WASHINGTON—In a fresh sign of his growing concerns about inflation, Chair Jerome Powell said Wednesday that the Federal Reserve can’t be sure that price increases will slow in the second half of next year as many economists expect. Powell told the House Financial Services Committee that most economists regard the current price spikes, which have sent consumer inflation to a three-decade high, as largely a response to the pandemic’s persistent disruptions to supply and demand. As Americans have spent more time at home, they have ramped up spending on furniture, appliances, laptop computers. Soaring demand for such goods, combined with parts shortages, have resulted in supply chain snarls and higher prices. In the past, Powell, who was nominated last week to a second four-year term by President Joe Biden, has frequently expressed his belief that these supply-and-demand imbalances should fade as the pandemic eases, which would reduce inflation. But on …