Audi Hamilton Island Race Week Day 5 - Weather forecast

by Kenn Batt on 23 Aug 2013 Audi Hamilton Island Race Week Day 5 - Weather forecast issued by Kenn Batt at 0615hr for Friday 23rd August 2013.

Nil Warnings current.

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

Observations:
At 0600hr the surface wind at Hamilton Island was 160 deg at 7kn, gusting 9kn (temp 18C, pressure 1018hPa). 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:MD150 DR(170-130) MS08 SR05-12kn
1000:MD130 DR(150-110) MS10 SR06-14kn
1200:MD110 DR(130-090) MS11 SR07-15kn
1400:MD100 DR(120-080) MS12 SR08-16kn
1600:MD090 DR(110-070) MS14 SR09-17kn
1800:MD080 DR(100-060) MS12 SR08-15kn

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
All the computer models that count are in good agreement.

The gradient wind is expected to change from about 160deg at 10kn this morning to lighter and more variable this afternoon. As such the surface wind is expected to commence from 150deg and trend left during the day under the influence of the sea breeze component.

The difficult questions are still however:
1. How far left? The weak sea breeze component should assist the left trend. For your racing period a max left of 060deg is possible.
2. How quickly will the direction trend left? I’m happy with forecast above however there is a 65% 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 85% chance that the wind speeds could be about 2kn stronger from mid-afternoon? The sea breeze component will again assist here.

Wind direction and speed is expected to be more variable than forecast above over parts of the race course (especially within the general Whitsunday Passage area) 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 sunny.
Maximum land temperature at Hamilton Island: 23 degrees. Proserpine: 26deg
Sea Temperature: about 22 degrees.

Wind Waves:
0.2 to 0.6 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 0.38m at 0601hr, High of 3.10m at 1211hr, and Low 0.52m at 1802hr. (know your time offsets for other locations around the race courses)

Outlook
Saturday: Light and variable at first becoming SE to E at 8 to 14 kn (ave) during the late morning and afternoon. Dry and sunny.