moved past its peak, although the first part of the fleet is currently under its strongest sector. Over the next 12 hours the trade winds are expected to subside. However the extent of the trade winds is quite large and so it will dominate the weather for the next 2-3 days. With the decline in the strength of the surge the weather should also improve with increasing fine breaks between the trade wind showers.

Mid range trade winds are then expected through to Suwarrow which should produce good sailing conditions through to the atoll

The weather for the first part of leg 7 - from Bora Bora to Suwarrow, is likely to be dominated by the passage of a fresh to strong and gusty ESE trade wind surge generated by a strong high pressure system of approximately 1038 hPa centred near 39S 145W. Although the latest satellite imagery shows good weather between Bora Bora and Suwarrow, ahead of and along the leading edge of the trade wind surge there are likely to be increasing showers and gusty wind conditions. These are to be expected over Bora Bora in the lead up morning before the race start and for the first few hours of the race. There is the possibility of an isolated thunderstorm embedded in this band of wet weather.

SHORT RANGE SYNOPSIS VALID 03/28/1200 UTC to 03/30/1200 UTC Trade winds of light-moderate strength the next 2 days as the WARC fleet wraps up their Pacific crossing. Pressure gradient between 2 S Pacific highs and equatorial low pressure along ITCZ remains similar to slightly stronger than past few days. SE Pacific high (1022mb) centered near 28S/088W is stronger & building further NW. S central Pacific high (1030mb) moving E from 42S/130W is less blocked from strengthening gradient by surface trough from 12S/140W to 15S/130W to 18S/120W. Also, ITCZ currently between 02N-04N from 125W-145W moving S by 1-2 deg. This slight strengthening of gradient likely causes a small increase in trade wind strength next 2 days. Still mostly fine WX with greatest chance of shwrs & increased clds remaining to SW of Marquesas.

SHORT RANGE SYNOPSIS VALID 03/26/1200 UTC to 03/28/1200 UTC Trade winds remain light next 24 hrs then slight strengthening possible. Similar large-scale WX picture to last FX with light pressure gradient between 2 S Pacific highs & equatorial low pressure. SE Pacific high pressure (1018mb) near 30S/085W is to weak & too far E to strengthen winds in region of WARC. South central Pacific high (1031mb) near 45S/135W is blocked from strengthening gradient by weak trough that is from 14S/140W to 15S/130W to 25S/120W. Gradient & winds may strengthen slightly in 24-48 hrs as ITCZ currently between 04N-06N from 125W-145W moves S by 1-2 deg & SE Pacific high begins strengthening. Generally fine WX with greatest chance of shwrs & increased clds to SW of Marquesas.

SHORT RANGE SYNOPSIS VALID 03/24/1200 UTC to 03/26/1200 UTC Trade winds generally light with slight fluctuation during next 2 days. The pressure gradient between areas of high pressure in S Pacific and low pressure along ITCZ remains slack for several reasons. High (1018mb) meandering near 30S/085W is weak & too far E to strengthen pressure gradient in region of WARC fleet. High (1035mb) near 43S/143W is too far W & blocked from tightening gradient by trough from 30S/110W to 15S/140W to 12S/150W. Also, ITCZ has moved slightly N between 04N-05N from 120W-130W & between 03N-04N from 130W-140W. All these factors causing light trade winds. Trades look strongest first 12-18 hrs of FX then slight easing. Lightest wind generally S of 08-09S, especially W of 130W.

SHORT RANGE SYNOPSIS VALID 03/24/1200 UTC to 03/26/1200 UTC Trade winds generally light with slight fluctuation during next 2 days. The pressure gradient between areas of high pressure in S Pacific and low pressure along ITCZ remains slack for several reasons. High (1018mb) meandering near 30S/085W is weak & too far E to strengthen pressure gradient in region of WARC fleet. High (1035mb) near 43S/143W is too far W & blocked from tightening gradient by trough from 30S/110W to 15S/140W to 12S/150W. Also, ITCZ has moved slightly N between 04N-05N from 120W-130W & between 03N-04N from 130W-140W. All these factors causing light trade winds. Trades look strongest first 12-18 hrs of FX then slight easing. Lightest wind generally S of 08-09S, especially W of 130W.  

SHORT RANGE SYNOPSIS VALID 03/22/1200 UTC to 03/24/1200 UTC Trade winds fluctuating next 2 days. Pressure gradient between S Pacific highs & equatorial low pressure near ITCZ varies along route of WARC because of S Pacific Low. ITCZ between 01N-03N from 110W-120W & between 00N-02N from 120W-140W, only slight 1-2 deg movement expected next 48 hrs. S Pacific highs meandering near 30S/095W (1022mb) & 40S/140W (1035mb). S Pacific low (1008mb) currently near 30S/120W at the base of a trough extending to 20S/135W & 15S/140W. Low moving SE to near 40S/100W in 24-26 hrs. Low weakens pressure gradient between ITCZ & S Pacific highs causing lower wind speeds along route of WARC next 24-36 hrs, especially W of 125W. Winds start increasing when low moves further SE. Trough also causing increased cld & threat of shwrs near Marquesas.

SHORT RANGE SYNOPSIS VALID 03/20/1200 UTC to 03/22/1200 UTC Consistent trade wind sailing with no large fluctuations in speed or direction expected next 2 days. A few much appreciated reports from the fleet & satellite information tend to indicate wind speeds at the mid to high end of the fcast range the past 2 days. This occurred because the pressure gradient between S Pacific high pressure & equatorial low pressure increased slightly from the ITCZ moving S by 1-2 degrees to between 01N-03N from 110W-130W & between 02N-04N from 130W-140W. Little movement of ITCZ & S Pacific high (1033mb) remaining near 40S/100W next 2 days so only slight fluctuations in wind speed & direction. Surface trough of low pressure from 09S/160W to 15S/140W to 25S/137W weakening gradient and causing lowest wind speeds S of 09S from 125W-140W.

SHORT RANGE SYNOPSIS VALID 03/18/1200 UTC to 03/20/1200 UTC Reliable Trade Wind sailing continues through the next 2 days. The pressure gradient between S Pacific high pressure & equatorial low pressure near ITCZ has fluctuated a little the past 24 hrs. This caused slight increase of wind speeds along much of WARC route, particularly from 115W-125W between 05S-09S. S Pacific high (1031mb) currently near 42S/120W & ITCZ between 05N-06N from 080W-100W & between 03N-05N from 100W-140W. Little movement of ITCZ next 2 days but high shifts slightly SE & this may ease wind speeds slightly in 36 hrs. Lighter trade winds W of about 127W as W/NW flank of high is weakened by remnants of Tropical Cyclone Tomas moving from 40S/160W to 55S/125W in 24 hrs & trough of low pressure from 10S/160W to 12S/145W to 20S/135W.

SHORT RANGE SYNOPSIS VALID 03/16/1200 UTC to 03/18/1200 UTC Trade winds along much of WARC route remain steady next two days as pressure gradient stays fairly stable between S Pacific high pressure & equatorial low pressure near ITCZ. Large S Pacific high (1028mb) meanders near 38S/120W next 48 hours. High currently extends over S Pacific region between 080W-160W. However, W/NW flank of high is weakened in 36-48 hours as surface low pressure trough develops from 10S/150W to 30S/130W & Tropical Cyclone Tomas tracks from Fiji to near 35S/165W. This may ease trades very slightly along WARC route W of 130W. ITCZ is between 05N-06N from 080W-0110W &
between 03N-05N from 110W-140W. Little movement expected next 2 days so most organized & widespread shwr/tsm activity remaining N of WARC FX zones.  

SHORT RANGE SYNOPSIS VALID 03/14/1200 UTC to 03/16/1200 UTC The reliable trade winds continue because of a stable pressure gradient between S Pacific high pressure & equatorial low pressure near ITCZ. SE Pacific high pressure (1026mb) near 36S/097W dissipating in 36-48 hrs as central Pacific high becomes dominate center of circulation near 35S/120W. Trade winds may back (trend left) slightly in 36-48 hrs in response to further W center of high pressure. ITCZ is still displaced to N, between 05N-07N from 080W-095W & between 04N-06N from 095W-130W. ITCZ may move S by 1-2 deg next 2 days but most organized & widespread shwr/tsm acvity remaining N of WARC FX zones.

SHORT RANGE SYNOPSIS VALID 03/12/1200 UTC to 03/14/1200 UTC Consistent trade winds next 2 days along the WARC route. Pressure gradient between S Pacific high pressure & equatorial low pressure near ITCZ only fluctuates slightly. SE Pacific high (1029mb) weakening near 36S/095W next 48 hrs as strong low pressure (978mb) moves E from 45S/128W. However, a central Pacific high moving E from 34S/148W reinforces the high pressure area to the S of the WARC route. ITCZ has moved further N & likely remains between 05N-07N from 080W-100W & 04N-06N from 100W-125W. This means most organized & widespread shwr/tsm activity is N of WARC FX zones.

SHORT RANGE SYNOPSIS VALID 03/10/1200 UTC to 03/12/1200 UTC Trade winds persist but ease slightly along the route of the WARC. Pressure gradient between South Pacific high & equatorial low pressure along ITCZ is driving trade winds. South Pacific high (1026mb) currently near 34S/092W & ITCZ between 04N-05N from 080W-100W & 03N-04N from 100W-120W. ITCZ fluctuating little next 48 hrs but high starts weakening in 24-36 hrs as South Pacific low pressure moves east to near 45S/125W. Weakening high relaxes the pressure gradient & eases wind speeds slightly in all WARC FX zones in 24-30 hours. 

SHORT RANGE SYNOPSIS VALID 02/17/1200 GMT to 02/19/1200 GMT A mostly straightforward forecast because of the steady trade winds. Trades from Mar 7th-9th driven by pressure gradient between S Pacific high & equatorial low pressure associated with ITCZ. S Pacific high centered near 33S/103W on 7th moving to 35S/87W by 9th. ITCZ likely between 03N-05N from 77W-105W on 7th & slightly further S by 9th. Generally expect a SE'ly wind of 08-15 kts near the Galapagos. Wind speeds increase from N to S. Between 93W-105W expect 03-10 kts shifting ESE/SSE to the N of Equator, 10-18 kts from the SE/SSE between the Equator-03S, and 13-20 kts from the SE between 03S-08S. Greatest chance of shwrs/tsms near equator with lower risk the further south you travel.

SHORT RANGE SYNOPSIS VALID 02/21/1200 GMT to 02/23/1200 GMT Light winds persist along the route from La Libertad to the Galapagos because of a weak pressure gradient between the S Pacific high & equatorial low pressure. Continue to expect 08-13 kts with occasional periods under 05 kts. There will be a gradual backing of the flow as you head west, from the SSW/S off the coast of mainland Ecuador to SE/ESE near the Galapagos.

SHORT RANGE SYNOPSIS VALID 02/19/1200 GMT to 02/21/1200 GMT - The fleet continues to enjoy a mainly steady, albeit light, gradient wind from La Libertad to Galapagos. The pressure gradient between S Pacific high pressure and equatorial low pressure continues to drive winds in this region between 01N-04S. Generally expect 08-12 kts with occasional lulls. Winds remain highest to the south and lowest to the north. Expect a gradual backing of the flow from the S off the coast of Ecuador to SE/ESE near the Galapagos.

SHORT RANGE SYNOPSIS VALID 02/17/1200 GMT to 02/19/1200 GMT
Large-scale weather picture typical for this time of year during the next
few days and much of this leg. High pressure generally over Mexico & in S
Pacific with lower pressure along Equator associated with ITCZ & in N
Colombia with semi-permanent low center.