JPMorgan Chase boss Jamie Dimon, the son of a stockbroker and as steeped in financial market knowledge as any executive on Wall Street, has seen more than a few of his forecasts fall flat recently. The culprit has generally been what the late Donald Rumsfeld famously described as “unknown unknowns”.

Dimon expressed caution about the U.S. economic outlook last month, warning that the full impact of tariffs, immigration, geopolitics, and President Donald Trump's tax and spending policies remains un...