The first point of contention will be the $99.50 level which acted as support for the first half of August.
After that buyers will be taking shares off panicked holders, particularly those who bought late through the July-August rally. But if the SPY can't close the day above $99.50 it will hand initiative back to the sellers.
Dr. Declan Fallon, Senior Market Technician, Zignals.com the free stock alerts, market alerts, and stock charts website