In various media and blogs much has been made of a Bipartisan Policy Center (BPC) study that projects there will be $172 billion in federal revenues in August against $307 billion in authorized expenditures. The $172 billion is supposedly enough for debt payments, Social Security and more -- problem for those accounts, at least, supposedly solved. However, this does not account for the timing of the August cash flows. If the incoming funds arrive mostly late in August there may not be enough to avoid defaults early in August.
Update, July 14:
"Powell’s [Jay Powell, undersecretary of the Treasury for Finance under President George H.W. Bush] research also shows that the daily cash-flow outlook would be just as difficult [as the over all monthly shortage] to manage. For example, if the cash shortage begins on Aug. 3, as projected by Treasury, the government may find itself unable to make a $23 billion Social Security payment due to go out that day." (LINK)