VIX and VXX as leading indicators in a sideways market

Today I noticed that SPY had been an almost horizontal line for over two days, stuck at 111.25.  It seems like this is often a setup for a fairly sharp sell-off when it appears a rally has run out of gas and fear gets an upper hand.   The S&P value was going nowhere, but fairly early in the day today, around 10:00, the VXX started climbing.  Intraday the VXX gives a pretty good indication of close in VIX futures action.  The VIX doesn't start to move up until quite a bit later, but look at 2:30 on the intra-day SPY chart.   SPY rallies a little, but the VIX isn't buying it--it's going up too.   In this case anyway it appears the VXX signaled that traders were getting nervous, well before the decline started.

I was very tempted to buy some SDS -- double short SPY in the early afternoon, but chickened out.
Click to enlarge.