Here's the marine forecast for tonight's storm moving through Chicago:
NORTH GALES TO 40 KT BECOMING NORTHEAST 15 TO 25 KT AFTER MIDNIGHT. PARTLY CLOUDY. WAVES 7 TO 11 FT SUBSIDING TO 5 TO 8 FT.
Where does this come from?
In theory, the main source of wind is air moving from a high pressure system to a low one. Let's look at the pressure map.
Yep, there is a HI pressure system sitting to the NW of us and two LO pressure systems—one directly south of us and another directly east of the HI. HI pressure system is much like a giant mountain of atmosphere made of air much like real mountains are made of earth and rock. Unlike earth and rock, air is not solid and whenever a mountain is formed next to a valley (the LO area) there is a flow from high to low pressure areas. That is wind. So, being in between the two pressure systems, we should be experiencing NNW winds. Let's look at the wind maps, then.
Not exactly, the wind through Chicago area is distinctly from the NNE, not NNW which is the direction that connects the two centers. Why is that?
Straight flow of air from the HI to the LOW is modified by the rotation of the Earth (technical term is Coriolis Effect). Instead of flowing directly from the center of the HI to the center of the LO, in the northern hemisphere, the wind around a HI pressure system spirals in the clock-wise direction and then counter-clock-wise into the center of the LO pressure system. It's the opposite on the other side of the equator. The wind map shows exactly that--an inverted S curving around the HI on top and into the LO on the bottom.
Let's get down to the water level now. Wind blowing over the open water, the friction between the two causes waves. Here's the map of Lake Michigan, then:
It shows small waves on the side of the lake from where the wind is blowing. As it moves across the open lake (fetch) over time the effects compound and the waves grow larger. One foot waves on the north end up as 9-footers on the south some 300 miles later.
So there you have it. Here's what I actually saw on Oak Street Beach this afternoon: