The fine will be doubled after 15 days. If you still do not pay, it will be sent to a collections agency (alliance one), who will add their own fees and charges on top of the ticket.
Once you have more than 3 unpaid tickets for the same car, things get pretty bad. Any further parking tickets are considered "scofflaw violations", which means the car will be impounded regardless of the severity of the parking violation. Also, your registration will be flagged with the state DOL, and you will not be able to renew your tabs until the tickets are paid.
Seattle offers payment plans, which can help you avoid the increased fine for late payment. Go down to the city courthouse in person, and ask about them at the payment window.
At the very least, you can always request a mitigation hearing. You can explain the circumstances at the hearing and hope for a reduced fine. More importantly, you don't have to pay the fine until AFTER the hearing, so it will buy you some time to scrounge up the money.