The St. Louis Fed's Financial Stress Index has shot upward in the past few months.
Instead of going into the details about how the index is composed, I'll let you click through to the St. Louis Fed's explanation here.
The last three times that we've seen spikes like this were during the U.S. financial crisis of 2008, during the acceleration of the Greek crisis in May 2010 and in August 2011, when the U.S. credit rating was downgraded by S&P.
The latest stress spike began in the second week of May and has continued unabated.
We already know that markets are in turmoil, so this isn't exactly news. But one thing to remember is that the past spikes in the index were swiftly followed by Fed action that relieved the stress. In June 2010, the Fed extended swap lines to central banks around the globe. In August 2011, the Fed announced that rates would remain low for at least two more years.
Assuming this isn't a completely spurious correlation, the recent rise in financial stress may mean that we should expect something new from the Federal Reserve soon.
—By CNBC's John Carney. Follow him on Twitter @Carney.