The New York Stock Exchange is seen on May 7, 2013 in New York City. - 

There's some rare good news expected for the finances of the federal government this afternoon. The Congressional Budget Office estimates the Treasury will report a $112 billion surplus for the month of April. So far in 2013, the federal government is running a deficit $230 billion less than it was last year.

Chris Low chief economist at FTN Financial explains how this is possible.

Follow David Brancaccio at @DavidBrancaccio