Controller reported aircraft descended below assigned altitude resulted in a CFIT event.

Narrative

Aircraft X was issued a clearance of a speed reduction to 170 then maintain 90 with a good readback. I was spending extra time issuing reported tailwinds and turbulence when I got a report from tower of moderate to severe turbulence on final. My priority immediately went to issues that report to then 2 aircraft that were about to fly through that same area. This took my eyes off Aircraft X in the downwind who I observed at 79 in an 85 MVA. I issued an immediate climb to 90 and an immediate turn to a lower MVA along with a low altitude alert. They continued in the downwind and then landed at ZZZ without any further issues. They were issued the BRASHER warning on the ground. Recommendation: This is the first time I have had this happen in the downwind here in 6 years. The only thing I could have done differently is catch it sooner. Maybe if tower had coordinated with the supervisor or flow about the turbulence I would have been able to. However; I understand a severe report deserves swift coordination and I agree with how tower went the quickest route.