Hi. Yes, you will need to file an extension. As you will not meet the requirements for either the Physical Presence Test (PPT) or the Bonafide Residence Test (BFR) until at the earliest Nov, 2018, you cannot file your return until one of the tests is met.
You will need to file Extension Form 2350 which is not supported by TurboTax. You can download it from the IRS site and complete it and mail it in. I would keep a copy of it. Please click on the following link. https://www.irs.gov/pub/irs-pdf/f2350.pdf . Even if you think you will use PPT I would request an extension until January 30, 2019 (in case you end up being in the US more than 35 days in a 12 month period).
I would also suggest you complete the regular extension Form 4868 in case something comes up and you need to return to the US from abroad before either test is met.
Please see attached for additional informaton. Tax Guide for US Citizens Abroad
