Charles Schwab's (SCHW) Chief Investment Strategist Liz Ann Sonders says the bull market still has some steam.

"No one has been a big believer in this bull market and that's a positive," she said. "One of the reasons we have remained fairly constructive this entire bull market is because of how many disbelievers there are. I think the sentiment environment is probably the most important prop for the market."

The S&P 500 trades at 2,166, just shy of its record high close of 2,175 reached on July 22.

Plus, the U.S. Citigroup Economic Surprise Index, which tracks whether economic data have been worse or better-than-expected, has risen in the past month to 38.6, sending the S&P 500 higher. A level below zero signals economic data has been falling short of analysts' forecasts.

"Better or worse matters more for markets than good or bad," Sonders says, adding that the improvement in economic data is all that's needed to ignite the stock market. 

In terms of how the Federal Reserve will process this in its July policy statement on Wednesday, Sonders isn't expecting a rate hike announcement, but is curious for any language that sets the stage for a September rate hike.

"We do think September is on the table if the data does continue to come out better than expectations," she said. "We're pretty convinced that in [the Fed's] mind, raising rates one more time this year is the right decision."

In December, the Fed expected to raise rates four times in 2016, a forecast that was lowered in March to two rate increases in 2016. That forecast was reiterated when the Fed met in June. Sonders is neutral on stocks, meaning her clients should stay with their current long-term allocations to stocks and use volatility to take advantage of pullbacks and re-balance their portfolio.