For the FOMC tonight, it's all about forward guidance. At Yellen's congressional testimony two weeks ago, the Fed Chair neither ruled out a September hike, nor signalled September was a done deal.
"A September hike is still expected, but analysts make the argument that aesthetics matter to a data dependent Fed", says RBC capital markets.
The non-committal message might also be what is conveyed in tonight's statement.
"As a result, the Fed will opt to keep their options open and use the seven weeks between the September and July meetings to firm up their decision", added RBC capital markets.