The U.S. national debt has increased by more than half a trillion dollars in less than six months, amid predictions that the U.S. is on the verge of returning to another explosion in government borrowing.

The debt rose to an all-time high of $21.4 trillion on Thursday, according to government data posted Friday. That's more than $500 billion above the $20.88 trillion seen March 7, just a bit more than five months ago.

While the debt continues to rise, the pace of growth has slowed for much of 2018. It only took about six months for the debt to jump from $20 trillion to $21 trillion, a mark it hit in mid-March.

Growth of about $500 billion over the last five months is a relative improvement, one that reflects the surge of tax receipts that flowed into the government starting in April, when individual tax returns are due.

But there are signs the debt situation is getting worse. The Treasury Department says the government is on track to have a total budget deficit of about $850 billion in the current fiscal year that ends Sept. 30.

Fiscal 2019 is expected to be worse: The Congressional Budget Office predicts a budget deficit of just under $1 trillion.

The budget deficit is just one factor in the total national debt, which also expands due to federal lending programs that aren't taken into account in the budget deficit figure.

Under current law, the government is allowed to borrow as much as it wants through March 1, 2019, which is still more than six months away. After that, the debt ceiling will take effect again, and limit borrowing to whatever the size of the national debt is at that point.

Congress in recent years has avoided this limit by suspending the debt ceiling again.