You can always go standby on the next flight. United, like most airlines has a flat-tire rules - which I believe in United's case is 2 hours past time of flight. So you should be fine. Nonetheless, I live up in Mukilteo (the city attatched to the buttom of Everett) - and regardless of what day I leave (whether is day before Turkey day or day after Christmas), the past two years, I've simply left Mukilteo 2 hours prior to my flight and never had a problem. If you're feeling worried, leave 2.5-3 hours. It usually takes no more than 45 mins to get to Seatac. And... if snow is screwing up the roads in Seattle, it's invariably screwing up the airports.