Traders have tried to buy US dollars ahead of Fed events for the past two months and been burned repeatedly.

If Yellen comes out a signals a more hawkish Fed (even a slightly more hawkish stance) that’s probably all the US dollar needs to start a broad, long rally. If not then dollar bulls will (and the Treasury bears) will take yet another beating. I get the sense that we’ll see a bit of US dollar selling in the hour ahead of the Fed decision as nerves begin to fray.