Every chairman of the US Federal Reserve seems to get hit by a crisis in their first year. For Paul Volcker in 1979 it was raging inflation. For Alan Greenspan it was the 1987 “Black Monday” meltdown. And for Ben Bernanke in 2006 it was the bursting of the US housing bubble. Whatever might blindside Janet Yellen, she starts off with a problem that affected none of her predecessors: the Fed has run out of ammunition. Moreover the one remaining weapon the Fed thinks it has – the hocus-pocus of “forward guidance” – is a gun that fires blanks.

Given how much emphasis Ms Yellen puts on forward guidance, her first conundrum is plain. A widely forecast year of US take-off is once again in danger of faltering before it happens. US job creation since November has averaged 116,000 a month – well below escape velocity. Manufacturing growth has stalled. And export growth is slowing sharply. Some of this might be seasonal. Last month’s chilling “polar vortex” was hardly a good time for America’s jobseekers to pound the streets. But it is inconsistent with the strong rebound that was expected in 2014.

Should the poor numbers persist, Ms Yellen’s first option – to press pause on the Fed’s two-month-old tapering – would be hard to take even if she wanted to. At his final meeting in January, Mr Bernanke wound down quantitative easing by another $10bn to $65bn a month.