Why forecast tools are not navigation tools

Planning information

Planning information describes what conditions are expected to be. It includes weather and wave forecasts, the raw output of computer weather models, tide and current predictions repackaged for convenience, and summaries such as activity or suitability scores. It is good for comparing days, choosing between locations and spotting an obviously unsuitable forecast early.

CatchWindow belongs here. Its location pages show wind, weather, waves, tides, currents where a station applies, water temperature, sunrise and sunset, solunar periods and 0 to 10 activity scores, and its map shows wind and current layers along with boat ramps, campsites and fuel docks drawn from OpenStreetMap. All of that is for planning. It is not a navigation product and is not a substitute for the official sources described below.

Navigation information

Navigation information is what a person relies on to move a vessel safely. It comes from bodies with a formal responsibility for it, usually national hydrographic offices, maritime authorities and meteorological services.

Official nautical charts, paper or electronic, showing depths, hazards, buoys, lights and channels.

Notices to mariners and local navigation warnings, which report changes and corrections: a moved buoy, a new wreck, a light that is out.

Official tide tables and tidal current tables or atlases for the area.

Marine forecasts and warnings issued by the national weather service.

Instructions and local information from harbour and waterway authorities.

These sources are not infallible either, but they are produced to defined standards and corrected through known channels. Which publications apply, and what should be carried aboard, varies by country and region, so check the rules that apply.

Why forecasts and predictions can be wrong or late

Weather models divide the atmosphere and ocean into a grid and calculate forward from an imperfect picture of the present. Features smaller than the grid, such as a gap between hills that funnels wind, a sea breeze along one stretch of shore or a single thunderstorm, may be smoothed out or missed. Timing errors are common: a wind shift that arrives two hours early is a small error for a model and a large one for a small boat.

Tide predictions are calculated from astronomy and from past measurements at a specific station. They are dependable for what they cover, but they do not include the weather. Strong wind and unusually high or low atmospheric pressure can raise or lower the actual water level and shift its timing. Current predictions are generally less certain than height predictions.

Then there is the data itself. Any tool that gathers information from other sources can show something out of date if a feed is delayed, a station is offline or a phone has no signal. Map data contributed by volunteers, including the position of a ramp or a fuel dock, can be incomplete or out of date.

What a suitability score can and cannot tell

A suitability score compresses several forecast values into one number so that days and places can be compared at a glance. That is its whole job. A high score says that the forecast inputs line up well for the activity in general. A low score says they do not. A score cannot know:

the size, type and condition of the boat or craft

the experience, fitness and number of people aboard

local hazards such as bars, rocks, shipping, a difficult entrance or patches of rough water where current runs over an uneven bottom

whether a warning has been issued since the data was last updated

what the water actually looks like on arrival

So a score is never a safety declaration. A day rated 9 for fishing may be entirely wrong for a small open boat. Use it to shortlist, never to decide.

The decision belongs to the person in charge

On the water, responsibility rests with the skipper, or with paddlers for themselves and for the group they lead. No forecast provider, app or website shares it. That is not a statement about the law, which differs from country to country; it is simply how good seamanship works. The person in charge is the only one who can see the actual conditions, knows the boat and crew, and can turn back.

Good judgment means being prepared to act against a favourable forecast. If the sky, the wind or the sea looks worse than predicted, the observation wins. If a plan depends on a forecast being exactly right, the plan needs more margin.

Using both together

The two kinds of tool work well in sequence. Several days ahead, use planning tools to pick a likely day and place. The day before, read the official marine forecast and any warnings, look up the official tide and current tables, and study the chart for the route. On the day, check the official forecast again before leaving, then compare what is visible at the water's edge with what was expected.

Where two sources disagree, plan for the less favourable one and give the official source priority. The article on why forecasts disagree explains where the differences come from, and the pre-trip conditions checklist sets out the full routine.

In practice

Treat CatchWindow and tools like it as a way to narrow down when to go and what to look at more closely. Treat official charts, notices, tables, forecasts and warnings as what to navigate by. Keep the final decision with the person who will be on the water, and when the picture is unclear, do not go.