What's behind the narrowing in the Brent-WTI spread?

The collapse in the differential between the prices of Brent crude and West Texas Intermediate (WTI) primarily reflects the prospect of a further easing of the logistical constraints that have been holding back the traditional US benchmark. This spread narrowing should therefore soon become permanent. In contrast, we would give very little weight to explanations based on the relative strength of the US economy, higher interest rate expectations, or developments in the Middle East. But Brent might also now be more vulnerable to swings in investor sentiment towards commodities more generally, and the Brent-WTI spread itself has become a focus of speculation too.