Jitters are rising beyond expressed overbought technical conditions that rebuilt in recent days; and beyond the grief (and needed assistance) not only to the twin Texas and Florida disasters; but now to the feared direct hit of yet another hurricane, as Maria slammed Puerto Rico.
 

That State, already very financially strapped, now has to contend with what is by any measure going to be historically heavy damage for modern times. Combine assistance to the islands, what may yet come as Maria heads over the Bahamas and (hopefully) skirts most of the Southeast US (less clear as regards the Mid-Atlantic and even the Northeast), plus a serious rethinking at all kinds of governmental, insurance and development levels with regard to future construction, and you do have a focus on truly urgent matters. 
 

On top of all that you had a little-noticed temblor off of Southern California; that was eclipsed by the truly disastrous one in Mexico.
 

All these issues will require American financial assistance and compassion, as they eclipse other issues that are more political in tone.

However at the moment that's not the focus. Rather it's the Fed Meeting we just concluded. Generally it isn't expected to see a 'rate hike' surprise in this sluggish economy; but it has some traders worried about a 'revelation' of the Fed moving to lighten its Balance Sheet as so many (especially FX or currency traders) fear. If that tone is not predominant you might get a brief relief bounce; and if it is (as FX volume implies plus Euro strength); maybe that will be seen as a negative catalyst. For the moment traders will watch it and the Yellen news conference that follows. 

In sum:

The market remains vulnerable while trying to hold together. So far it is a sluggish economy with no big changes to core inflation; and no big shift in market action although it can become more sensitive to anything now.

The Fed is poised to outline and initialize more normalization of policy as it relates to unwinding the balance sheet and that matters perhaps more now than nominal rate hikes at these low levels. 

There is no change in our overall view that sees the market also poised for a correction, the magnitude of which will not likely be a complete debacle as some fear, but an overdue correction... barring an exogenous event of which there are a couple which potentially threaten. Some believe that's the North Korean issue; while most suspect passivity in the face of that rogue nation's moves is really the greater threat to Northeast Asian security.

There is greater risk of decline forthcoming almost regardless of the coming Fed pronouncement, which there is a brief relief rally follow or not; which of course will vary with whatever statement they make make.