[It’s not just the stock market that’s flashing warnings. The bond market is too.]

Howard Silverblatt, a senior index analyst for S&P Dow Jones Indices who tracks and catalogs market performance, said stocks’ swings had been larger and more frequent this year than at any other time since 2011. On 15 days in 2018, the value of the S&P 500 changed more than 3 percent. Such a large change did not occur on a single day in 2017, and happened on only five days in 2016. In 2011, when ratings agencies downgraded United States government debt, roiling the stock market, the days on which the S&P lost or gained 3 percent numbered 24.

“It’s higher today than it has been for the last several years, and you can feel it,” Mr. Silverblatt said of the market’s volatility.

[Read more about investors’ biggest concerns, and about how technology stocks fell into a bear market last week.]

Adding to the recent jitters are concerns about Mr. Trump’s response to the drop in stock prices. Last week, Mr. Trump used Twitter to vent his frustration over the Fed’s decision to keep raising interest rates and sought guidance from aides about whether he could fire Mr. Powell. Despite the president’s frustrations, a number of White House officials have tried to reassure investors that Mr. Powell’s job is safe.

Traders have had their own concerns with Mr. Powell as well: At a news conference on Dec. 19 he used a word that is still ringing in market participants’ ears — “autopilot” — to describe the Fed’s likely approach to monetary tightening.

This suggested to investors that the Fed might not be willing to change its plans to further raise rates and drain money from the financial system, even if market turmoil persisted. Other Fed officials quickly moved to assuage those worries after the Dec. 19 announcement, but Mr. Donabedian said he expected anxiety about the Fed’s flexibility to persist until at least the end of the next meeting of the Fed board on Jan. 30.

Ultimately, he said, deep worries about the future, rather than more superficial factors like low trading volume, were behind the market tumult lately.

“There are a lot of people searching for something else, some technical factor,” he said. “I really think this is driven by rising uncertainty about the fundamentals: earnings, the economy and interest rates.”