Saturday 11 April 2009, 12:00 GMT

Low pressure, which has been dominating the conditions through much of this week, will finally track east today, as a new high builds in from the southwest. Over the next few days, this high will become established over Rio, producing light winds and clear skies.

Looking a little closer the conditions today will depend on how quickly the new high builds into Rio, and how much the sea breeze develops. With mostly clear skies expected, and a moderate to weak pressure gradient, the sea breeze will be the driving force through the day. The sea breeze will peak mid afternoon, fading again as the sun sets. Likewise the sea breeze will fade as the teams sail offshore, backing left to a weak SE'ly to ESE'ly pressure gradient around the southern periphery of the new high.

Forecast Discussion: In the Bay, look for generally light winds, peaking mid afternoon with the thermal push. In the morning look for a weak, 2-4 knot, NNW'ly drainage flow. This breeze will fade quickly backing left and decreasing to a midday lull. The sea breeze should fill to 5-9 knots out of the S by early afternoon, but you may see some big wholes as this new breeze fills. Best case scenario, look for the S'ly sea breeze to peak in the low teens today.

Outside the Bay, the winds will be lighter, with a more notable left shift, as the high builds and the influence from the sea breeze decreases. Still, the more clearing you have, the longer the sea breeze is likely to hold, and the more S'ly the afternoon winds will be. For the evening there will be a real decreasing trend with the winds backing left and likely fading to light and variable by Sunset.

Weather: Some lingering morning clouds, clearing through the day. Otherwise look for a sunny (hazy) afternoon, with cumulus development over the mainland, and clearing over the ocean. The high temperature is expected to be a little bit cooler, staying around 26C.

Detailed Wind Forecast for (4/11/2009) for the in-port racing area:
Wind Speed Wind Direction (true)
Time(LT) Mean Range Mean Range
0900 03 01-06 270 220-320 . more N early, backing to SW
1000 02 01-06 230 220-260 . midday lull, filling from SW
1100 03 01-06 215 200-240
1200 05 02-07 205 190-230
1300 06 04-09 205 180-220
1400 09 07-12 200 180-220 . peak gusts in the low teens
1500 09 07-12 195 180-220
1600 08 06-11 195 170-210 . left shift towards ESE'ly
1700 07 05-10 190 160-200
1800 05 04-09 175 150-190 . decreasing to light & variable

HEDGE: Forecast Confidence -Average.
1. There is good model agreement but once again, the local thermal gradient will have a big influence over the developing conditions. With enough midday clearing the sea breeze could fill to the low teens; however, even in the best case scenario, the sea breeze will be patchy, giving the locals the advantage.

2. The drainage flow will not be very significant today, but the longer it lasts, the later the sea breeze will fill. Otherwise, the more clearing you have in the morning the more left shifted the morning breeze will be - filling somewhere between NNW'ly and SW'ly. The more left shifted the morning winds the earlier the sea breeze will fill.

3. Look for any morning overcast to clear, with lingering haze in the course area. The skies should clear to the east (offshore), while some new cumulus clouds develop over the mainland.

4. You may see a convergence zone develop between any early drainage flow and the developing sea breeze. In this case there may be a line of cumulus clouds moving west towards the coast, with the sea breeze filling in behind this line of clouds.

5. The more clearing there is offshore, and the more the cumulus clouds develop inland, the stronger the afternoon sea breeze is likely to be. Best case scenario the sea breeze will build to the low teens.

6. As the fleet leaves the Bay, and starts leg 6, the winds are likely to decrease. Initially the flow outside the bay will be S'ly to SSE'ly influenced by the sea breeze; however, as the sun goes down the winds are likely to back left decreasing to light and variable.

EXTENDED: As high pressure builds to the south, Rio can expect several days of fine weather following the start. Expect a good amount of sunshine and the low risk of showers with a temperature that reaches the upper 20s C. The familiar pattern of morning drainage flow to an onshore afternoon thermal breeze will occur. Speeds should generally remain under 10 knots during the period.