While American citizens cannot enter the EU countries, you can fly to Europe on a different passport, such as a Polish one. There are two ways to do it: on regular flights or “rescue” flights.
Situation changes, so check your connection to see if you can fly on a regular connection. If regular flights are not allowed, Polish citizens and people who have the right to enter Poland can get on a rescue flight operated by Polish Airlines LOT (see info at the bottom of the page).
For regular flights, it’s the usual. You can book a flight on a regular airline such as Delta using your Polish passport or a passport from another EU country. Going to Poland requires a layover somewhere in Europe as usual, but typically is faster. You can also book a regular flight on the Polish Airlines LOT starting from Chicago, New York or Washington DC and connect to it from Seattle on a domestic flight. This is typically slower than above.
Flying with an expired passport. Renewing Polish passports has been quite difficult for WA residents in 2020. However, the Polish Airlines LOT have a policy to allow Polish citizens with an expired passport to board flights to Poland. To avoid complications with other airlines, buy a domestic ticket to Chicago, New York or Washington, DC and a separate LOT ticket from there to Poland.
Update as of January 1, 2021: it seems that the rescue flights are off. The link below for the page with rescue flight schedule now redirect to a page with a regular flight info.
For rescue flights, you can book a seat onboard a special charter flight by the Polish Airlines LOT. LOT flies these only from Chicago and New York JFK for Polish citizens and other people who have the right to enter Poland. Tickets for the charter flights are not available at the LOT website or travel agents – to make a reservation you need to contact LOT directly. Check for available dates and the booking application requirements at the LOT website here.
More: Border regulations for entering Poland (in Polish), Coronovirus regulations and restrictions in Poland (in Polish)