The numbers that matter, and the ones people usually misread
This guide covers the marine numbers behind the optional Water tools in Weather Bolt (formerly HarborCast). If you boat, kayak, fish, or spend time on the water, these are the ones to know.
Every marine forecast is packed with numbers, and not all of them carry the same weight. Two forecasts can both say "10-15 knots" and mean very different days on the water depending on what else is in the report. Here's what the main ones actually tell you.
Sustained wind speed is the steady background wind. Gusts are short bursts well above that. A forecast reading "15 knots, gusting to 25" is a much rougher day than a steady 15 knots with no gust line at all, even though the sustained number looks the same. The gust spread is often the better predictor of how uncomfortable a day will actually feel, especially for a smaller boat.
Height is the obvious number, but period, the time between wave crests, changes everything about how that height feels. Two-foot waves eight seconds apart are a gentle rolling day. Two-foot waves at three seconds apart is a short, choppy, tiring ride that beats up a small boat far more than the height alone suggests. A longer period spreads the same energy out over more time, which is why an experienced boater checks period before deciding a forecast is fine just because the height number looks small.
As a rough guide: periods above 7-8 seconds tend to feel manageable even with a bit more height. Periods under 4-5 seconds can feel rough even when the height is modest, especially beam-on to the waves rather than bow-on.
A small craft advisory isn't a subjective warning, it's issued at specific, published thresholds, generally sustained winds in the 20s of knots or seas building past a set height for the local waters. It's aimed at smaller vessels, but "small" is relative to conditions, not a fixed boat length, a 30-foot boat can still have a genuinely bad day under an advisory that a 60-foot boat handles easily. The advisory tells you conditions have crossed a real threshold; it doesn't tell you whether your specific boat and experience level are fine with that threshold. That part is still a judgment call.
Visibility gets less attention than wind and waves, but fog rolling in unexpectedly is one of the more common ways a routine trip turns stressful, particularly near shipping channels or busy inlets. A forecast noting reduced visibility, even without any wind or wave warning attached, is worth taking seriously on its own.
The same wind and wave forecast can be a non-issue offshore and genuinely dangerous at an inlet, because outgoing tide running against wind and swell stacks waves up higher and steeper right at the inlet mouth. Checking the tide state for the time you'll actually be crossing, not just the general marine forecast, is one of the more overlooked steps in planning a trip that starts or ends at a coastal inlet.