Every wave on all five lakes, hour by hour and five days ahead, from NOAA's wave model. Tap the water for a reading.
This is a forecast model, not a measurement. NOAA's Great Lakes wave model (GLWU, a version of WAVEWATCH III) covers all five lakes with one grid of 2.5 km squares. It grows waves from the forecast wind, hour by hour, and runs four times a day. The page reads the model's own files: wave height every hour for two days and every three hours after that, direction and period every three to six hours.
The height is the significant wave height: the average of the tallest third of the waves. It is close to what an experienced sailor would call the size of the sea. Single waves run bigger. In a long storm the tallest wave of the day can be close to twice the significant height, which is why a TV forecast of the "top waves" can read higher than this map.
Wind makes the waves, and distance lets them grow. A north wind runs down the full 300 miles of Lake Michigan and builds the biggest seas on the lakes; the same wind across Lake Erie, 50 miles wide, has no room. The lines show which way the waves travel. Crests are drawn far wider apart than the real ones (100 to 150 m on a big day) so you can see them at all; they brighten and sharpen as the sea builds, and whitecaps appear past about 6 feet. Streaks show the same direction as moving lines.
The wind over the land comes from a second NOAA model, the global GFS (squares of a quarter degree, about 25 km, every three hours), because the wave model stops at the shore. The pale lines drift the way the wind blows and run faster and brighter where it blows harder. Tap the land for its speed. The wind on the water is what builds the waves, so the land lines show where it comes from before it reaches the lake.
Close to shore the 2.5 km squares are too coarse to show the surf, and bays the model leaves out borrow their nearest open water. For the current under the waves, see Lake Erie, in Motion.