The tyre must have at least 1.6mm across the middle 3/4 of the tyre's width, so if it's for example 145mm wide then a section of 108mm along the middle needs to have legal tread. If the worn section has canvas showing then it's illegal.

I would say by looking at it that it's borderline legal, however you need to get the tape measure out, but just by looking at it I certainly wouldn't want to drive on it especially with the heavy snow that is forecast tomorrow for the most of the country!