9:40 PM Wed 24 Aug 2011 GMT
Audi Hamilton Island Race Week day five weather forecast: Warnings: A Strong Wind Warning is current.
Synoptic Discussion: A high pressure centre [central pressure 1026 hPa] in the Tasman Sea extends a firm ridge along the Qld coast. This high is expected weaken over the next couple of days. A trough in the Coral Sea that has been moving toward the Qld/NSW coast, is expected to stall and weaken. A cold front is moving through far southern parts.
Observations: At 0600hr the surface wind at Hamilton Island was 140 deg at 24kn, gusting 28kn (temp 19C, pressure 1019hPa) indicative of the synoptic wind plus some topographic forcing. At Hardy Reef (further offshore at 19.7S 149.2E) the wind was SSE at 23kn (ave). At Creal Reef (20.53S 150.38E) the wind was SSE at 25kn.
Isolated weak showers were observed just to the south of the Whitsundays on weather radar.
Forecast Winds for Surprise Rock (Does not include effects of precipitation)
1000:MeanDir:140d DirRange:160-120d MeanSp:22kn Gust:27kn
1300:MeanDir:140d DirRange:160-120d MeanSp:21kn Gust:26kn
1600:MeanDir:130d DirRange:150-110d MeanSp:20kn Gust:25kn
1900:MeanDir:130d DirRange:150-110d MeanSp:19kn Gust:24kn
NOTE: Heavy showers can produce short period squalls (up to 35kn possible) and short period wind shifts up to 90deg from the mean.
Key: MeanDir: Mean wind direction Degrees True, DirRange: Wind direction range Deg T , MeanSp: mean wind speed in Knots, Gust: Maximum wind speed possible in the 10secs leading up to the hour in Knots.
Discussion: The high pressure system has weakened significantly over the last 24hr. This has essentially been in response to the movement of a cold front through far southern parts (amongst other factors). The trough in the Coral Sea is expected to stall and weaken. As such the big question today will be:
? How strong the winds? I'm expecting an easing trend with the strongest winds this morning on and close to Hamilton Island (due to topographic forcing). Hopefully the forecast above reflects this? There is a chance however that the speeds drop to be about 2-3kn lighter than forecast from about late morning??
The next question will be:
? How far left the wind direction? I'm thinking 110deg by 1600hr.Chance of 100deg though?
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Audi Hamilton Island Race Week 2011 day five weather: Lastest surface chart (left) and Forecast chart for 4pm today (right) -
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Wind may be 2-3kn stronger at times than above in Dent/Fitzallen/Whitsunday Passages and around headlands due to channeling/funneling/upslope acceleration (topographic forcing)?
Wind may also be channeled/funnelled and hence locally stronger between islands and out of some bays. Wind may be lighter and more variable in the lee of any land mass.
Natural oscillations: 15 to 20 deg away from the effects of the land, up to 90deg over short time periods with the passage of any heavy shower cell.
Weather: Cloudy at times with the slight chance of a light shower or two.
Maximum land temperature at Hamilton Island: 21-22 degrees.
Wind Waves: 1.2 to 2.2metres, less in lee of land, more in open water and when wind wave opposes tidal current.
Swell Waves: from 120deg up to about 0.8m offshore??
(Wave heights quoted are Significant wave heights).
Current: A strong ebb till early afternoon, becoming a strong flood later this afternoon. Be extremely careful in channels, etc.
Remember: Tide floods to the south and ebbs to the north in the Whitsundays but there can be huge localized variations.
Tide at Shute Harbour: High 2.46m at 0751hr, Low 0.56m at 1341hr, High 3.51m at 2023hr
Sea Temperature: about 22 degrees.
Outlook: Friday: Partly cloudy. Chance of a shower. SE-E winds at 12-16kn (ave)
Audi Hamilton Island Race Week
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by Kenn Batt
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Double Is Pt, QLD
