The consensus economic projections of Fed officials published one year ago show that they expected the economy to grow 2.5 percent in 2018 and 2.1 percent in 2019. If anything, they were too pessimistic about 2018, which now looks likely to be north of 3 percent.

But slower growth in 2019 and 2020 has been expected among mainstream economic forecasters ever since the tax legislation took shape a year ago.

“Nothing in economics and markets happens in a straight line or without lags and feedback effects,” said Blu Putnam, chief economist at the CME Group. “There are some big headwinds all coming from events in 2018 that we know about, but whose impacts are yet to be fully felt or appreciated.”

And knowing something bad will probably happen is not the same as knowing when. It evidently took another confusing series of developments in economic diplomacy between the United States and China for it to become clear.

“Most economists are good at analyzing fundamentals, but they know better than to forecast direction and timing in the same sentence,” Mr. Putnam said.

Part of the agita on financial markets in the last few weeks has come from fears that the Fed has been underestimating these risks, and is dead set on raising interest rates several more times next year even as the economy is still trying to adjust to the early rounds of increases.

Those fears have eased some as Fed officials have signaled open-mindedness and flexibility about the path ahead in the last few days. But a tricky period for the Fed is only beginning.