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sail-world.com -- America's Cup: Predictwind - Weird winds for Day 7

America's Cup: Predictwind - Weird winds for Day 7    
Wed, 18 Sep 2013


Predictwind indicates a significant shift in wind direction for Day 7 Tuesday for Races 11 and 12 of the 2013 America's Cup. The first race starts at 1.15pm on Tuesday, local time with the second race scheduled for 1415hrs.

Today we again show a new feature of Predictwind - course routing optimised for wind and tide on the AC race course. However this has not been adjusted in location for the new wind direction - it is included to show that the effect of the tide is reasonably even across the breadth of the course. See bottom of story for the course variations.

Officially the breeze is being forecast at 20-25kts. Today is the third race day which will be held in an ebbing (outgoing tide) which will see a wind against tide condition in the main channel of the course - maybe building standing waves which will require a more seamanlike approach to mark rounding at Mark 1 and Mark 3 in particular. At the bottom of the course there is minimal tidal effect on the sea condition. The ebb tide is predicted at 2.5kts which lowers the wind limit under the unique process adopted for this regatta.

The key feature of Predictwind's forecast is the shift in direction as much as the wind strength.

Today winds at our San Francisco location point show the winds have swung to the west on the San Francisco shore. However the key factors are a stronger wind driven by an advancing low pressure system which is both increasing strength offshore as well as affecting the direction.

The headland on the northern or Marin County shore of the course - which supports one end of the Golden Gate Bridge is also a significant factor in the wind forecast as the breeze splits to pass around the high headland. In fact earlier in the day this headland is predicted to form an area of rotating breeze.


To perhaps best understand what is happening with the breeze, we have included a high level wind map for the greater San Francisco area, done at a less intense resolution to show the flows of the breeze. As Predictwind takes topography into account the effect of the headland is taken into account.

The key feature of this map is the way the prevailing wind direction is sucked at a different angle through the Golden Gate - which is a local effect only.

A further factor is that inland temperatures are expected to be less today, and will be less of a driver for the breeze, additionally there is no sea-fog creating cooler than usual sea sir - which is normally sucked in like a cold drink on a hot day, by the inland areas. That factor is also missing today.

With the racing at its regular start time of 1315hrs the two feeds produced by Predictwind indicate that winds will be averaging around 17-19kts, with the breeze picking up sharply around 1100hrs. The key point is that on the graph image the two weather feeds produced by Predictwind are not very close together for the hours leading into the race start suggesting the the certainty of the prognosis is not good. but at race start time they do come together increasing the certainty of the forecast. While winds do increase slightly for the start of the second race, Race 12, the breeze is also forecast to drop earlier than normal - probably due to the lack of the temperature driving factors mentioned above.




The above image shows the wind speed and direction in three shots starting at 1100hrs, 1200hrs and finally 1300hrs with breezes expected to be 19kts at the start of Race 11 - these are only average speeds and gusts will be higher, but not enough to exceed the wind limit. The significant effect is the stronger wind in red to the west of the Golden Gate Bridge, shown in red.

If this breeze advances more quickly or fans across the course more widely than expected then racing could be cancelled for exceeding the wind limit.

A feature of the wind image map for both 1300 and 1400hrs (above and below) effect of the surrounding topography, which is factored into the detailed 1km forecast option used to compile these predictions. As mentioned above this splits the breeze. The wind source to the west is also sucked between the two high headlands on which the Bridge is constructed, before fanning out into San Francisco Bay.

A similar but reverse effect happens with the tide flowing from east to west, against the wind, and with the massive body of water which forms San Francisco Bay trying to exit under the Golden Gate Bridge - a little like a bath draining down a plughole - with the plughole being the entrance to the Bay under the Golden Gate Bridge. This tide is predicted to be flowing against the wind at 2.5kts today


The second image (above) shows a graphic representation of the wind build - confirming the numbers for the prognosis. If the breeze builds later than predicted, then the light winds on one the left of the graph will extend through to the race start time. Winds are expected to start building at 1100hrs to a peak of 19kts at race start time.


The top image shows the wind speed and direction at 1400hrs with breezes expected to remain at 20kts at the start of Race 12 - these are only average speeds and gusts will be higher, maybe exceeding the wind limit of 23kts minus the unique subtraction of the tidal strength at this regatta, which reduces the wind limit to 20.3kts, today.)


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 Tuesday and Wednesday. Typically the forecast for Wednesday is revised upwards closer to the time of the race start. Note that sea conditions described in this chart are based on open water and are not descriptive of an enclosed harbour


The courses optimised by Predictwind for an AC72 show that the optimum course only has a small variation between the two races. The faint purple line shows the recommended course for Race 11 and the red dotted line shows the course recommended for Race 12. The mark positions used in this projection are only very approximate - and will vary on the course set for the day. At the bottom of the upwind leg the model shows that the boats can use the full width of the course due to the evenness of the tide.




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 image with the funneling of the breeze.

Developed by former Olympian, Jon Bilger (470, 1992), PredictWind?nid=114603 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?nid=114603 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?nid=114603. 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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