America's Cup: Predictwind - Fresh breezes again for Day 4

The America’s Cup Race Course, framed by the Golden Gate Bridge

Predictwind indicates that fresh winds winds will again prevail for Thursday, the sixth and seventh races for the 2013 America's Cup. The first race starts at 1.15pm on Thursday, local time with the second race scheduled for 1415hrs. The 18fters have the Bridge to Bridge Classic today starting at 1630hrs, which will be sailed in breezes expected to be around 22kts.

With the racing at its regular start time of 1315hrs the two feeds produced by Predictwind indicate that winds will be averaging around 17 kts, with the breeze having been up all night at around 10kts. The key point is that on the graph image the two weather feeds produced by Predictwind are very close together at race time, suggesting the the certainty of the prognosis is high. Before that there is a little disagreement as to rate of increase of the breeze strength. Today there seems to be no fog which will hold the breeze down a little, weather is also overcast restricting the inland heating which drives the Frisco 'Doctor'. When there is fog, the cold air on the sea gets angrily sucked in by the hotter inland systems, increasing the strength of the breeze. Unfortunately the fog is not easy to predict or measure and so it is a random factor in the forecast. Winds do increase 22kts, to be close to the limit of 23kts just over two hours after the conclusion of racing.

Wind map for September 12, 2013 San Francisco at 1315hrs - Start of Race 6

The above image shows the wind speed and direction at 1300hrs with breezes expected to be 17kts at the start of Race 6 - 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.

Wind graph for September 12, 2013 San Francisco at 1315hrs - Start of Race 6

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.

Wind map for September 12, 2013 San Francisco at 1415hrs - Start of Race 7

The top image shows the wind speed and direction at 1400hrs with breezes expected to rise to 19kts at the start of Race 7 - these are only average speeds and gusts will be higher, but not enough to exceed 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 12, 2013 San Francisco at 1415hrs - Start of Race 7

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 wind will be slightly lighter.

The third chart (below) shows the wind expectations for Tuesday and 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 12 - 13, 2013 San Francisco

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.