LONDON—Investment banks in May largely stuck to their forecasts that oil prices will rise in 2019’s final two quarters, predicting that supply risks will make up for sharp selloffs in recent weeks. Brent crude, the global benchmark, was expected to average close to $70 a barrel for the year, although the banks edged down their average estimate to $69.73 a barrel from $69.93 in April, according to a poll of 10 institutions conducted by The Wall Street Journal. West Texas Intermediate, the U.S. oil benchmark, was expected to average $61.14 a barrel this year, down 0.6% from April’s forecast o...