Tidal current forecast sneak preview

By Mike Dvorak on November 26, 2014

As we mentioned last month in our article about winning our division in the SSS Vallejo 2 using the Sail Tactics wind and tidal current forecast, we’ve been hard at work to develop a better tide forecast (more formally, a tidal current forecast) for the San Francisco Bay. In this post, I want to tell you more details of our tide forecast and also give you a sneak preview of the tide forecast interface. We’re aiming to launch the tide forecast before the end of the new year, and we’re hoping to get reader feedback about the user interface and map visualizations, which are previewed below.

The first big test of the tide forecast will be the SSS Three Bridge Fiasco, only two months away. We’ve been working around the clock to get the tide forecast up and running, well before the Three Bridge so users can get acquainted. We’re also going to be offering tidal current GRIBs, for racers that want to try out weather routing inside the Bay. Imagine being able to wake up on the morning of the Three Bridge, download the Sail Tactics wind+tide GRIBs and use your weather routing software to tell you the best way to go around Treasure Island, Blackaller, and Red Rock. That’s what we’re aiming to do this year and we are confident that one of our customers can pull off a Three Bridge win using our wind and tidal current GRIBs. Of course, you can always just eyeball weather route, like I do when I’m on a boat without instruments, just by looking at the Sail Tactics wind and tide forecast maps.

Overview map of the Sail Tactics tide forecast at the lowest zoom level to help you develop your overall strategy for a race. For example, you might say, “It’s flooding and we need to go from the St. Francis YC to Point Bonita, and we’ll be fighting the flood the entire way.” These overview maps help you answer the first strategy question on most boats, which is “What is the tide doing today?” This tidal current forecast is from Fri, Oct 17 at 6:15 am PDT.

What makes the Sail Tactics tide forecasts different?

The Sail Tactics San Francisco Bay tidal current predictions will provide high resolution forecasts for surface currents throughout the Central Bay, South Bay, San Pablo Bay and the Gulf of the Farallones. The forecasts were developed by our tide forecaster, Rusty Holleman while doing his PhD research at UC Berkeley. Rusty will be updating the forecasts daily from Wednesday through Sunday, with outlook forecasts covering Monday and Tuesday.

What makes our tide forecasts different is that the tidal current predictions come from a physics-based computer model developed locally at Stanford University. The computer model, affectionately called SUNTANS, was originally developed by Stanford Professor Oliver Fringer to study three-dimensional currents in Monterey Bay. The oceanographic research community has used SUNTANS for numerous scientific investigations in the coastal ocean, bays, and estuaries. Commercial uses have included dredging studies, salt pond restoration modeling, and even current predictions for the America’s Cup. Rusty has also recently applied this software to generate educational salinity hindcasts and forecasts for the Exploratorium’s Bay Observatory.

The middle-zoom tidal current forecast helps you refine your overall strategy for the race. Based on additional tidal current information available to us in this zoom level, returning to our hypothetical race from St. Francis YC to Point Bonita, we might say, “There appears to be some current relief by the South Tower on our way to Point Bonita. If we go to the North Tower, we’ll be right in the teeth of the flood.” This tidal current forecast is from Fri, Oct 17 at 6:15 am PDT.

Rusty’s PhD research at UC Berkeley focused on applications of the SUNTANS model to San Francisco Bay. Rusty and I met when I was a postdoc in the same research group at Cal and we began enthusiastically talking about creating a tide forecast for sailors on the San Francisco Bay. During his time at Cal, Rusty customized and tweaked the SUNTANS model to better capture the physical processes dominant in San Francisco Bay. For example, predicting tidal flows on the extensive mud flats in South Bay and San Pablo Bay required careful examination and tuning of the model’s underlying equations and assumptions.

“Using physics-based equations, rather than statistics-based tide tables, we incorporate non-tidal effects such as river flow and large-scale weather patterns.”
 — Rustly Holleman, Sail Tactics Tide Forecaster

Another significant challenge in simulating a body of water like San Francisco Bay is to distribute the spatial resolution of the tide model efficiently. Areas like the Golden Gate Bridge require grid spacing as fine as 50 meters, while 30 miles out into the Pacific this resolution is unnecessary. SUNTANS tackles this problem by solving the equations of motion on a triangular grid, which can smoothly transition from high to low resolution. The triangular grid for the Sail Tactics current predictions has been customized to the needs of Bay Area racers, providing excellent detail around typical courses, while remaining computationally efficient enough to be updated daily.

The highest-zoom level of the tidal current forecast helps you back up your strategy with numbers. In our example of trying to get from the St. Francis YC to Point Bonita during a flood, this zoom level lets you make some calculations to support your strategy. Based on information in this zoom level, you might say, “The South Tower relief is actually a gyre, pulling us [west] towards the Gate. The flood near the North Tower is about 0.8 miles wide, or nearly the width of the Golden Gate Bridge with a max speed of 3.1 knots. We can either attempt to beat into the teeth of the flood for about 1.5 miles of tacking… or get relief all the way to the bridge and then cut across [north] at the Gate, which will bring us to building ebb on the north shore of the Gate.” This tidal current forecast is from Fri, Oct 17 at 6:15 am PDT.

So what makes this different from using tide tables or an app on your phone that can tell you the tides for years out? The Sail Tactics current predictions work by taking a wide range of environmental factors (depth maps, ocean tides, river flows, wind, even wastewater outfalls) and using the equations of fluid mechanics to predict how these factors interact to affect currents. Because we will be running the current predictions daily, from Wednesday-Sunday, you’ll be getting the most up-to-date current predictions possible. Using physics-based equations, rather than statistics-based tide tables, we incorporate non-tidal effects such as river flow and large-scale weather patterns.

What is the best way to visualize the new tidal currents? Tell us!

What I’ve shown here is my first cut at visualizing Rusty’s tidal current forecasts for the San Francisco Bay. I thought it might be fun to ask you, the potential user of the forecast, what you would like to see in the forecast maps? Please leave your ideas in the comments below! We eventually plan on offering multiple viewing options (even tides+winds), so even if we don’t implement your idea in the first round, it might be in a future revision.


Originally published at www.sailtactics.com on November 26, 2014.