This graphic depicts what happens. Think of like water disturbed in a pond or lake.
The satellite image below shows a gravity wave that moved over the lake earlier in the morning. The rippled clouds are a clear indication of a gravity wave over far southern Lake Michigan. There was also some evidence along the Michigan shore near Muskegon, but it is hard to see with the observations on the map.
The next satellite image shows the back edge of the gravity wave that caused the drizzle over southern Wisconsin. The wave extends north-south in this image, through the center of the lake.
The radar loop below shows the drizzle traveling across the area, with the white line on the first frame indicating the sharp edge of the front of the gravity wave. It also shows an interesting result of the gravity wave to the north…as the wave interacted with some left over snow showers, it clearly enhances the reflectivity on the radar. The extra lift caused the snow showers to intensify as the wave intersected them. There were even several reports of thunder and lightning at this time.
More information about gravity waves is available if you search on the web.