Audi Hamilton Island Race Week Day 6 forecast

by Kenn Batt on 26 Aug 2011
Audi Hamilton Island Race Week day six forecast - Warnings: Nil current. Synoptic Discussion: A broad high pressure area in the Tasman Sea extends a weak ridge along the Queensland coast.

Observations: At 0700hr the surface wind at Hamilton Island was 150 deg at 11kn, gusting 14kn (temp 20C, pressure 1018hPa) indicative of the synoptic wind plus some topographic forcing. At Hardy Reef (further offshore at 19.7S 149.2E) the wind was SE at 17kn (ave). At Creal Reef (20.53S 150.38E) the wind was SSE at 12kn.

Forecast Winds for Surprise Rock (Does not include effects of any precipitation)

1000:MeanDir:140d DirRange:160-120d MeanSp:13kn Gust:17kn
1300:MeanDir:130d DirRange:150-110d MeanSp:13kn Gust:17kn
1600:MeanDir:120d DirRange:140-100d MeanSp:12kn Gust:17kn
1900:MeanDir:110d DirRange:130-080d MeanSp:12kn Gust:17kn

NOTE: Heavy showers can produce short period squalls (up to 25kn 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 even more over the last 24hrs. This has essentially been in response to the movement of a cold front through southern states. The slacker pressure gradient over the area poses some difficult questions today:

1. How strong the winds? I’m expecting a difficult day in this area. My thinking is a 12-14kn average speed range? Could be 2kn less? There is also a slight chance however that the speed will be in the 13-16kn for your racing period, as the forecast gradient wind is 20kn for today? Any shower activity close to or over the race course will see some even more difficult conditions (gusty and erratic winds) albeit over short time periods?

2. How far left any persistent wind shift? I’m thinking 100deg by about 1600hr.

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 30 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 some showers developing this morning.

Maximum land temperature at Hamilton Island: 21-22 degrees.

Wind Waves: 0.5 to 1.2metres, less in lee of land, more in open water, in gusts and when wind wave opposes tidal current.

Swell Waves: from 120deg up to about 0.3m offshore??

(Wave heights quoted are Significant wave heights).

Current: A strong ebb till early afternoon, followed by 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 2.63m at 0832hr, Low 0.39m at 1421hr, High 3.69m at 2102hr

Sea Temperature: about 22 degrees.

Outlook Saturday: Cloudy with showers tending to rain at times. Chance of a thunderstorm. SE-ENE winds at 8-14kn (ave)

Audi Hamilton Island Race Week website