America's Cup: Predictwind - Two races expected on Day 8

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 shows a much simpler forecast for Day 8 Wednesday for Races 11 and 12 of the 2013 America's Cup. The first race starts at 1.15pm on Wednesday, 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. 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

Officially the breeze is being forecast in the 14-18kts range - below the expected limit, tide adjusted of 20.3kts (similar to yesterday.

Today is the fourth 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 to 3 kts which lowers the wind limit under the unique process adopted for this regatta. That mean the wind limit will be 20.5kts for the first race and just 20kts for the second.

The key feature of Predictwind's forecast is a return to a normal pattern, without the local variations of yesterday which resulted in strong winds on the edge of the course, creeping across the width of the course and precluding racing because of the wind limits.

Today winds at our San Francisco location point show the winds have swung to the west on the San Francisco shore.

High level wind map for September 18, 2013 San Francisco at 1400hrs

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. Predictwind takes topography into account swinging the breeze through the Golden Gate, and creating some interesting effects to the north of the course.

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.

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. Inland temperatures are the main driver of the breeze strength today.

With the racing at its regular start time of 1315hrs the two feeds produced by Predictwind indicate that winds will be averaging around 17kts, with the breeze picking up around 1000hrs. The key point is that on the graph image the two weather feeds produced by Predictwind are very close together for the hours leading into the race start suggesting the the certainty of the prognosis is good.

Wind map for September 18, 2013 San Francisco at 1315hrs - Start of Race 11

The above image shows the wind speed and direction 1300hrs with breezes expected to be 17kts 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, yesterday of the the stronger wind in red to the west of the Golden Gate Bridge, shown in red, is now not present at all.


Wind graph for September 18, 2013 San Francisco at 1315hrs - Start of Race 11 - 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 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 17kts at race start time.

Wind map for September 18, 2013 San Francisco at 1415hrs - Start of Race 12

The top image shows the wind speed and direction at 1400hrs with breezes expected to remain at 19-20kts 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 20.5kts for Race 11 and 20kts for Race 12, today.)

Wind graph for September 18, 2013 San Francisco at 1415hrs - Start of Race 12 - 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 Wednesday and Thursday. Typically the forecast for Thursday 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 18, 2013 San Francisco for Wednesday and Thursdayy and Wednesday

The courses optimised by Predictwind for an AC72 show that the optimum course only has a 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.

The point here is that downwind is there is little variation in the options. But upwind for Race 11 the model shows that the fastest course is to the south, or cityside shore. while in Race 12 the full breadth of the course can be used by competitors allowing the trailing boat more latitude on tactical options to escape a cover.

Downwind course projection September 17, 2013 - faint purple line is for Race 11, Red dotted line is for Race 12

Upwind course projection September 185 2013 - faint purple line is for Race 11, Red dotted line is for Race 12


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), 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.
http://www.sail-world.com/114649