The data transfers to your US servers for EU citizens are now covered using the EU/US privacy shield.
In the European Parliament resolution on the adequacy of the protection afforded by the EU-US Privacy Shield (2018/2645(RSP)) I see this :
"34. Takes the view that the current Privacy Shield arrangement does not provide the adequate level of protection required by Union data protection law and the EU Charter as interpreted by the European Court of Justice;
- Considers that, unless the US is fully compliant by 1 September 2018, the Commission has failed to act in accordance with Article 45(5) GDPR; calls therefore on the Commission to suspend the Privacy Shield until the US authorities comply with its terms;"
Is there a way to sign on this transfert using a Standard Contractual Clauses.