U.S. market indicators are signaling that the economy could slow in 2019, former deputy Treasury Secretary Roger Altman told CNBC on Monday as markets closed early ahead of Christmas.

The major averages saw the worst Christmas Eve trading session in history as the Dow Jones Industrial Average shed over 650 points and the entered bear market territory.

Altman, founder and senior chairman of investment advisory firm Evercore, said that while the market was not signaling a recession, it was reflecting an "anticipated slowing."

"I think the outlook for 2019 is weakening," he told CNBC's "Closing Bell." "If you look at a whole series of factors like bond spreads, like the trends right now in consumer net worth, like the index of leading indicators, you see incipient weakness."

The weakness, exacerbated by falling commodity prices and turmoil in Washington, could lead to gross domestic product estimate cuts for the year ahead, Altman said. Gross domestic product, or GDP, is a key economic measure that tallies the market value of all goods and services in a given region over a period of time.

Altman warned that U.S. investors could see 2019 GDP forecasts slashed "quite a bit, maybe by a full percentage point." Current estimates have been declining but hover in the high 1 percent to mid-2 percent range.

"The narrative of synchronized global growth, … I think, has removed itself," he said. "We don't have that now."

And while the former Treasury official saw Treasury Secretary Steven Mnuchin's call with top bank executives over the weekend as "commonplace and appropriate," Mnuchin's subsequent statement that the U.S. markets had "ample liquidity" was somewhat miscalculated, Altman told CNBC.

Calling it "unusual" and something investors would only expect to see during times of credit crisis or other emergencies, Altman said it was "not a wise thing to do."

"Either way you slice it, it was a mistake," he said.

Turbulence at the upper ranks in Washington isn't helping, either, the investment chief added.

From the president's attacks on the Fed to the government shutdown, the "Washington overlay as a whole" is "obviously putting downward pressure on valuations and the market, and almost all of that is needless."