I run an 18fter. If they post 20mph winds I'm not going to waste the $$ to get beat up. Yes you can fish in that with the wind from most any direction. But it's going to be rough, and the longer it blows. The rougher it is going to get. Most tourney's send the guys out in 20 mph as it usually is like 3 to 4's and building up to 6 after a few hours. Also depending on who is giving wave hgts, they typically measure the wave hgt from water median hgt as a base not the trough/bottom of the wave. Most boaters measure the waves as top to bottom of the waves.
Also if there is a 20+ mph from the S, the air is very warm (70s' let's say)while the water is cold ( 40+) in the spring of the year. The warm air skips over the cold surface water and goes upward to where you can be out a few miles and there will not be but a breeze. Happens most every year depending. Fishing around the islands can really help as you only have short open area's and can usually find calmer water and fish with one side or the other to help block the wind. Most folks hate an east wind the most.