'The America’s Cup Race Course, framed by the Golden Gate Bridge - today the wind blows from right to left, and the ebbing tide flows from left to right against the windflow. The blue sea that can be seen behind the City to the right, is not ocean but the greater expanse of the San Francisco Bay - which exits through the relatively narrow channel under the Bridge.'
Richard Gladwell ©
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A fresh San Fransisco breeze is expected on Day 15, the Final Day, of the 34th America's Cup, for Wednesday, Race 19 of the 2013 America's Cup. The final race is due to start at 1.15pm on Wednesday, local time.
Towards the bottom of this story we feature Predictwind's course routing optimised for wind and tide on the AC race course, and using an AC72 performance profile. It is included to show that the effect of the tide on the race course and race strategies. See bottom of story for the course variations.
For the official line on how the current factor is determined click here Today the tide is flooding (coming in through the Golden Gate) and will negate the previous advantage that exists with an ebbing tide.
With the final race of the regatta set for its regular start time of 1315hrs the wind feeds produced by Predictwind indicate that winds will be around 15-17kts. The feeds produced by Predictwind are close to agreement on the strength at the race start time, and there is only small build in the breeze over the morning.
Officials will add the strength of the tide to set the new wind limit, which will take it just over 24kts. On the basis of this forecast, the winds will be well under that level.
The forecast does not take account of the influence of fog, which can have several effects on the race course winds.
The above image shows the wind speed and direction 1300hrs with breezes expected to be 15-17kts at the start of Race 19 - these are only average speeds and gusts will be higher. The only issue of concern is the more northerly wind direction in some areas of San Francisco Bay, which could affect the set of the course.
The second image (above) shows a graphic representation of the wind build - confirming the numbers for the prognosis.
The third chart (below) shows the wind expectations for Wednesday. Note that sea conditions described in this chart are based on open water and are not descriptive of an enclosed harbour
Wind-predictions for September 25, 2013 San Francisco for Wednesday - PredictWind.com
The courses optimised by Predictwind for an AC72 show that the optimum course only. The red dotted line shows the Leg 2 course recommended for Race 19. The mark positions used in this projection are only very approximate - and will vary on the course set for the day.
In the second course projection, for Leg 3 the tide is expected to be in flood (incoming) for this race, and boats will need to avoid its effects upwind by taking the city shore.
Downwind course projection September 25, 2013 - Red dotted line is for Race 19 - PredictWind.com
Upwind course projection September 25, 2013 - Red dotted line is for Race 19 - PredictWind.com
Two weather feeds are used by Predictwind, the key to the accuracy of the forecast lies with the graphs, as when the two lines for the two feeds are quite close together and following each other in the trend, then the likelihood of the forecast is high. If there is separation to any marked degree, then there still some issues to be resolved but usually 24 hours from the race they are well aligned.
The Predictwind model takes various weather feeds and then polishes these, part of this process is factoring in the effect of local topography on the breeze - well demonstrated by the images with the funneling of the breeze.
Developed by former Olympian, Jon Bilger (470, 1992), PredictWind application was used by the Swiss team Alinghi to win the 2003 and 2007 America's Cups, and has become the leading wind system used by racing and recreational sailors around the world. It is available on a subscription basis and also with a free forecast option.
A mobile app for Android and iPhone is available and PredictWind is probably the most used app on the Sail-World iPhone.
Real time local wind observations have been added as further functionality.
You can check the weather anywhere in the world, and get a five day forecast, plus a variety of other features and functions of PredictWind. The app is synchronised with your PC, so that your latest set of locations are always available without resetting each time you change a device.
by Richard Gladwell
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1:27 PM Wed 25 Sep 2013GMT
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