20 seconds is the period of the swell. How this translates to the beach is longer swell periods mean more time between waves and surge during the sets. I would think that this would bode better for diving than a 10-12 sec period of similar wave heights. The longer the period the faster the swell trains are moving, meaning that as the swell decays the periods should get shorter.
Swell models for this event have been all over the map. Some people claiming triple over head surf, some saying solid but not huge. And NOAA, which I find usually overestimates, seems to be forecasting on the smaller side. One way or another there will be solid swell in the water coming from a pretty westerly direction, which as Chuck said should help provide some shadowing inside the Monterey are of the Bay.
This first swell is supposed to be peaking on Fri. night/Sat. morning. As for Monday, they are forecasting the another bump in swell on Sun. afternoon, which probably doesn't help if you are hoping for things to clear up. We are still going to try and dive of our boat on Sat. We do have reservations at Lobos on Sun., but I have a feeling that it won't be very good for diving.
That's my two cents, for what it's worth.
Dan