How to read a marine forecast

Four numbers decide whether a day on the lake is a pleasure or a pounding: wind speed, gusts, wave height, and — the one most people skip — wave period. Here's what each one is really telling you.

Wind speed is in knots, and it's the engine of everything

Marine forecasts report wind in knots (nautical miles per hour), not the miles per hour you see on a land forecast. One knot is about 1.15 mph, so 20 knots is roughly 23 mph. Wind is the number to watch first, because on an enclosed body of water like the Great Lakes the wind is what builds the waves. There's no ocean swell rolling in from a storm a thousand miles away — the chop you feel was made by the wind you're standing in, often within the last few hours.

A rough rule of thumb for a small or mid-size recreational boat:

These are guides, not guarantees — direction and fetch (covered in a separate guide) can make the same wind speed far better or far worse.

Gusts are the number that actually bites

A steady 12 knots with gusts to 25 is a very different day from a steady 12 with no gusts. Gusts are what heel a sailboat over, what slam the bow down, and what catch you off guard while you're netting a fish. When the gap between the sustained wind and the gust is more than about 5–10 knots, the day will feel unsettled and unpredictable even if the average looks modest. Always read the gust figure, not just the sustained wind.

Wave height: it's an average, and the biggest ones are bigger

Forecast and buoy wave heights are usually given as significant wave height — the average height of the largest one-third of the waves. That definition has a sting in the tail: if the forecast says 3 feet, plenty of waves will be 3 feet, but roughly one in ten will be over 4 feet, and the occasional one can approach twice the significant height. Plan for the big ones, not the average.

Wave period is the number everyone ignores — and it decides the ride

Wave period is the number of seconds between one wave crest and the next. It rarely gets top billing, but it's the difference between rolling gently over swells and getting rattled to pieces. Two feet at 8 seconds is a lazy, comfortable roll. Two feet at 3 seconds is a short, steep, slapping chop that throws spray and beats your kidneys.

On shouldiboat.com the hour-by-hour table shows wave height with the seconds-between-waves right beside it (for example, 2 ft @ 4s) so you can judge steepness at a glance, not just size.

The nearshore forecast: read it like a boater, not a meteorologist

The National Weather Service issues a plain-text Nearshore Marine Forecast for each stretch of Great Lakes shoreline, broken into periods (TODAY, TONIGHT, and the next few days). A typical period reads:

Read it in order of what matters: the wind direction and speed (and how it's changing), then the wave height, then the weather. "Becoming W 15 to 20 kt in the afternoon" is the key phrase here — the morning may be fine and the afternoon may not, which is exactly the kind of turn that catches boaters out. shouldiboat.com splits each nearshore period into clean wind / waves / weather rows and pairs it with an hour-by-hour timeline so you can see when the day changes.

This guide is general education, not an official forecast. Always check the official NWS marine forecasts and use your own judgment on the water.