The Wire hitmakers were wrapping up their performance at the city's Forum venue when Welch, wearing a white T-shirt bearing the trio's name, stepped into the spotlight and decided to crowdsurf.
Bassist Este Haim took the Florence and the MAChine singer's lead and threw herself into the audience too.
Welch's impromptu stage dive wasn't the only surprise during the show - the Haim stars were also presented with their first ever gold disc for selling more than 100,000 units of their debut album Days Are Gone in the U.K. The project hit the top of the British albums chart in October (13).
Welch is no stranger to the Haim sisters - the band supported the Dog Days Are Over star during her U.K. tour last year (12).