Audi Hamilton Island Race Week Day 4 - Weather forecast

by Kenn Batt on 22 Aug 2013 Audi Hamilton Island Race Week - Daily Weather Forecast issued by Kenn Batt at 0600hr for Thursday 22nd August 2013.

Nil Warnings current.

Synoptic Discussion:
A weak ridge of high pressure lies over the race area.

Observations:
At 0600hr the surface wind at Hamilton Island was 180 deg at 09kn, gusting 11kn (temp 18C, pressure 1017hPa). At Hardy Reef (further offshore at 19.7S 149.2E) the wind was 110deg at 07kn.

Forecast Winds for Surprise Rock (to E of Hamilton Island) (Read Discussion below)

0800:MD160 DR(180-140) MS08 SR05-12kn
1000:MD140 DR(160-120) MS10 SR06-14kn
1200:MD130 DR(150-110) MS11 SR07-15kn
1400:MD110 DR(130-090) MS12 SR08-16kn
1600:MD100 DR(120-080) MS13 SR09-17kn
1800:MD090 DR(110-070) MS12 SR08-16kn

Note: The wind in the Whitsunday Passage, whilst in the directional range of 180-140deg, could be about 2kn stronger than forecast above and could be about 10-20deg more right at times due to funnelling.

Key: First column is mean wind direction in deg Magnetic (MD). It is the 10min average (mean) value at a height of 10m above the water leading up to the hour quoted. The second column is the directional range (DR) of the wind direction in deg Mag. This takes into account the natural oscillation of the wind and is a function of the atmospheric stability, etc. The third column is the mean speed (MS) in knots (kn) and is the average 10min value leading up to the hour quoted at a height of 10m above the water. The last column is the wind speed range (SR) in knots and is the lowest wind speed to highest wind speed in the 10min leading up to forecast hour.

Discussion
The gradient wind is expected to shift from about 160deg at 10kn this morning to 120deg at 12kn during the afternoon. As such the surface wind is expected to commence from 150deg and trend left during the day.
The difficult questions are however:
1. How far left? The weak sea breeze component should assist the left trend. For your racing period a max left of 090deg is possible.
2. How quickly will the direction trend left? I’m happy with forecast above however there is a 60% chance that the direction shifts more slowly than above.
3. How strong the winds? Pretty happy with the wind ranges above, however there is a 55% chance that the wind speeds could be about 2kn stronger this afternoon? The sea breeze component should assist here.

Wind direction and speed is expected to be more variable than forecast above over parts of the race course due to changing topographical profiles i.e. wind speed enhancement (+2 to 5kn) due to funneling out of bays, some speed enhancement and wind directional changes around headlands and in passages. Lighter and more variable winds in the lee of islands, etc

Natural wind direction oscillations: around 20 to 30 deg, more/less due to topographical influences.

Weather
Dry and partly cloudy.
Maximum land temperature at Hamilton Island: 23 degrees.
Sea Temperature: about 21-22 degrees.

Wind Waves:
0.4 to 0.8 metres.

Heights less in lee of land and more when wind wave opposes tidal current.
(Wave heights quoted are Significant wave heights).

Tidal Current: A strong flood first up this morning followed by slack water and then a strong ebb developing this afternoon.
Be extremely careful in narrow channels, etc.

Remember: In general tidal currents flood to the south and ebb to the north in the Whitsundays. There are many localized variations however so be very vigilant.

Tide at Shute Harbour:
Low 0.31m at 0520hr, High of 3.14m at 1127hr, and Low 0.29m at 1716hr.

(know your time offsets for other locations around the race courses)

Outlook
Friday and Saturday: SE to E at 8 to 14 kn (ave).