If you've deployed in the last 10 years to a combat zone you probably made some TSP contributions that were tax exempt. About 10% of my own TSP balance today is tax exempt. Earnings from that portion of my portfolio will be taxed as ordinary income when I take them out. If I could move the tax exempt portion to the Roth TSP, however, the earnings might grow tax free. That's very attractive, particularly for military members still on active duty.
Unfortunately it isn't possible at this time. I spoke with a the TSP representative yesterday who informed me that today there isn't an option to convert traditional TSP contributions to Roth TSP. She advised me to continue to monitor the TSP website for updates and changes.
This brings us to the point of this article. I previously commented about contributing to a Roth IRA compared to the Roth TSP. In most cases you'll have more flexibility with the IRA.
NOTE: IRS Pub 590 reminds us that your traditional or Roth IRA contributions are limited to the taxable compensation for the year or $5,500 ($6,500 if 50 years old and older) whichever is lower (corrected per comment by Jonathan). Congress recognized that if you are deployed for the entire year you might not have any taxable compensation, so they passed legislation that still allows you to contribute to either a Roth of Traditional IRA. (See IRS.gov "Miscellaneous Provisions — Combat Zone Service" question 34 for the explanation.