In the Portland and Vancouver WA areas lawns do not go completely dormant over the winter. Fall is the best time to prepare your lawn for the coming spring with a fall winter fertilizer and a treatment of lime.
Fertilize your lawn with a slow release Nitrogen product usually sold as a winterizer or Fall-Winter fertilizer. This would typically be a 18-4-12 fertilizer with slow release Nitrogen. . This way the nitrogen is available for the coming spring as your lawn wakes up and produces a green healthy lawn quicker.
The fall is also a great time to apply lime to the lawn to control acidity. A lime treatment will control the soil pH (acidity) and help the grass roots absorb the fertilizer.