Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

Despite the uncertainty hanging over the market linked to the situation in Syria and the possibility of a U.S.-led military strike, U.S. stocks opened little changed today, with the S&P 500 (SNPINDEX:^GSPC) and the narrower, price-weighted Dow Jones Industrial Average (DJINDICES:^DJI) up 0.3% and 0.2%, respectively, as of 10:10 a.m. EDT.

After a recent spate of downbeat economic indicators, today's revision in the second-quarter GDP data comes as a pleasant surprise. According to the latest estimates published by the Department of Commerce, the economy grew at a 2.5% annualized rate during the three months to June 30 -- a substantial improvement over the initial 1.7% estimate. The new estimate was also higher than the 2.2% consensus forecast in a Reuters poll of economists.

Where did the improvement come from? New trade data showed that exports grew at the fastest rate in more than two years, despite the fact that the trade-weighted U.S. dollar index strengthened during the second quarter. Retailers also added to their inventories at a faster clip.

Using the new estimate, the U.S. economy grew at an annualized rate of roughly 1.7% in the first half of the year. According to the Federal Reserve's latest published estimates, which were released at the conclusion of the June Federal Open Market Committee meeting, Chairman Ben Bernanke and his colleagues expect full-year growth in a range of 2.3% to 2.6% for 2013.

In order for the economy to achieve even the lower bound of that range, it would need to grow at an average rate near 2.9% (annualized). Clearly, this would imply a pickup in growth, even from last quarter's level. That looks like a bit of a reach to me, but far from impossible; after all, GDP grew 2.8% in the second half of 2010.

Today's upward growth revision may raise the likelihood that the Fed will begin to scale back its $85 billion-per-month bond purchases next month, even if, to this observer, that would seem to be odd timing, given that next month will already be saddled with considerable uncertainty linked to political squabbles over government funding and the federal debt limit. Either way, I'm expecting a volatile September. Investors would do well to steel themselves for that possibility and remain focused on valuations and long-term investing goals.

Fool contributor Alex Dumortier, CFA has no position in any stocks mentioned; you can follow him on LinkedIn. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.