Having spent time in SoCal myself, I can tell you the transit system in Seattle is very good so don't bother with Uber/Lyft unless you're going somewhere off the beaten path.
Also, the rain isn't as big a deal as it is in SoCal where it really comes down. Here it mists, sprinkles, drizzles, spits and everything else but what you'd think of as rain. Locals don't use umbrellas because they're so seldom needed.
Your normal SoCal Winter clothes will be fine for the game. You can grab a cheap rain poncho the day of if you're afraid of getting wet.
Last thing is to remember: Jesus Christ Made Seattle Under Pressure. Starting at the south end of downtown the streets are named: jefferson, james, cherry, columbia, madison, marion, seneca, spring, union, university, pike and pine. That should keep you from getting lost as you wander around.