Unless you consider it truly essential to visit some Portuguese beaches in the following weeks, coming to Portugal - as an American citizen - is only possible if there is actual essential reason for now. Individuals wanting to travel for tourism will still have to wait a while to see if the current rules (see the legislation) ease for Americans coming to Portugal. If you hold a Portuguese passport, you can travel now.
Essential travel is considered a trip motivated by work, studies, family reunions, health or humanitarian reasons.
For an American citizen traveling for essential reasons, you still have some rules to follow. Passengers on flights departing from the United States must present, at the time of departure, proof of a Covid-19 test (only PCR tests are accepted) with a negative result, obtained up to 72 hours prior to departure.
In an exceptional case of the passenger arriving in Portugal without having been previously tested, they will be refused entry to the country and ask to return to the US on a flight covered by the respective airline.
Even if the traveler tested negative, everyone entering the Portuguese airports will be subjected to an infrared temperature screening - at no cost. However, if the traveler shows a significant temperature, the Portuguese airport authorities will test them and bill them. In the meantime of waiting for the results, the traveler will have to remain in quarantine.
Finally, when returning to the US after a stay in this European country, no quarantine is mandatory if no symptoms are presented.