America's Cup: Predictwind - Better conditions for two races on Day 6

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.

Predictwind indicates that more moderate winds winds will prevail for Sunday, Races 9 and 10 of the 2013 America's Cup. The first race starts at 1.15pm on Sunday, 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. See bottom of story for the course variations.

Officially the breeze is being forecast at 20-21kts. Today is the second 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.

With the racing at its regular start time of 1315hrs the two feeds produced by Predictwind indicate that winds will be averaging around 20kts, with the breeze having been up all night at around 15kts. The key point is that on the graph image the two weather feeds produced by Predictwind are very close together for the five hours leading into the race start suggesting the the certainty of the prognosis is high. There is only a small amount of breeze build before the start, although for Race 2, winds are predicted to be under the limit of 23 kts with the feeds showing 21kts.

Today there was no fog over San Francisco. Skies are clear and blue, meaning the air out to sea will not be as cold and likely to be sucked in to the same extent by the hotter inland systems, increasing the strength of the breeze. Peak breeze today is forecast to be 21kts - another indicator that winds during race time will not build further than forecast.

Wind map for September 15, 2013 San Francisco at 1315hrs - Start of Race 9

The above image shows the wind speed and direction at 1300hrs with breezes expected to be 22kts at the start of Race 8 - these are only average speeds and gusts will be higher, but not enough to exceed the wind limit. There is a little variation in the forecast indicating a higher degree of certainty as to the outcome.

The key feature of the forecast is close agreement between the two feeds as to the rate of build of the breeze, and around race start they are consistent in their projection.

A feature of the wind image map for both 1300 and 1400hrs (above and below) is the lowered intensity of the breeze in the actual race area, from yesterday - driven by the surrounding topography, which is factored into the detailed 1km forecast option used to compile these predictions. Surrounding the race area, winds are significantly lighter in strength at both times. The wind source, to the west of Golden Gate Bridge is also less strong than the strength to the east of the landmark - again a function of the wind being sucked between the two high headlands on which the Bridge is constructed, before fanning out into San Francisco Bay and losing intensity.

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.

Wind graph for September 15, 2013 San Francisco at 1315hrs - Start of Race 9 - indicated by the vertical yellow line. Read the actual data above the graph section

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 soon one the left of the graph will extend through to the race start time. Winds are expected to start building at 1000hrs to a peak of 21kts four hours later.

Wind map for September 15, 2013 San Francisco at 1415hrs - Start of Race 10

The top image shows the wind speed and direction at 1400hrs with breezes expected to remain at 21ktskts at the start of Race 10 - these are only average speeds and gusts will be higher, maybe exceeding the wind limit of 23kts.

The key feature of the forecast is agreement between the two feeds as to the rate of build of the breeze, and again around race start they are consistent in their projection.

Wind graph for September 14, 2013 San Francisco at 1415hrs - Start of Race 10 - indicated by the vertical yellow line. Read the actual data above the graph section

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 Sunday and Monday. Typically the forecast for Monday 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

Windpredictions for September 15, 2013 San Francisco for Sunday and Monday

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 9 and the red dotted line shows the course recommended for Race 10. 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, however as the boats move to the top of Leg 3 the shoreside is more favoured, meaning that the lead boat will be encouraged to sail very strategically against the trailing boat as well as covering from a tactical perspective. The reason for the favour given to the shoreside is not clear, but is most likely to be a back eddy effect of the tide - which goes against the flow in the main channel. This effect is more pronounced in the second race of the day, sailed an hour later.

Downwind course projection September 15, 2013 - faint purple line is for Race 9, Red dotted line is for Race 10


Upwind course projection September 15, 2013 - faint purple line is for Race 9, Red dotted line is for Race 10


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 strength of the ebbing tide and standing waves can be seen in this buoy just to seaward of the Golden Gate Bridge.

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), www.predictwind.com!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 www.predictwind.com!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 www.predictwind.com!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.